focus on CS8625, convert to error

.editorconfig

@@ -151,9 +151,14 @@ dotnet_diagnostic.MEF006.severity = silent
 
 dotnet_diagnostic.IDE2003.severity = silent
 
-#cleared null error types
+#nullability diagnostics
 dotnet_diagnostic.CS8597.severity = error
-dotnet_diagnostic.CS8605.severity = error
+dotnet_diagnostic.CS8600.severity = warning
+dotnet_diagnostic.CS8601.severity = warning
+dotnet_diagnostic.CS8602.severity = warning
+dotnet_diagnostic.CS8603.severity = warning
+dotnet_diagnostic.CS8604.severity = warning
+dotnet_diagnostic.CS8605.severity = warning
 dotnet_diagnostic.CS8606.severity = error
 dotnet_diagnostic.CS8607.severity = error
 dotnet_diagnostic.CS8608.severity = error
@@ -164,9 +169,16 @@ dotnet_diagnostic.CS8612.severity = error
 dotnet_diagnostic.CS8613.severity = error
 dotnet_diagnostic.CS8614.severity = error
 dotnet_diagnostic.CS8615.severity = error
+dotnet_diagnostic.CS8617.severity = warning
+dotnet_diagnostic.CS8618.severity = warning
+dotnet_diagnostic.CS8619.severity = warning
+dotnet_diagnostic.CS8620.severity = warning
 dotnet_diagnostic.CS8621.severity = error
 dotnet_diagnostic.CS8622.severity = error
 dotnet_diagnostic.CS8624.severity = error
+dotnet_diagnostic.CS8625.severity = error
+dotnet_diagnostic.CS8629.severity = warning
+dotnet_diagnostic.CS8631.severity = warning
 dotnet_diagnostic.CS8633.severity = error
 dotnet_diagnostic.CS8634.severity = error
 dotnet_diagnostic.CS8643.severity = error
@@ -193,17 +205,5 @@ dotnet_diagnostic.CS8775.severity = error
 dotnet_diagnostic.CS8777.severity = error
 
 #nullable error types still not cleaned up
-dotnet_diagnostic.CS8600.severity = warning
-dotnet_diagnostic.CS8601.severity = warning
-dotnet_diagnostic.CS8602.severity = warning
-dotnet_diagnostic.CS8603.severity = warning
-dotnet_diagnostic.CS8604.severity = warning
-dotnet_diagnostic.CS8605.severity = warning
-dotnet_diagnostic.CS8615.severity = warning
-dotnet_diagnostic.CS8617.severity = warning
-dotnet_diagnostic.CS8618.severity = warning
-dotnet_diagnostic.CS8619.severity = warning
-dotnet_diagnostic.CS8620.severity = warning
-dotnet_diagnostic.CS8625.severity = warning
-dotnet_diagnostic.CS8629.severity = warning
-dotnet_diagnostic.CS8631.severity = warning
+
+

ICSharpCode.BamlDecompiler/BamlElement.cs

@@ -41,13 +41,13 @@ namespace ICSharpCode.BamlDecompiler
 			String = value;
 		}
 
-		public readonly XElement Element;
-		public readonly string String;
+		public readonly XElement? Element;
+		public readonly string? String;
 
 		public static implicit operator XamlNode(XElement value) => new XamlNode(value);
 		public static implicit operator XamlNode(string value) => new XamlNode(value);
-		public static implicit operator XElement(XamlNode node) => node.Element;
-		public static implicit operator string(XamlNode node) => node.String;
+		public static implicit operator XElement?(XamlNode node) => node.Element;
+		public static implicit operator string?(XamlNode node) => node.String;
 	}
 
 	internal class BamlElement
@@ -55,7 +55,7 @@ namespace ICSharpCode.BamlDecompiler
 		public BamlNode Node { get; }
 		public XamlNode Xaml { get; set; }
 
-		public BamlElement Parent { get; set; }
+		public BamlElement? Parent { get; set; }
 		public IList<BamlElement> Children { get; }
 
 		public BamlElement(BamlNode node)

ICSharpCode.BamlDecompiler/Handlers/Blocks/ConstructorParametersHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.ConstructorParametersStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var doc = new BamlElement(node);
 			doc.Xaml = new XElement(ctx.GetPseudoName("Ctor"));

ICSharpCode.BamlDecompiler/Handlers/Blocks/DocumentHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.DocumentStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var doc = new BamlElement(node);
 			doc.Xaml = new XElement(ctx.GetPseudoName("Document"));

ICSharpCode.BamlDecompiler/Handlers/Blocks/ElementHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.ElementStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (ElementStartRecord)((BamlBlockNode)node).Header;
 			var doc = new BamlElement(node);

ICSharpCode.BamlDecompiler/Handlers/Blocks/KeyElementStartHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		BamlRecordType IHandler.Type => BamlRecordType.KeyElementStart;
 
-		BamlElement IHandler.Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		BamlElement? IHandler.Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			XamlResourceKey.Create(node);
 			return null;

ICSharpCode.BamlDecompiler/Handlers/Blocks/PropertyArrayHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyArrayStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyArrayStartRecord)((BamlBlockNode)node).Header;
 			var doc = new BamlElement(node);

ICSharpCode.BamlDecompiler/Handlers/Blocks/PropertyComplexHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyComplexStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyComplexStartRecord)((BamlBlockNode)node).Header;
 			var doc = new BamlElement(node);

ICSharpCode.BamlDecompiler/Handlers/Blocks/PropertyDictionaryHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyDictionaryStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyDictionaryStartRecord)((BamlBlockNode)node).Header;
 			var doc = new BamlElement(node);

ICSharpCode.BamlDecompiler/Handlers/Blocks/PropertyListHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyListStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyListStartRecord)((BamlBlockNode)node).Header;
 			var doc = new BamlElement(node);

ICSharpCode.BamlDecompiler/Handlers/Records/AssemblyInfoHandler.cs

@@ -28,6 +28,6 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.AssemblyInfo;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/AttributeInfoHandler.cs

@@ -28,6 +28,6 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.AttributeInfo;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/ConnectionIdHandler.cs

@@ -28,7 +28,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.ConnectionId;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (ConnectionIdRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/ConstructorParameterTypeHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.ConstructorParameterType;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (ConstructorParameterTypeRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/ContentPropertyHandler.cs

@@ -28,7 +28,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.ContentProperty;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (ContentPropertyRecord)((BamlRecordNode)node).Record;
 			// TODO: What to do here?

ICSharpCode.BamlDecompiler/Handlers/Records/DefAttributeHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.DefAttribute;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (DefAttributeRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/DefAttributeKeyStringHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.DefAttributeKeyString;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			XamlResourceKey.Create(node);
 			return null;

ICSharpCode.BamlDecompiler/Handlers/Records/DefAttributeKeyTypeHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.DefAttributeKeyType;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			XamlResourceKey.Create(node);
 			return null;

ICSharpCode.BamlDecompiler/Handlers/Records/DeferableContentStartHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.DeferableContentStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (DeferableContentStartRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/LineNumberAndPositionHandler.cs

@@ -28,6 +28,6 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.LineNumberAndPosition;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/LinePositionHandler.cs

@@ -28,6 +28,6 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.LinePosition;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/LiteralContentHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.LiteralContent;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (LiteralContentRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/OptimizedStaticResourceHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.OptimizedStaticResource;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (OptimizedStaticResourceRecord)((BamlRecordNode)node).Record;
 			var key = XamlResourceKey.FindKeyInSiblings(node);

ICSharpCode.BamlDecompiler/Handlers/Records/PIMappingHandler.cs

@@ -28,6 +28,6 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PIMapping;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/PresentationOptionsAttributeHandler.cs

@@ -30,7 +30,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PresentationOptionsAttribute;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PresentationOptionsAttributeRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/PropertyCustomHandler.cs

@@ -179,7 +179,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 			return property.IsAttachedTo(type);
 		}
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyCustomRecord)((BamlRecordNode)node).Record;
 			var serTypeId = ((short)record.SerializerTypeId & 0xfff);

ICSharpCode.BamlDecompiler/Handlers/Records/PropertyHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public virtual BamlRecordType Type => BamlRecordType.Property;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/PropertyTypeReferenceHandler.cs

@@ -22,10 +22,9 @@
 
 using System.Xml.Linq;
 
