diff --git a/ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs b/ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs
index 2f5ea6fdc..050924c9e 100644
--- a/ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs
+++ b/ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -94,7 +96,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// </remarks>
 		public bool IsDescendantOf(ILInstruction possibleAncestor)
 		{
-			for (ILInstruction ancestor = this; ancestor != null; ancestor = ancestor.Parent)
+			for (ILInstruction? ancestor = this; ancestor != null; ancestor = ancestor.Parent)
 			{
 				if (ancestor == possibleAncestor)
 					return true;
@@ -102,33 +104,33 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public ILInstruction GetCommonParent(ILInstruction other)
+		public ILInstruction? GetCommonParent(ILInstruction other)
 		{
 			if (other == null)
 				throw new ArgumentNullException(nameof(other));
 
-			ILInstruction a = this;
-			ILInstruction b = other;
+			ILInstruction? a = this;
+			ILInstruction? b = other;
 
 			int levelA = a.CountAncestors();
 			int levelB = b.CountAncestors();
 
 			while (levelA > levelB)
 			{
-				a = a.Parent;
+				a = a!.Parent;
 				levelA--;
 			}
 
 			while (levelB > levelA)
 			{
-				b = b.Parent;
+				b = b!.Parent;
 				levelB--;
 			}
 
 			while (a != b)
 			{
-				a = a.Parent;
-				b = b.Parent;
+				a = a!.Parent;
+				b = b!.Parent;
 			}
 
 			return a;
@@ -153,13 +155,13 @@ namespace ICSharpCode.Decompiler.IL
 
 			while (levelA > levelB)
 			{
-				a = a.Parent;
+				a = a.Parent!;
 				levelA--;
 			}
 
 			while (levelB > levelA)
 			{
-				b = b.Parent;
+				b = b.Parent!;
 				levelB--;
 			}
 
@@ -172,8 +174,8 @@ namespace ICSharpCode.Decompiler.IL
 
 			while (a.Parent != b.Parent)
 			{
-				a = a.Parent;
-				b = b.Parent;
+				a = a.Parent!;
+				b = b.Parent!;
 			}
 
 			// now a and b have the same parent or are both root nodes
@@ -183,7 +185,7 @@ namespace ICSharpCode.Decompiler.IL
 		private int CountAncestors()
 		{
 			int level = 0;
-			for (ILInstruction ancestor = this; ancestor != null; ancestor = ancestor.Parent)
+			for (ILInstruction? ancestor = this; ancestor != null; ancestor = ancestor.Parent)
 			{
 				level++;
 			}
@@ -240,7 +242,7 @@ namespace ICSharpCode.Decompiler.IL
 		protected private void MakeDirty()
 		{
 #if DEBUG
-			for (ILInstruction inst = this; inst != null && !inst.IsDirty; inst = inst.parent)
+			for (ILInstruction? inst = this; inst != null && !inst.IsDirty; inst = inst.parent)
 			{
 				inst.IsDirty = true;
 			}
@@ -289,7 +291,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		protected void InvalidateFlags()
 		{
-			for (ILInstruction inst = this; inst != null && inst.flags != invalidFlags; inst = inst.parent)
+			for (ILInstruction? inst = this; inst != null && inst.flags != invalidFlags; inst = inst.parent)
 				inst.flags = invalidFlags;
 		}
 
@@ -425,7 +427,7 @@ namespace ICSharpCode.Decompiler.IL
 			internal ChildrenCollection(ILInstruction inst)
 			{
 				Debug.Assert(inst != null);
-				this.inst = inst;
+				this.inst = inst!;
 			}
 
 			public int Count {
@@ -485,7 +487,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// </summary>
 		public struct ChildrenEnumerator : IEnumerator<ILInstruction>
 		{
-			ILInstruction inst;
+			ILInstruction? inst;
 			readonly int end;
 			int pos;
 
@@ -494,7 +496,7 @@ namespace ICSharpCode.Decompiler.IL
 				Debug.Assert(inst != null);
 				this.inst = inst;
 				this.pos = -1;
-				this.end = inst.GetChildCount();
+				this.end = inst!.GetChildCount();
 #if DEBUG
 				inst.StartEnumerator();
 #endif
@@ -502,7 +504,7 @@ namespace ICSharpCode.Decompiler.IL
 
 			public ILInstruction Current {
 				get {
-					return inst.GetChild(pos);
+					return inst!.GetChild(pos);
 				}
 			}
 
@@ -556,7 +558,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// </remarks>
 		public void ReplaceWith(ILInstruction replacement)
 		{
-			Debug.Assert(parent.GetChild(ChildIndex) == this);
+			Debug.Assert(parent!.GetChild(ChildIndex) == this);
 			if (replacement == this)
 				return;
 			parent.SetChild(ChildIndex, replacement);
@@ -621,7 +623,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// </summary>
 		public IEnumerable<ILInstruction> Ancestors {
 			get {
-				for (ILInstruction node = this; node != null; node = node.Parent)
+				for (ILInstruction? node = this; node != null; node = node.Parent)
 				{
 					yield return node;
 				}
@@ -683,7 +685,7 @@ namespace ICSharpCode.Decompiler.IL
 				child.ReleaseRef();
 		}
 
-		ILInstruction parent;
+		ILInstruction? parent;
 
 		/// <summary>
 		/// Gets the parent of this ILInstruction.
@@ -709,7 +711,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// 
 		/// Note that is it is possible (though unusual) for a stale position to reference an orphaned node.
 		/// </remarks>
-		public ILInstruction Parent {
+		public ILInstruction? Parent {
 			get { return parent; }
 		}
 
@@ -732,7 +734,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// 
 		/// Precondition: this node must not be orphaned.
 		/// </remarks>
-		public SlotInfo SlotInfo {
+		public SlotInfo? SlotInfo {
 			get {
 				if (parent == null)
 					return null;
@@ -748,7 +750,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <param name="newValue">New child</param>
 		/// <param name="index">Index of the field in the Children collection</param>
 		protected internal void SetChildInstruction<T>(ref T childPointer, T newValue, int index)
-			where T : ILInstruction
+			where T : ILInstruction?
 		{
 			T oldValue = childPointer;
 			Debug.Assert(oldValue == GetChild(index));
@@ -861,7 +863,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// If the method returns true, it adds the capture groups (if any) to the match.
 		/// If the method returns false, the match object may remain in a partially-updated state and
 		/// needs to be restored before it can be reused.</returns>
-		protected internal abstract bool PerformMatch(ILInstruction other, ref Match match);
+		protected internal abstract bool PerformMatch(ILInstruction? other, ref Match match);
 
 		/// <summary>
 		/// Attempts matching this instruction against a list of other instructions (or a part of said list).
diff --git a/ICSharpCode.Decompiler/IL/Transforms/IILTransform.cs b/ICSharpCode.Decompiler/IL/Transforms/IILTransform.cs
index e813ebaed..35c5fd7fb 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/IILTransform.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/IILTransform.cs
@@ -16,16 +16,15 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
-using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Threading;
 
 using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.DebugInfo;
 using ICSharpCode.Decompiler.TypeSystem;
-using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.IL.Transforms
 {
@@ -44,16 +43,16 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 	{
 		public ILFunction Function { get; }
 		public IDecompilerTypeSystem TypeSystem { get; }
-		public IDebugInfoProvider DebugInfo { get; }
+		public IDebugInfoProvider? DebugInfo { get; }
 		public DecompilerSettings Settings { get; }
 		public CancellationToken CancellationToken { get; set; }
 		public Stepper Stepper { get; set; }
 		public Metadata.PEFile PEFile => TypeSystem.MainModule.PEFile;
 
-		internal DecompileRun DecompileRun { get; set; }
-		internal ResolvedUsingScope UsingScope => DecompileRun?.UsingScope.Resolve(TypeSystem);
+		internal DecompileRun? DecompileRun { get; set; }
+		internal ResolvedUsingScope? UsingScope => DecompileRun?.UsingScope.Resolve(TypeSystem);
 
-		public ILTransformContext(ILFunction function, IDecompilerTypeSystem typeSystem, IDebugInfoProvider debugInfo, DecompilerSettings settings = null)
+		public ILTransformContext(ILFunction function, IDecompilerTypeSystem typeSystem, IDebugInfoProvider? debugInfo, DecompilerSettings? settings = null)
 		{
 			this.Function = function ?? throw new ArgumentNullException(nameof(function));
 			this.TypeSystem = typeSystem ?? throw new ArgumentNullException(nameof(typeSystem));
@@ -62,7 +61,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			Stepper = new Stepper();
 		}
 