-using ICSharpCode.Decompiler.TypeSystem;
-
 using ICSharpCode.BamlDecompiler.Baml;
 using ICSharpCode.BamlDecompiler.Xaml;
+using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.BamlDecompiler.Handlers
 {
@@ -33,7 +32,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyTypeReference;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyTypeReferenceRecord)((BamlRecordNode)node).Record;
 			var attr = ctx.ResolveProperty(record.AttributeId);

ICSharpCode.BamlDecompiler/Handlers/Records/PropertyWithExtensionHandler.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyWithExtension;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyWithExtensionRecord)((BamlRecordNode)node).Record;
 			var extTypeId = ((short)record.Flags & 0xfff);

ICSharpCode.BamlDecompiler/Handlers/Records/PropertyWithStaticResourceIdHandler.cs

@@ -32,7 +32,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.PropertyWithStaticResourceId;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (PropertyWithStaticResourceIdRecord)((BamlRecordNode)node).Record;
 			var doc = new BamlElement(node);
@@ -43,8 +43,8 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 			doc.Xaml.Element.AddAnnotation(elemAttr);
 			parent.Xaml.Element.Add(doc.Xaml.Element);
 
-			BamlNode found = node;
-			XamlResourceKey key;
+			BamlNode? found = node;
+			XamlResourceKey? key;
 			do
 			{
 				key = XamlResourceKey.FindKeyInAncestors(found.Parent, out found);

ICSharpCode.BamlDecompiler/Handlers/Records/StaticResourceIdHandler.cs

@@ -27,12 +27,12 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.StaticResourceId;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (StaticResourceIdRecord)((BamlRecordNode)node).Record;
 
-			BamlNode found = node;
-			XamlResourceKey key;
+			BamlNode? found = node;
+			XamlResourceKey? key;
 			do
 			{
 				key = XamlResourceKey.FindKeyInAncestors(found.Parent, out found);

ICSharpCode.BamlDecompiler/Handlers/Records/StaticResourceStartHandler.cs

@@ -29,7 +29,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.StaticResourceStart;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (StaticResourceStartRecord)((BamlBlockNode)node).Record;
 			var key = XamlResourceKey.FindKeyInSiblings(node);

ICSharpCode.BamlDecompiler/Handlers/Records/TextHandler.cs

@@ -28,7 +28,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.Text;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (TextRecord)((BamlRecordNode)node).Record;
 
@@ -42,7 +42,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.TextWithId;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (TextWithIdRecord)((BamlRecordNode)node).Record;
 

ICSharpCode.BamlDecompiler/Handlers/Records/TypeInfoHandler.cs

@@ -28,13 +28,13 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 	{
 		public BamlRecordType Type => BamlRecordType.TypeInfo;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 
 	internal class TypeSerializerInfoHandler : IHandler
 	{
 		public BamlRecordType Type => BamlRecordType.TypeSerializerInfo;
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent) => null;
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent) => null;
 	}
 }

ICSharpCode.BamlDecompiler/Handlers/Records/XmlnsPropertyHandler.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.BamlDecompiler.Handlers
 			}
 		}
 
-		public BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent)
+		public BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent)
 		{
 			var record = (XmlnsPropertyRecord)((BamlRecordNode)node).Record;
 			foreach (var asmId in record.AssemblyIds)

ICSharpCode.BamlDecompiler/IHandlers.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.BamlDecompiler
 	internal interface IHandler
 	{
 		BamlRecordType Type { get; }
-		BamlElement Translate(XamlContext ctx, BamlNode node, BamlElement parent);
+		BamlElement? Translate(XamlContext ctx, BamlNode node, BamlElement? parent);
 	}
 
 	internal interface IDeferHandler

ICSharpCode.BamlDecompiler/Rewrite/ConnectionIdRewritePass.cs

@@ -250,7 +250,7 @@ namespace ICSharpCode.BamlDecompiler.Rewrite
 			}
 		}
 
-		bool MatchFieldAssignment(ILInstruction inst, [NotNullWhen(true)] out FieldAssignment field)
+		bool MatchFieldAssignment(ILInstruction inst, [NotNullWhen(true)] out FieldAssignment? field)
 		{
 			field = null;
 			if (!inst.MatchStFld(out _, out var fld, out var value) || !value.MatchCastClass(out var arg, out _)

ICSharpCode.BamlDecompiler/Xaml/NamespaceMap.cs

@@ -29,17 +29,17 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 {
 	internal class NamespaceMap
 	{
-		public string XmlnsPrefix { get; set; }
+		public string? XmlnsPrefix { get; set; }
 		public string FullAssemblyName { get; set; }
 		public string XMLNamespace { get; set; }
-		public string CLRNamespace { get; set; }
+		public string? CLRNamespace { get; set; }
 
 		public NamespaceMap(string prefix, string fullAssemblyName, string xmlNs)
 			: this(prefix, fullAssemblyName, xmlNs, null)
 		{
 		}
 
-		public NamespaceMap(string prefix, string fullAssemblyName, string xmlNs, string clrNs)
+		public NamespaceMap(string? prefix, string fullAssemblyName, string xmlNs, string? clrNs)
 		{
 			XmlnsPrefix = prefix;
 			FullAssemblyName = fullAssemblyName;

ICSharpCode.BamlDecompiler/Xaml/XamlProperty.cs

@@ -82,7 +82,7 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 			return true;
 		}
 
-		public XName ToXName(XamlContext ctx, XElement parent, bool isFullName = true)
+		public XName ToXName(XamlContext ctx, XElement? parent, bool isFullName = true)
 		{
 			var typeName = DeclaringType.ToXName(ctx);
 			XName name;

ICSharpCode.BamlDecompiler/Xaml/XamlResourceKey.cs

@@ -78,7 +78,7 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 		public BamlElement KeyElement { get; set; }
 		public IList<BamlNode> StaticResources { get; }
 
-		public static XamlResourceKey FindKeyInSiblings(BamlNode node)
+		public static XamlResourceKey? FindKeyInSiblings(BamlNode node)
 		{
 			var children = node.Parent.Children;
 			var index = children.IndexOf(node);
@@ -90,9 +90,9 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 			return null;
 		}
 
-		public static XamlResourceKey FindKeyInAncestors(BamlNode node) => FindKeyInAncestors(node, out var found);
+		public static XamlResourceKey? FindKeyInAncestors(BamlNode node) => FindKeyInAncestors(node, out var found);
 
-		public static XamlResourceKey FindKeyInAncestors(BamlNode node, out BamlNode found)
+		public static XamlResourceKey? FindKeyInAncestors(BamlNode node, out BamlNode? found)
 		{
 			BamlNode n = node;
 			do

ICSharpCode.BamlDecompiler/Xaml/XamlType.cs

@@ -37,7 +37,7 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 		public string TypeNamespace { get; }
 		public string TypeName { get; }
 
-		public XNamespace Namespace { get; private set; }
+		public XNamespace? Namespace { get; private set; }
 		public IType ResolvedType { get; set; }
 
 		public XamlType(IModule assembly, string fullAssemblyName, string ns, string name)
@@ -45,7 +45,7 @@ namespace ICSharpCode.BamlDecompiler.Xaml
 		{
 		}
 
-		public XamlType(IModule assembly, string fullAssemblyName, string ns, string name, XNamespace xmlns)
+		public XamlType(IModule assembly, string fullAssemblyName, string ns, string name, XNamespace? xmlns)
 		{
 			Assembly = assembly;
 			FullAssemblyName = fullAssemblyName;

ICSharpCode.BamlDecompiler/XamlContext.cs

@@ -27,10 +27,9 @@ using System.Reflection.Metadata;
 using System.Threading;
 using System.Xml.Linq;
 
-using ICSharpCode.Decompiler.TypeSystem;
-
 using ICSharpCode.BamlDecompiler.Baml;
 using ICSharpCode.BamlDecompiler.Xaml;
+using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.BamlDecompiler
 {
@@ -207,8 +206,8 @@ namespace ICSharpCode.BamlDecompiler
 					continue;
 				var xmlNs = attr.FixedArguments[0].Value as string;
 				var typeNs = attr.FixedArguments[1].Value as string;
-				Debug.Assert((object)xmlNs != null && (object)typeNs != null);
-				if ((object)xmlNs == null || (object)typeNs == null)
+				Debug.Assert((object?)xmlNs != null && (object?)typeNs != null);
+				if ((object?)xmlNs == null || (object?)typeNs == null)
 					continue;
 
 				if (typeNamespace == typeNs)
@@ -221,7 +220,7 @@ namespace ICSharpCode.BamlDecompiler
 			return possibleXmlNs.FirstOrDefault();
 		}
 
-		public XName GetKnownNamespace(string name, string xmlNamespace, XElement context = null)
+		public XName GetKnownNamespace(string name, string xmlNamespace, XElement? context = null)
 		{
 			var xNs = GetXmlNamespace(xmlNamespace);
 			XName xName;

ICSharpCode.BamlDecompiler/XmlnsDictionary.cs

@@ -22,11 +22,10 @@
 
 using System.Collections.Generic;
 
+using ICSharpCode.BamlDecompiler.Xaml;
 using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.TypeSystem;
 
-using ICSharpCode.BamlDecompiler.Xaml;
-
 namespace ICSharpCode.BamlDecompiler
 {
 	internal class XmlnsScope : List<NamespaceMap>
@@ -58,7 +57,7 @@ namespace ICSharpCode.BamlDecompiler
 
 		public XmlnsDictionary() => CurrentScope = null;
 
-		public XmlnsScope CurrentScope { get; set; }
+		public XmlnsScope? CurrentScope { get; set; }
 
 		public void PushScope(BamlElement element) => CurrentScope = new XmlnsScope(CurrentScope, element);
 

ICSharpCode.Decompiler.PowerShell/GetDecompiledProjectCmdlet.cs

@@ -25,7 +25,7 @@ namespace ICSharpCode.Decompiler.PowerShell
 		readonly object syncObject = new object();
 		int completed;
 		string fileName;
-		ProgressRecord progress;
+		ProgressRecord? progress;
 
 		public void Report(DecompilationProgress value)
 		{

ICSharpCode.Decompiler.Tests/Semantics/OverloadResolutionTests.cs

@@ -258,7 +258,7 @@ namespace ICSharpCode.Decompiler.Tests.Semantics
 				get { throw new NotImplementedException(); }
 			}
 
-			public override IType GetInferredReturnType(IType[] parameterTypes)
+			public override IType GetInferredReturnType(IType[]? parameterTypes)
 			{
 				return inferredReturnType;
 			}

ICSharpCode.Decompiler/CSharp/OutputVisitor/ITokenWriter.cs

@@ -46,7 +46,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// <summary>
 		/// Writes a primitive/literal value
 		/// </summary>
-		public abstract void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None);
+		public abstract void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None);
 
 		public abstract void WritePrimitiveType(string type);
 
@@ -129,7 +129,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			decoratedWriter.WriteToken(role, token);
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			decoratedWriter.WritePrimitiveValue(value, format);
 		}

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertMissingTokensDecorator.cs

@@ -138,7 +138,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			base.WriteIdentifier(identifier);
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			Expression? node = nodes.Peek().LastOrDefault() as Expression;
 			var startLocation = locationProvider.Location;

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertRequiredSpacesDecorator.cs

@@ -148,7 +148,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			lastWritten = LastWritten.Whitespace;
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			if (lastWritten == LastWritten.KeywordOrIdentifier)
 			{

ICSharpCode.Decompiler/CSharp/OutputVisitor/InsertSpecialsDecorator.cs

@@ -82,7 +82,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		/// <summary>
 		/// Writes all specials from start to end (exclusive). Does not touch the positionStack.
 		/// </summary>
-		void WriteSpecials(AstNode start, AstNode end)
+		void WriteSpecials(AstNode start, AstNode? end)
 		{
 			for (AstNode? pos = start; pos != end; pos = pos.NextSibling)
 			{
@@ -118,7 +118,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			WriteSpecialsUpToRole(role, null);
 		}
 
-		void WriteSpecialsUpToRole(Role role, AstNode nextNode)
+		void WriteSpecialsUpToRole(Role role, AstNode? nextNode)
 		{
 			if (positionStack.Count == 0)
 			{

ICSharpCode.Decompiler/CSharp/OutputVisitor/TextWriterTokenWriter.cs

@@ -233,7 +233,7 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			return writer.ToString();
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			if (value == null)
 			{

ICSharpCode.Decompiler/CSharp/Resolver/AwaitResolveResult.cs

@@ -42,20 +42,20 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// <summary>
 		/// Property representing the IsCompleted property on the awaiter type. Can be null if the awaiter type or the property was not found, or when awaiting a dynamic expression.
 		/// </summary>
-		public readonly IProperty IsCompletedProperty;
+		public readonly IProperty? IsCompletedProperty;
 
 		/// <summary>
 		/// Method representing the OnCompleted method on the awaiter type. Can be null if the awaiter type or the method was not found, or when awaiting a dynamic expression.
 		/// This can also refer to an UnsafeOnCompleted method, if the awaiter type implements <c>System.Runtime.CompilerServices.ICriticalNotifyCompletion</c>.
 		/// </summary>
-		public readonly IMethod OnCompletedMethod;
+		public readonly IMethod? OnCompletedMethod;
 
 		/// <summary>
 		/// Method representing the GetResult method on the awaiter type. Can be null if the awaiter type or the method was not found, or when awaiting a dynamic expression.
 		/// </summary>
-		public readonly IMethod GetResultMethod;
+		public readonly IMethod? GetResultMethod;
 
-		public AwaitResolveResult(IType resultType, ResolveResult getAwaiterInvocation, IType awaiterType, IProperty isCompletedProperty, IMethod onCompletedMethod, IMethod getResultMethod)
+		public AwaitResolveResult(IType resultType, ResolveResult getAwaiterInvocation, IType awaiterType, IProperty? isCompletedProperty, IMethod? onCompletedMethod, IMethod? getResultMethod)
 			: base(resultType)
 		{
 			if (awaiterType == null)

ICSharpCode.Decompiler/CSharp/Resolver/CSharpConversions.cs

@@ -792,7 +792,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		#endregion
 
 		#region Implicit Constant-Expression Conversion
-		bool ImplicitConstantExpressionConversion(ResolveResult rr, IType toType)
+		bool ImplicitConstantExpressionConversion(ResolveResult? rr, IType toType)
 		{
 			if (rr == null || !rr.IsCompileTimeConstant)
 				return false;
@@ -983,7 +983,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			return Conversion.UserDefinedConversion(selected[0].Method, isLifted: selected[0].IsLifted, isImplicit: isImplicit, isAmbiguous: true, conversionBeforeUserDefinedOperator: ExplicitConversion(source, mostSpecificSource), conversionAfterUserDefinedOperator: ExplicitConversion(mostSpecificTarget, target));
 		}
 
-		Conversion UserDefinedImplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
+		Conversion UserDefinedImplicitConversion(ResolveResult? fromResult, IType fromType, IType toType)
 		{
 			// C# 4.0 spec §6.4.4 User-defined implicit conversions
 			var operators = GetApplicableConversionOperators(fromResult, fromType, toType, false);
@@ -1025,7 +1025,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			}
 		}
 
-		Conversion UserDefinedExplicitConversion(ResolveResult fromResult, IType fromType, IType toType)
+		Conversion UserDefinedExplicitConversion(ResolveResult? fromResult, IType fromType, IType toType)
 		{
 			// C# 4.0 spec §6.4.5 User-defined explicit conversions
 			var operators = GetApplicableConversionOperators(fromResult, fromType, toType, true);
@@ -1112,7 +1112,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			return NullableType.GetUnderlyingType(type);
 		}
 
-		List<OperatorInfo> GetApplicableConversionOperators(ResolveResult fromResult, IType fromType, IType toType, bool isExplicit)
+		List<OperatorInfo> GetApplicableConversionOperators(ResolveResult? fromResult, IType fromType, IType toType, bool isExplicit)
 		{
 			// Find the candidate operators:
 			Predicate<IMethod> opFilter;

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -1712,7 +1712,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			return null;
 		}
 
-		ResolveResult? LookInUsingScopeNamespace(ResolvedUsingScope usingScope, INamespace n, string identifier, IReadOnlyList<IType> typeArguments, bool parameterizeResultType)
+		ResolveResult? LookInUsingScopeNamespace(ResolvedUsingScope? usingScope, INamespace n, string identifier, IReadOnlyList<IType> typeArguments, bool parameterizeResultType)
 		{
 			if (n == null)
 				return null;
@@ -2222,7 +2222,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 
 		#region ResolveInvocation
 