-		public ILTransformContext(ILTransformContext context, ILFunction function = null)
+		public ILTransformContext(ILTransformContext context, ILFunction? function = null)
 		{
 			this.Function = function ?? context.Function;
 			this.TypeSystem = context.TypeSystem;
@@ -89,13 +88,13 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// Unlike <c>context.Stepper.Step()</c>, calls to this method are only compiled in debug builds.
 		/// </summary>
 		[Conditional("STEP")]
-		internal void Step(string description, ILInstruction near)
+		internal void Step(string description, ILInstruction? near)
 		{
 			Stepper.Step(description, near);
 		}
 
 		[Conditional("STEP")]
-		internal void StepStartGroup(string description, ILInstruction near = null)
+		internal void StepStartGroup(string description, ILInstruction? near = null)
 		{
 			Stepper.StartGroup(description, near);
 		}
diff --git a/ICSharpCode.Decompiler/IL/Transforms/Stepper.cs b/ICSharpCode.Decompiler/IL/Transforms/Stepper.cs
index 76aa63ff0..f3f85386a 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/Stepper.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/Stepper.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -58,8 +60,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		public class Node
 		{
-			public string Description { get; set; }
-			public ILInstruction Position { get; set; }
+			public string Description { get; }
+			public ILInstruction? Position { get; set; }
 			/// <summary>
 			/// BeginStep is inclusive.
 			/// </summary>
@@ -70,6 +72,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			public int EndStep { get; set; }
 
 			public IList<Node> Children { get; } = new List<Node>();
+
+			public Node(string description)
+			{
+				Description = description;
+			}
 		}
 
 		readonly Stack<Node> groups;
@@ -88,12 +95,12 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		/// 
 		/// May throw <see cref="StepLimitReachedException"/> in debug mode.
 		/// </summary>
-		public void Step(string description, ILInstruction near = null)
+		public void Step(string description, ILInstruction? near = null)
 		{
 			StepInternal(description, near);
 		}
 
-		private Node StepInternal(string description, ILInstruction near)
+		private Node StepInternal(string description, ILInstruction? near)
 		{
 			if (step == StepLimit)
 			{
@@ -102,8 +109,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				else
 					throw new StepLimitReachedException();
 			}
-			var stepNode = new Node {
-				Description = $"{step}: {description}",
+			var stepNode = new Node($"{step}: {description}") {
 				Position = near,
 				BeginStep = step,
 				EndStep = step + 1
@@ -117,7 +123,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return stepNode;
 		}
 
-		public void StartGroup(string description, ILInstruction near = null)
+		public void StartGroup(string description, ILInstruction? near = null)
 		{
 			groups.Push(StepInternal(description, near));
 		}
diff --git a/ICSharpCode.Decompiler/TypeSystem/IAssembly.cs b/ICSharpCode.Decompiler/TypeSystem/IAssembly.cs
index d2191da74..da8148f90 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IAssembly.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IAssembly.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 using ICSharpCode.Decompiler.Metadata;
@@ -40,7 +42,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Resolves this metadata module.
 		/// </summary>
-		IModule Resolve(ITypeResolveContext context);
+		IModule? Resolve(ITypeResolveContext context);
 	}
 
 	/// <summary>
@@ -51,7 +53,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Gets the underlying metadata file. May return null, if the IAssembly was not created from a PE file.
 		/// </summary>
-		PEFile PEFile { get; }
+		PEFile? PEFile { get; }
 
 		/// <summary>
 		/// Gets whether this assembly is the main assembly of the compilation.
@@ -97,7 +99,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the type definition for a top-level type.
 		/// </summary>
 		/// <remarks>This method uses ordinal name comparison, not the compilation's name comparer.</remarks>
-		ITypeDefinition GetTypeDefinition(TopLevelTypeName topLevelTypeName);
+		ITypeDefinition? GetTypeDefinition(TopLevelTypeName topLevelTypeName);
 
 		/// <summary>
 		/// Gets all non-nested types in the assembly.
diff --git a/ICSharpCode.Decompiler/TypeSystem/IAttribute.cs b/ICSharpCode.Decompiler/TypeSystem/IAttribute.cs
index 53b61b0c8..45d07b092 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IAttribute.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IAttribute.cs
@@ -16,12 +16,11 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System.Collections.Generic;
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Reflection.Metadata;
 