-		IList<ResolveResult> AddArgumentNamesIfNecessary(ResolveResult[] arguments, string[] argumentNames)
+		IList<ResolveResult> AddArgumentNamesIfNecessary(ResolveResult[] arguments, string[]? argumentNames)
 		{
 			if (argumentNames == null)
 			{
@@ -2239,7 +2239,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			}
 		}
 
-		private ResolveResult ResolveInvocation(ResolveResult target, ResolveResult[] arguments, string[] argumentNames, bool allowOptionalParameters)
+		private ResolveResult ResolveInvocation(ResolveResult target, ResolveResult[] arguments, string[]? argumentNames, bool allowOptionalParameters)
 		{
 			// C# 4.0 spec: §7.6.5
 

ICSharpCode.Decompiler/CSharp/Resolver/LambdaResolveResult.cs

@@ -65,7 +65,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// This method determines the return type inferred from the lambda body, which is used as part of C# type inference.
 		/// Use the <see cref="ReturnType"/> property to retrieve the actual return type as determined by the target delegate type.
 		/// </remarks>
-		public abstract IType GetInferredReturnType(IType[] parameterTypes);
+		public abstract IType GetInferredReturnType(IType[]? parameterTypes);
 
 		/// <summary>
 		/// Gets the list of parameters.
@@ -138,7 +138,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 
 		public override ResolveResult Body { get; }
 
-		public override IType GetInferredReturnType(IType[] parameterTypes)
+		public override IType GetInferredReturnType(IType[]? parameterTypes)
 		{
 			// We don't know how to compute which type would be inferred if
 			// given other parameter types.

ICSharpCode.Decompiler/CSharp/Resolver/MemberLookup.cs

@@ -259,18 +259,18 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 			public readonly IType DeclaringType;
 
 			// When a nested type is hidden, it is simply removed from the list.
-			public List<IType> NestedTypes;
+			public List<IType>? NestedTypes;
 
 			// When members are hidden, they are merely marked as hidden.
 			// We still need to store the hidden methods so that the 'override' processing can
 			// find them, so that it won't introduce the override as a new method.
-			public readonly List<IParameterizedMember> Methods;
+			public readonly List<IParameterizedMember>? Methods;
 			public bool MethodsAreHidden;
 
-			public IMember NonMethod;
+			public IMember? NonMethod;
 			public bool NonMethodIsHidden;
 
-			public LookupGroup(IType declaringType, List<IType> nestedTypes, List<IParameterizedMember> methods, IMember nonMethod)
+			public LookupGroup(IType declaringType, List<IType>? nestedTypes, List<IParameterizedMember>? methods, IMember? nonMethod)
 			{
 				this.DeclaringType = declaringType;
 				this.NestedTypes = nestedTypes;

ICSharpCode.Decompiler/CSharp/Resolver/MethodGroupResolveResult.cs

@@ -83,7 +83,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		readonly string methodName;
 
 		public MethodGroupResolveResult(ResolveResult? targetResult, string methodName,
-			IReadOnlyList<MethodListWithDeclaringType> methods, IReadOnlyList<IType> typeArguments)
+			IReadOnlyList<MethodListWithDeclaringType> methods, IReadOnlyList<IType>? typeArguments)
 			: base(SpecialType.NoType)
 		{
 			if (methods == null)
@@ -143,10 +143,10 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// List of extension methods, used to avoid re-calculating it in ResolveInvocation() when it was already
 		/// calculated by ResolveMemberAccess().
 		/// </summary>
-		internal List<List<IMethod>> extensionMethods;
+		internal List<List<IMethod>>? extensionMethods;
 
 		// the resolver is used to fetch extension methods on demand
-		internal CSharpResolver resolver;
+		internal CSharpResolver? resolver;
 
 		/// <summary>
 		/// Gets all candidate extension methods.

ICSharpCode.Decompiler/CSharp/Resolver/OverloadResolution.cs

@@ -130,7 +130,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		readonly CSharpConversions conversions;
 		//List<Candidate> candidates = new List<Candidate>();
 		Candidate bestCandidate;
-		Candidate bestCandidateAmbiguousWith;
+		Candidate? bestCandidateAmbiguousWith;
 		IType[] explicitlyGivenTypeArguments;
 		bool bestCandidateWasValidated;
 		OverloadResolutionErrors bestCandidateValidationResult;
@@ -994,7 +994,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				return GetArgumentsWithConversions(null, GetBestCandidateWithSubstitutedTypeArguments());
 		}
 
-		IList<ResolveResult> GetArgumentsWithConversions(ResolveResult targetResolveResult, IParameterizedMember bestCandidateForNamedArguments)
+		IList<ResolveResult> GetArgumentsWithConversions(ResolveResult? targetResolveResult, IParameterizedMember? bestCandidateForNamedArguments)
 		{
 			var conversions = this.ArgumentConversions;
 			ResolveResult[] args = new ResolveResult[arguments.Length];
@@ -1072,9 +1072,9 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		/// <param name="returnTypeOverride">
 		/// If not null, use this instead of the ReturnType of the member as the type of the created resolve result.
 		/// </param>
-		public CSharpInvocationResolveResult CreateResolveResult(ResolveResult targetResolveResult, IList<ResolveResult>? initializerStatements = null, IType? returnTypeOverride = null)
+		public CSharpInvocationResolveResult CreateResolveResult(ResolveResult? targetResolveResult, IList<ResolveResult>? initializerStatements = null, IType? returnTypeOverride = null)
 		{
-			IParameterizedMember member = GetBestCandidateWithSubstitutedTypeArguments();
+			IParameterizedMember? member = GetBestCandidateWithSubstitutedTypeArguments();
 			if (member == null)
 				throw new InvalidOperationException();
 

ICSharpCode.Decompiler/CSharp/Resolver/TypeInference.cs

@@ -94,11 +94,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 		}
 		#endregion
 
-		TP[] typeParameters;
-		IType[] parameterTypes;
-		ResolveResult[] arguments;
-		bool[,] dependencyMatrix;
-		IReadOnlyList<IType> classTypeArguments;
+		TP[]? typeParameters;
+		IType[]? parameterTypes;
+		ResolveResult[]? arguments;
+		bool[,]? dependencyMatrix;
+		IReadOnlyList<IType>? classTypeArguments;
 
 		#region InferTypeArguments (main function)
 		/// <summary>

ICSharpCode.Decompiler/CSharp/SequencePointBuilder.cs

@@ -76,7 +76,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			/// <summary>
 			/// The function containing this sequence point.
 			/// </summary>
-			internal ILFunction Function;
+			internal ILFunction? Function;
 
 			public StatePerSequencePoint(AstNode primaryNode)
 			{

ICSharpCode.Decompiler/CSharp/Syntax/CSharpTokenNode.cs

@@ -100,7 +100,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			}
 		}
 
-		public CSharpTokenNode(TextLocation location, TokenRole role)
+		public CSharpTokenNode(TextLocation location, TokenRole? role)
 		{
 			this.startLocation = location;
 			if (role != null)

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/IndexerExpression.cs

@@ -54,7 +54,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 		}
 
-		public IndexerExpression(Expression target, IEnumerable<Expression>? arguments)
+		public IndexerExpression(Expression? target, IEnumerable<Expression>? arguments)
 		{
 			AddChild(target, Roles.TargetExpression);
 			if (arguments != null)

ICSharpCode.Decompiler/CSharp/Syntax/PatternMatching/Match.cs

@@ -27,7 +27,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 	{
 		// TODO: maybe we should add an implicit Match->bool conversion? (implicit operator bool(Match m) { return m != null; })
 
-		List<KeyValuePair<string, INode>> results;
+		List<KeyValuePair<string, INode?>>? results;
 
 		public bool Success {
 			get { return results != null; }
@@ -36,7 +36,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		internal static Match CreateNew()
 		{
 			Match m;
-			m.results = new List<KeyValuePair<string, INode>>();
+			m.results = new List<KeyValuePair<string, INode?>>();
 			return m;
 		}
 
@@ -88,7 +88,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		{
 			if (groupName != null && node != null)
 			{
-				results.Add(new KeyValuePair<string, INode>(groupName, node));
+				results.Add(new KeyValuePair<string, INode?>(groupName, node));
 			}
 		}
 
@@ -96,7 +96,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching
 		{
 			if (groupName != null)
 			{
-				results.Add(new KeyValuePair<string, INode>(groupName, null));
+				results.Add(new KeyValuePair<string, INode?>(groupName, null));
 			}
 		}
 	}

ICSharpCode.Decompiler/CSharp/Syntax/Role.cs

@@ -69,7 +69,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	public class Role<T> : Role where T : class?
 	{
 		readonly string name; // helps with debugging the AST
-		readonly T nullObject;
+		readonly T? nullObject;
 
 		/// <summary>
 		/// Gets the null object used when there's no node with this role.
@@ -79,7 +79,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		/// Roles used for non-collections should always have a null object, so that no AST property returns null.
 		/// However, if a role used for collections only, it may leave out the null object.
 		/// </remarks>
-		public T NullObject {
+		public T? NullObject {
 			get { return nullObject; }
 		}
 
@@ -97,7 +97,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			this.nullObject = null!;
 		}
 
-		public Role(string name, T nullObject)
+		public Role(string name, T? nullObject)
 		{
 			this.name = name ?? throw new ArgumentNullException(nameof(name));
 			this.nullObject = nullObject;

ICSharpCode.Decompiler/CSharp/Transforms/ContextTrackingVisitor.cs

@@ -28,8 +28,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public abstract class ContextTrackingVisitor<TResult> : DepthFirstAstVisitor<TResult>
 	{
-		protected ITypeDefinition currentTypeDefinition;
-		protected IMethod currentMethod;
+		protected ITypeDefinition? currentTypeDefinition;
+		protected IMethod? currentMethod;
 
 		protected void Initialize(TransformContext context)
 		{
@@ -45,7 +45,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 
 		public override TResult VisitTypeDeclaration(TypeDeclaration typeDeclaration)
 		{
-			ITypeDefinition oldType = currentTypeDefinition;
+			ITypeDefinition? oldType = currentTypeDefinition;
 			try
 			{
 				currentTypeDefinition = typeDeclaration.GetSymbol() as ITypeDefinition;

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -139,7 +139,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 		}
 
 		readonly Dictionary<ILVariable, VariableToDeclare> variableDict = new Dictionary<ILVariable, VariableToDeclare>();
-		TransformContext context;
+		TransformContext? context;
 
 		public void Run(AstNode rootNode, TransformContext context)
 		{

ICSharpCode.Decompiler/CSharp/Transforms/NormalizeBlockStatements.cs

@@ -12,7 +12,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	{
 		TransformContext context;
 		bool hasNamespace;
-		NamespaceDeclaration singleNamespaceDeclaration;
+		NamespaceDeclaration? singleNamespaceDeclaration;
 
 		public override void VisitSyntaxTree(SyntaxTree syntaxTree)
 		{

ICSharpCode.Decompiler/CSharp/Transforms/PatternStatementTransform.cs

@@ -35,7 +35,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	public sealed class PatternStatementTransform : ContextTrackingVisitor<AstNode>, IAstTransform
 	{
 		readonly DeclareVariables declareVariables = new DeclareVariables();
-		TransformContext context;
+		TransformContext? context;
 
 		public void Run(AstNode rootNode, TransformContext context)
 		{

ICSharpCode.Decompiler/CSharp/Transforms/PrettifyAssignments.cs

@@ -39,7 +39,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </remarks>
 	class PrettifyAssignments : DepthFirstAstVisitor, IAstTransform
 	{
-		TransformContext context;
+		TransformContext? context;
 
 		public override void VisitAssignmentExpression(AssignmentExpression assignment)
 		{

ICSharpCode.Decompiler/CSharp/Transforms/ReplaceMethodCallsWithOperators.cs

@@ -41,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			MemberName = "TypeHandle"
 		};
 
-		TransformContext context;
+		TransformContext? context;
 
 		public override void VisitInvocationExpression(InvocationExpression invocationExpression)
 		{

ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs

@@ -37,8 +37,8 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 	/// </summary>
 	public class TransformFieldAndConstructorInitializers : DepthFirstAstVisitor, IAstTransform
 	{
-		TransformContext context;
-		Dictionary<IField, IL.ILVariable> fieldToVariableMap;
+		TransformContext? context;
+		Dictionary<IField, IL.ILVariable>? fieldToVariableMap;
 
 		public void Run(AstNode node, TransformContext context)
 		{

ICSharpCode.Decompiler/DebugInfo/ImportScopeInfo.cs

@@ -23,7 +23,7 @@ namespace ICSharpCode.Decompiler.DebugInfo
 {
 	class ImportScopeInfo
 	{
-		public readonly ImportScopeInfo Parent;
+		public readonly ImportScopeInfo? Parent;
 		public ImportScopeHandle Handle;
 		public readonly HashSet<string> Imports = new HashSet<string>();
 

ICSharpCode.Decompiler/Disassembler/MethodBodyDisassembler.cs

@@ -68,7 +68,7 @@ namespace ICSharpCode.Decompiler.Disassembler
 		/// </summary>
 		public IDebugInfoProvider DebugInfo { get; set; }
 
-		IList<DebugInfo.SequencePoint> sequencePoints;
+		IList<DebugInfo.SequencePoint>? sequencePoints;
 		int nextSequencePointIndex;
 
 		// cache info

ICSharpCode.Decompiler/Disassembler/OpCodeInfo.cs

@@ -31,7 +31,7 @@ namespace ICSharpCode.Decompiler.Disassembler
 		public readonly ILOpCode Code;
 		public readonly string Name;
 
-		string encodedName;
+		string? encodedName;
 
 		public OpCodeInfo(ILOpCode code, string name)
 		{

ICSharpCode.Decompiler/Disassembler/ReflectionDisassembler.cs

@@ -1912,16 +1912,16 @@ namespace ICSharpCode.Decompiler.Disassembler
 
 		}
 
-		sealed class EnumNameCollection<T> : IEnumerable<KeyValuePair<long, string>> where T : struct
+		sealed class EnumNameCollection<T> : IEnumerable<KeyValuePair<long, string?>> where T : struct
 		{
-			List<KeyValuePair<long, string>> names = new List<KeyValuePair<long, string>>();
+			List<KeyValuePair<long, string?>> names = new List<KeyValuePair<long, string?>>();
 
-			public void Add(T flag, string name)
+			public void Add(T flag, string? name)
 			{
-				this.names.Add(new KeyValuePair<long, string>(Convert.ToInt64(flag), name));
+				this.names.Add(new KeyValuePair<long, string?>(Convert.ToInt64(flag), name));
 			}
 
-			public IEnumerator<KeyValuePair<long, string>> GetEnumerator()
+			public IEnumerator<KeyValuePair<long, string?>> GetEnumerator()
 			{
 				return names.GetEnumerator();
 			}

ICSharpCode.Decompiler/FlowAnalysis/ControlFlowNode.cs

@@ -38,7 +38,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		/// <summary>
 		/// User data.
 		/// </summary>
-		public object UserData;
+		public object? UserData;
 
 		/// <summary>
 		/// Visited flag, used in various algorithms.
@@ -62,13 +62,13 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		/// Gets the immediate dominator (the parent in the dominator tree).
 		/// Null if dominance has not been calculated; or if the node is unreachable.
 		/// </summary>
-		public ControlFlowNode ImmediateDominator { get; internal set; }
+		public ControlFlowNode? ImmediateDominator { get; internal set; }
 
 		/// <summary>
 		/// List of children in the dominator tree.
 		/// Null if dominance has not been calculated; or if the node is unreachable.
 		/// </summary>
-		public List<ControlFlowNode> DominatorTreeChildren { get; internal set; }
+		public List<ControlFlowNode>? DominatorTreeChildren { get; internal set; }
 
 		/// <summary>
 		/// List of incoming control flow edges.
@@ -120,7 +120,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 		public bool Dominates(ControlFlowNode node)
 		{
 			// TODO: this can be made O(1) by numbering the dominator tree
-			ControlFlowNode tmp = node;
+			ControlFlowNode? tmp = node;
 			while (tmp != null)
 			{
 				if (tmp == this)

ICSharpCode.Decompiler/FlowAnalysis/ReachingDefinitionsVisitor.cs

@@ -331,7 +331,7 @@ namespace ICSharpCode.Decompiler.FlowAnalysis
 			for (int vi = 0; vi < storesByVar.Length; vi++)
 			{
 				if (activeVariables[vi])
-					storesByVar[vi] = new List<ILInstruction> { null };
+					storesByVar[vi] = new List<ILInstruction?> { null };
 			}
 			foreach (var inst in scope.Descendants)
 			{

ICSharpCode.Decompiler/IL/ControlFlow/DetectPinnedRegions.cs

@@ -57,7 +57,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 	/// </summary>
 	public class DetectPinnedRegions : IILTransform
 	{
-		ILTransformContext context;
+		ILTransformContext? context;
 
 		public void Run(ILFunction function, ILTransformContext context)
 		{

ICSharpCode.Decompiler/IL/ControlFlow/SwitchAnalysis.cs

@@ -24,7 +24,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// The variable that is used to represent the switch expression.
 		/// <c>null</c> while analyzing the first block.
 		/// </summary>
-		ILVariable switchVar;
+		ILVariable? switchVar;
 
 		/// <summary>
 		/// The variable to be used as the argument of the switch instruction.

ICSharpCode.Decompiler/IL/ControlFlow/SwitchDetection.cs

@@ -38,10 +38,10 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 	{
 		private readonly SwitchAnalysis analysis = new SwitchAnalysis();
 
-		private ILTransformContext context;
-		private BlockContainer currentContainer;
-		private ControlFlowGraph controlFlowGraph;
-		private LoopContext loopContext;
+		private ILTransformContext? context;
+		private BlockContainer? currentContainer;
+		private ControlFlowGraph? controlFlowGraph;
+		private LoopContext? loopContext;
 
 		/// <summary>
 		/// When detecting a switch, it is important to distinguish Branch instructions which will

ICSharpCode.Decompiler/IL/ControlFlow/YieldReturnDecompiler.cs

@@ -73,15 +73,15 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 
 		/// <summary>The field in the compiler-generated class holding the current state of the state machine</summary>
 		/// <remarks>Set in AnalyzeCtor() for MS, MatchEnumeratorCreationPattern() or AnalyzeMoveNext() for Mono</remarks>
-		IField stateField;
+		IField? stateField;
 
 		/// <summary>The backing field of the 'Current' property in the compiler-generated class</summary>
 		/// <remarks>Set in AnalyzeCurrentProperty()</remarks>
-		IField currentField;
+		IField? currentField;
 
 		/// <summary>The disposing field of the compiler-generated enumerator class.</summary>
 		/// <remarks>Set in ConstructExceptionTable() for assembly compiled with Mono</remarks>
-		IField disposingField;
+		IField? disposingField;
 
 		/// <summary>Maps the fields of the compiler-generated class to the original parameters.</summary>
 		/// <remarks>Set in MatchEnumeratorCreationPattern() and ResolveIEnumerableIEnumeratorFieldMapping()</remarks>
@@ -90,7 +90,7 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// <summary>This dictionary stores the information extracted from the Dispose() method:
 		/// for each "Finally Method", it stores the set of states for which the method is being called.</summary>
 		/// <remarks>Set in ConstructExceptionTable()</remarks>
-		Dictionary<IMethod, LongSet> finallyMethodToStateRange;
+		Dictionary<IMethod, LongSet>? finallyMethodToStateRange;
 
 		/// <summary>
 		/// For each finally method, stores the target state when entering the finally block,
@@ -106,17 +106,17 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 		/// <summary>
 		/// Local bool variable in MoveNext() that signifies whether to skip finally bodies.
 		/// </summary>
-		ILVariable skipFinallyBodies;
+		ILVariable? skipFinallyBodies;
 
 		/// <summary>
 		/// Local bool variable in MoveNext() that signifies whether to execute finally bodies.
 		/// </summary>
-		ILVariable doFinallyBodies;
+		ILVariable? doFinallyBodies;
 
 		/// <summary>
 		/// Set of variables might hold copies of the generated state field.
 		/// </summary>
-		HashSet<ILVariable> cachedStateVars;
+		HashSet<ILVariable>? cachedStateVars;
 
 		#region Run() method
 		public void Run(ILFunction function, ILTransformContext context)

ICSharpCode.Decompiler/IL/Instructions/DeconstructInstruction.cs

@@ -211,7 +211,7 @@ namespace ICSharpCode.Decompiler.IL
 			return input.MatchLdLoc(out inputVariable) || input.MatchLdLoca(out inputVariable);
 		}
 
-		internal static bool IsAssignment(ILInstruction inst, ICompilation typeSystem, [NotNullWhen(true)] out IType? expectedType, [NotNullWhen(true)] out ILInstruction? value)
+		internal static bool IsAssignment(ILInstruction inst, ICompilation? typeSystem, [NotNullWhen(true)] out IType? expectedType, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			expectedType = null;
 			value = null;

ICSharpCode.Decompiler/IL/Transforms/DeconstructionTransform.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 	/// </summary>
 	class DeconstructionTransform : IStatementTransform
 	{
-		StatementTransformContext context;
+		StatementTransformContext? context;
 		readonly Dictionary<ILVariable, int> deconstructionResultsLookup = new Dictionary<ILVariable, int>();
 		ILVariable[]? deconstructionResults;
 		ILVariable? tupleVariable;

ICSharpCode.Decompiler/IL/Transforms/ILInlining.cs

@@ -741,10 +741,10 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		internal readonly struct FindResult
 		{
 			public readonly FindResultType Type;
-			public readonly ILInstruction LoadInst; // ldloc or ldloca instruction that loads the variable to be inlined
-			public readonly ILInstruction CallArgument; // argument of call that needs to be promoted to a named argument
+			public readonly ILInstruction? LoadInst; // ldloc or ldloca instruction that loads the variable to be inlined
+			public readonly ILInstruction? CallArgument; // argument of call that needs to be promoted to a named argument
 
-			private FindResult(FindResultType type, ILInstruction loadInst, ILInstruction callArg)
+			private FindResult(FindResultType type, ILInstruction? loadInst, ILInstruction? callArg)
 			{
 				this.Type = type;
 				this.LoadInst = loadInst;

ICSharpCode.Decompiler/IL/Transforms/LockTransform.cs

@@ -294,7 +294,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return true;
 		}
 
-		bool MatchExitBlock(Block entryPoint, ILVariable flag, ILVariable obj)
+		bool MatchExitBlock(Block entryPoint, ILVariable? flag, ILVariable obj)
 		{
 			if (entryPoint.Instructions.Count != 2 || entryPoint.IncomingEdgeCount != 1)
 				return false;

ICSharpCode.Decompiler/IL/Transforms/NullableLiftingTransform.cs

@@ -42,7 +42,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 	struct NullableLiftingTransform
 	{
 		readonly ILTransformContext context;
-		List<ILVariable> nullableVars;
+		List<ILVariable>? nullableVars;
 
 		public NullableLiftingTransform(ILTransformContext context)
 		{

ICSharpCode.Decompiler/IL/Transforms/ReduceNestingTransform.cs

@@ -54,7 +54,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		private void Visit(BlockContainer container, Block continueTarget)
+		private void Visit(BlockContainer container, Block? continueTarget)
 		{
 			switch (container.Kind)
 			{
@@ -84,7 +84,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <param name="block"></param>
 		/// <param name="continueTarget">Marks the target block of continue statements.</param>
 		/// <param name="nextInstruction">The instruction following the end point of the block. Can only be null if the end point is unreachable.</param>
-		private void Visit(Block block, Block continueTarget, ILInstruction? nextInstruction = null)
+		private void Visit(Block block, Block? continueTarget, ILInstruction? nextInstruction = null)
 		{
 			Debug.Assert(block.HasFlag(InstructionFlags.EndPointUnreachable) || nextInstruction != null);
 
@@ -174,7 +174,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// For an if statement with an unreachable end point and no else block,
 		/// inverts to match IL order of the first statement of each branch
 		/// </summary>
-		private void ImproveILOrdering(Block block, IfInstruction ifInst, Block continueTarget)
+		private void ImproveILOrdering(Block block, IfInstruction ifInst, Block? continueTarget)
 		{
 			if (!block.HasFlag(InstructionFlags.EndPointUnreachable)
 				|| !ifInst.TrueInst.HasFlag(InstructionFlags.EndPointUnreachable)
@@ -368,7 +368,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Checks if an exit is a duplicable keyword exit (return; break; continue;)
 		/// </summary>
-		private bool CanDuplicateExit(ILInstruction exit, Block continueTarget, out ILInstruction keywordExit)
+		private bool CanDuplicateExit(ILInstruction exit, Block? continueTarget, out ILInstruction keywordExit)
 		{
 			keywordExit = exit;
 			if (exit != null && exit.MatchBranch(continueTarget))

ICSharpCode.Decompiler/IL/Transforms/TransformArrayInitializers.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 	/// </summary>
 	public class TransformArrayInitializers : IStatementTransform
 	{
-		StatementTransformContext context;
+		StatementTransformContext? context;
 
 		void IStatementTransform.Run(Block block, int pos, StatementTransformContext context)
 		{

ICSharpCode.Decompiler/IL/Transforms/TransformAssignment.cs

@@ -545,7 +545,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return true;
 		}
 
-		internal static bool IsImplicitTruncation(ILInstruction value, IType type, ICompilation compilation, bool allowNullableValue = false)
+		internal static bool IsImplicitTruncation(ILInstruction value, IType type, ICompilation? compilation, bool allowNullableValue = false)
 		{
 			return CheckImplicitTruncation(value, type, compilation, allowNullableValue) != ImplicitTruncationResult.ValuePreserved;
 		}
@@ -571,7 +571,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// Gets whether 'stobj type(..., value)' would evaluate to a different value than 'value'
 		/// due to implicit truncation.
 		/// </summary>
-		internal static ImplicitTruncationResult CheckImplicitTruncation(ILInstruction value, IType type, ICompilation compilation, bool allowNullableValue = false)
+		internal static ImplicitTruncationResult CheckImplicitTruncation(ILInstruction value, IType type, ICompilation? compilation, bool allowNullableValue = false)
 		{
 			if (!type.IsSmallIntegerType())
 			{

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		{
 			private readonly DisplayClass container;
 			private readonly IField field;
-			private ILVariable declaredVariable;
+			private ILVariable? declaredVariable;
 
 			public string Name => field.Name;
 
@@ -70,7 +70,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				Debug.Assert(declaredVariable == null || declaredVariable.StateMachineField == field);
 			}
 
-			public void Propagate(ILVariable variable)
+			public void Propagate(ILVariable? variable)
 			{
 				this.declaredVariable = variable;
 				this.CanPropagate = variable != null;
@@ -94,8 +94,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			public readonly ILVariable Variable;
 			public readonly ITypeDefinition Type;
 			public readonly Dictionary<IField, VariableToDeclare> VariablesToDeclare;
-			public BlockContainer CaptureScope;
-			public ILInstruction Initializer;
+			public BlockContainer? CaptureScope;
+			public ILInstruction? Initializer;
 
 			public DisplayClass(ILVariable variable, ITypeDefinition type)
 			{
@@ -105,8 +105,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 		}
 
-		ILTransformContext context;
-		ITypeResolveContext decompilationContext;
+		ILTransformContext? context;
+		ITypeResolveContext? decompilationContext;
 		readonly Dictionary<ILVariable, DisplayClass> displayClasses = new Dictionary<ILVariable, DisplayClass>();
 		readonly Dictionary<ILVariable, ILVariable> displayClassCopyMap = new Dictionary<ILVariable, ILVariable>();
 
@@ -502,7 +502,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return code;
 		}
 
-		VariableToDeclare AddVariable(DisplayClass result, StObj statement, IField field)
+		VariableToDeclare AddVariable(DisplayClass result, StObj? statement, IField field)
 		{
 			VariableToDeclare variable = new VariableToDeclare(result, field);
 			if (statement != null)

ICSharpCode.Decompiler/Metadata/LightJson/JsonValue.cs

@@ -24,7 +24,7 @@ namespace LightJson
 		public static readonly JsonValue Null = new JsonValue(JsonValueType.Null, default(double), null);
 
 		private readonly JsonValueType type;
-		private readonly object reference;
+		private readonly object? reference;
 		private readonly double value;
 
 		/// <summary>
@@ -139,7 +139,7 @@ namespace LightJson
 		/// The internal value reference of the JsonValue.
 		/// This value is used when the Json type is String, JsonObject, or JsonArray.
 		/// </param>
-		private JsonValue(JsonValueType type, double value, object reference)
+		private JsonValue(JsonValueType type, double value, object? reference)
 		{
 			this.type = type;
 			this.value = value;
@@ -357,7 +357,7 @@ namespace LightJson
 		public JsonObject? AsJsonObject {
 			get {
 				return this.IsJsonObject
-					? (JsonObject)this.reference
+					? (JsonObject?)this.reference
 					: null;
 			}
 		}
@@ -369,7 +369,7 @@ namespace LightJson
 		public JsonArray? AsJsonArray {
 			get {
 				return this.IsJsonArray
-					? (JsonArray)this.reference
+					? (JsonArray?)this.reference
 					: null;
 			}
 		}
@@ -382,7 +382,7 @@ namespace LightJson
 			get {
 				DateTime value;
 
-				if (this.IsString && DateTime.TryParse((string)this.reference, out value))
+				if (this.IsString && DateTime.TryParse((string?)this.reference, out value))
 				{
 					return value;
 				}
@@ -427,7 +427,7 @@ namespace LightJson
 			get {
 				if (this.IsJsonObject)
 				{
-					return ((JsonObject)this.reference)[key];
+					return ((JsonObject?)this.reference)[key];
 				}
 				else
 				{
@@ -438,7 +438,7 @@ namespace LightJson
 			set {
 				if (this.IsJsonObject)
 				{
-					((JsonObject)this.reference)[key] = value;
+					((JsonObject?)this.reference)[key] = value;
 				}
 				else
 				{
@@ -458,7 +458,7 @@ namespace LightJson
 			get {
 				if (this.IsJsonArray)
 				{
-					return ((JsonArray)this.reference)[index];
+					return ((JsonArray?)this.reference)[index];
 				}
 				else
 				{
@@ -469,7 +469,7 @@ namespace LightJson
 			set {
 				if (this.IsJsonArray)
 				{
-					((JsonArray)this.reference)[index] = value;
+					((JsonArray?)this.reference)[index] = value;
 				}
 				else
 				{

ICSharpCode.Decompiler/Output/TextTokenWriter.cs

@@ -363,7 +363,7 @@ namespace ICSharpCode.Decompiler
 			output.WriteLine();
 		}
 
-		public override void WritePrimitiveValue(object value, LiteralFormat format = LiteralFormat.None)
+		public override void WritePrimitiveValue(object? value, LiteralFormat format = LiteralFormat.None)
 		{
 			new TextWriterTokenWriter(new TextOutputWriter(output)).WritePrimitiveValue(value, format);
 		}

ICSharpCode.Decompiler/SRMExtensions.cs

@@ -628,7 +628,7 @@ namespace ICSharpCode.Decompiler
 		#endregion
 
 		public static unsafe BlobReader GetInitialValue(this FieldDefinition field, MetadataFile pefile,
-			ICompilation typeSystem)
+			ICompilation? typeSystem)
 		{
 			if (!field.HasFlag(FieldAttributes.HasFieldRVA))
 				return default;
@@ -647,7 +647,7 @@ namespace ICSharpCode.Decompiler
 
 		sealed class FieldValueSizeDecoder : ISignatureTypeProvider<int, GenericContext>
 		{
-			readonly MetadataModule module;
+			readonly MetadataModule? module;
 			readonly int pointerSize;
 
 			public FieldValueSizeDecoder(ICompilation? typeSystem = null)

ICSharpCode.Decompiler/Semantics/InvocationResolveResult.cs

@@ -42,7 +42,7 @@ namespace ICSharpCode.Decompiler.Semantics
 		/// </summary>
 		public readonly IList<ResolveResult> InitializerStatements;
 
-		public InvocationResolveResult(ResolveResult targetResult, IParameterizedMember member,
+		public InvocationResolveResult(ResolveResult? targetResult, IParameterizedMember member,
 									   IList<ResolveResult>? arguments = null,
 									   IList<ResolveResult>? initializerStatements = null,
 									   IType? returnTypeOverride = null)

ICSharpCode.Decompiler/TypeSystem/ApplyAttributeTypeVisitor.cs

@@ -175,11 +175,11 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		readonly bool hasDynamicAttribute;
 		readonly bool[] dynamicAttributeData;
 		readonly bool hasNativeIntegersAttribute;
-		readonly bool[] nativeIntegersAttributeData;
+		readonly bool[]? nativeIntegersAttributeData;
 		readonly TypeSystemOptions options;
 		readonly string[] tupleElementNames;
 		readonly Nullability defaultNullability;
-		readonly Nullability[] nullableAttributeData;
+		readonly Nullability[]? nullableAttributeData;
 		int dynamicTypeIndex = 0;
 		int tupleTypeIndex = 0;
 		int nullabilityTypeIndex = 0;
@@ -187,9 +187,9 @@ namespace ICSharpCode.Decompiler.TypeSystem
 
 		private ApplyAttributeTypeVisitor(ICompilation compilation,
 			bool hasDynamicAttribute, bool[] dynamicAttributeData,
-			bool hasNativeIntegersAttribute, bool[] nativeIntegersAttributeData,
+			bool hasNativeIntegersAttribute, bool[]? nativeIntegersAttributeData,
 			TypeSystemOptions options, string[] tupleElementNames,
-			Nullability defaultNullability, Nullability[] nullableAttributeData)
+			Nullability defaultNullability, Nullability[]? nullableAttributeData)
 		{
 			this.compilation = compilation ?? throw new ArgumentNullException(nameof(compilation));
 			this.hasDynamicAttribute = hasDynamicAttribute;

ICSharpCode.Decompiler/TypeSystem/FullTypeName.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		}
 
 		readonly TopLevelTypeName topLevelType;
-		readonly NestedTypeName[] nestedTypes;
+		readonly NestedTypeName[]? nestedTypes;
 
 		FullTypeName(TopLevelTypeName topLevelTypeName, NestedTypeName[] nestedTypes)
 		{

ICSharpCode.Decompiler/TypeSystem/Implementation/GetMembersHelper.cs

@@ -41,7 +41,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			return GetNestedTypes(type, null, filter, options);
 		}
 
-		public static IEnumerable<IType> GetNestedTypes(IType type, IReadOnlyList<IType> nestedTypeArguments, Predicate<ITypeDefinition> filter, GetMemberOptions options)
+		public static IEnumerable<IType> GetNestedTypes(IType type, IReadOnlyList<IType>? nestedTypeArguments, Predicate<ITypeDefinition> filter, GetMemberOptions options)
 		{
 			if ((options & GetMemberOptions.IgnoreInheritedMembers) == GetMemberOptions.IgnoreInheritedMembers)
 			{
@@ -53,9 +53,9 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			}
 		}
 
-		static IEnumerable<IType> GetNestedTypesImpl(IType outerType, IReadOnlyList<IType> nestedTypeArguments, Predicate<ITypeDefinition> filter, GetMemberOptions options)
+		static IEnumerable<IType> GetNestedTypesImpl(IType outerType, IReadOnlyList<IType>? nestedTypeArguments, Predicate<ITypeDefinition>? filter, GetMemberOptions options)
 		{
-			ITypeDefinition outerTypeDef = outerType.GetDefinition();
+			ITypeDefinition? outerTypeDef = outerType.GetDefinition();
 			if (outerTypeDef == null)
 				yield break;
 

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataNamespace.cs

@@ -34,7 +34,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		public string FullName { get; }
 		public string Name { get; }
 
-		public MetadataNamespace(MetadataModule module, INamespace parent, string fullName, NamespaceDefinition ns)
+		public MetadataNamespace(MetadataModule module, INamespace? parent, string fullName, NamespaceDefinition ns)
 		{
 			Debug.Assert(module != null);
 			Debug.Assert(fullName != null);

ICSharpCode.Decompiler/TypeSystem/Implementation/UnknownType.cs

@@ -38,7 +38,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		/// <param name="namespaceName">Namespace name, if known. Can be null if unknown.</param>
 		/// <param name="name">Name of the type, must not be null.</param>
 		/// <param name="typeParameterCount">Type parameter count, zero if unknown.</param>
-		public UnknownType(string namespaceName, string name, int typeParameterCount = 0, bool? isReferenceType = null)
+		public UnknownType(string? namespaceName, string name, int typeParameterCount = 0, bool? isReferenceType = null)
 		{
 			if (name == null)
 				throw new ArgumentNullException(nameof(name));

ICSharpCode.Decompiler/TypeSystem/TupleType.cs

@@ -53,10 +53,10 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Gets the names of the tuple elements.
 		/// </summary>
-		public ImmutableArray<string> ElementNames { get; }
+		public ImmutableArray<string?> ElementNames { get; }
 
 		public TupleType(ICompilation compilation, ImmutableArray<IType> elementTypes,
-			ImmutableArray<string> elementNames = default(ImmutableArray<string>),
+			ImmutableArray<string?> elementNames = default(ImmutableArray<string?>),
 			IModule? valueTupleAssembly = null)
 		{
 			this.Compilation = compilation;
@@ -64,7 +64,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			this.ElementTypes = elementTypes;
 			if (elementNames.IsDefault)
 			{
-				this.ElementNames = Enumerable.Repeat<string>(null, elementTypes.Length).ToImmutableArray();
+				this.ElementNames = Enumerable.Repeat<string?>(null, elementTypes.Length).ToImmutableArray();
 			}
 			else
 			{
@@ -73,7 +73,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			}
 		}
 
-		static ParameterizedType CreateUnderlyingType(ICompilation compilation, ImmutableArray<IType> elementTypes, IModule valueTupleAssembly)
+		static ParameterizedType CreateUnderlyingType(ICompilation compilation, ImmutableArray<IType> elementTypes, IModule? valueTupleAssembly)
 		{
 			int remainder = (elementTypes.Length - 1) % (RestPosition - 1) + 1;
 			Debug.Assert(remainder >= 1 && remainder < RestPosition);
@@ -92,7 +92,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			return type;
 		}
 
-		private static IType FindValueTupleType(ICompilation compilation, IModule valueTupleAssembly, int tpc)
+		private static IType FindValueTupleType(ICompilation compilation, IModule? valueTupleAssembly, int tpc)
 		{
 			var typeName = new TopLevelTypeName("System", "ValueTuple", tpc);
 			if (valueTupleAssembly != null)
@@ -236,7 +236,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			unchecked
 			{
 				int hash = UnderlyingType.GetHashCode();
-				foreach (string name in ElementNames)
+				foreach (string? name in ElementNames)
 				{
 					hash *= 31;
 					hash += name != null ? name.GetHashCode() : 0;
@@ -387,7 +387,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		public ImmutableArray<string> ElementNames { get; }
 
-		public IModuleReference ValueTupleAssembly { get; }
+		public IModuleReference? ValueTupleAssembly { get; }
 
 		public TupleTypeReference(ImmutableArray<ITypeReference> elementTypes)
 		{

ICSharpCode.Decompiler/Util/ResXResourceWriter.cs

@@ -45,12 +45,12 @@ namespace ICSharpCode.Decompiler.Util
 #endif
 	class ResXResourceWriter : IDisposable
 	{
-		private string filename;
-		private Stream stream;
-		private TextWriter textwriter;
-		private XmlTextWriter writer;
+		private string? filename;
+		private Stream? stream;
+		private TextWriter? textwriter;
+		private XmlTextWriter? writer;
 		private bool written;
-		private string base_path;
+		private string? base_path;
 
 		public static readonly string BinSerializedObjectMimeType = "application/x-microsoft.net.object.binary.base64";
 		public static readonly string ByteArraySerializedObjectMimeType = "application/x-microsoft.net.object.bytearray.base64";
@@ -131,7 +131,7 @@ namespace ICSharpCode.Decompiler.Util
 			writer.WriteString(base64);
 		}
 
-		void WriteBytes(string name, string type, byte[] value, int offset, int length, string comment)
+		void WriteBytes(string name, string? type, byte[] value, int offset, int length, string comment)
 		{
 			writer.WriteStartElement("data");
 			writer.WriteAttributeString("name", name);
@@ -165,7 +165,7 @@ namespace ICSharpCode.Decompiler.Util
 			writer.WriteEndElement();
 		}
 
-		void WriteString(string name, string value, string type, string comment)
+		void WriteString(string name, string value, string? type, string comment)
 		{
 			writer.WriteStartElement("data");
 			writer.WriteAttributeString("name", name);