-using ICSharpCode.Decompiler.Semantics;
-
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
@@ -39,7 +38,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the constructor being used.
 		/// This property may return null if no matching constructor was found.
 		/// </summary>
-		IMethod Constructor { get; }
+		IMethod? Constructor { get; }
 
 		/// <summary>
 		/// Gets whether there were errors decoding the attribute.
diff --git a/ICSharpCode.Decompiler/TypeSystem/ICodeContext.cs b/ICSharpCode.Decompiler/TypeSystem/ICodeContext.cs
index 6d9525c08..09152382b 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ICodeContext.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ICodeContext.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -34,4 +36,3 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		bool IsWithinLambdaExpression { get; }
 	}
 }
-
diff --git a/ICSharpCode.Decompiler/TypeSystem/ICompilation.cs b/ICSharpCode.Decompiler/TypeSystem/ICompilation.cs
index d41f350f4..abbbe5e2d 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ICompilation.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ICompilation.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 
@@ -63,7 +65,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// returns the global root namespace.
 		/// If no alias with the specified name exists, this method returns null.
 		/// </remarks>
-		INamespace GetNamespaceForExternAlias(string alias);
+		INamespace? GetNamespaceForExternAlias(string? alias);
 
 		IType FindType(KnownTypeCode typeCode);
 
diff --git a/ICSharpCode.Decompiler/TypeSystem/IDecompilerTypeSystem.cs b/ICSharpCode.Decompiler/TypeSystem/IDecompilerTypeSystem.cs
index f45bdd2c1..44991cd95 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IDecompilerTypeSystem.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IDecompilerTypeSystem.cs
@@ -16,10 +16,7 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System.Collections.Immutable;
-using System.Reflection.Metadata;
-
-using ICSharpCode.Decompiler.Metadata;
+#nullable enable
 
 namespace ICSharpCode.Decompiler.TypeSystem
 {
diff --git a/ICSharpCode.Decompiler/TypeSystem/IEntity.cs b/ICSharpCode.Decompiler/TypeSystem/IEntity.cs
index 4bd680a75..92452c64a 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IEntity.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IEntity.cs
@@ -16,7 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -46,14 +47,14 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// For members, this is the class that contains the member.
 		/// For nested classes, this is the outer class. For top-level entities, this property returns null.
 		/// </summary>
-		ITypeDefinition DeclaringTypeDefinition { get; }
+		ITypeDefinition? DeclaringTypeDefinition { get; }
 
 		/// <summary>
 		/// Gets/Sets the declaring type (incl. type arguments, if any).
 		/// This property will return null for top-level entities.
 		/// If this is not a specialized member, the value returned is equal to <see cref="DeclaringTypeDefinition"/>.
 		/// </summary>
-		IType DeclaringType { get; }
+		IType? DeclaringType { get; }
 
 		/// <summary>
 		/// The module in which this entity is defined.
diff --git a/ICSharpCode.Decompiler/TypeSystem/IEvent.cs b/ICSharpCode.Decompiler/TypeSystem/IEvent.cs
index ee9aa2b80..732840c62 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IEvent.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IEvent.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	public interface IEvent : IMember
@@ -24,8 +26,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		bool CanRemove { get; }
 		bool CanInvoke { get; }
 
-		IMethod AddAccessor { get; }
-		IMethod RemoveAccessor { get; }
-		IMethod InvokeAccessor { get; }
+		IMethod? AddAccessor { get; }
+		IMethod? RemoveAccessor { get; }
+		IMethod? InvokeAccessor { get; }
 	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/IField.cs b/ICSharpCode.Decompiler/TypeSystem/IField.cs
index 18f5a00b7..49a92fd26 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IField.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IField.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
diff --git a/ICSharpCode.Decompiler/TypeSystem/IFreezable.cs b/ICSharpCode.Decompiler/TypeSystem/IFreezable.cs
index b6dae14f0..2623eb1c2 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IFreezable.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IFreezable.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	public interface IFreezable
diff --git a/ICSharpCode.Decompiler/TypeSystem/IInterningProvider.cs b/ICSharpCode.Decompiler/TypeSystem/IInterningProvider.cs
index 0c670f214..7ef6a58fc 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IInterningProvider.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IInterningProvider.cs
@@ -16,7 +16,10 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 
 namespace ICSharpCode.Decompiler.TypeSystem
 {
@@ -47,52 +50,57 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// 
 		/// If the object is freezable, it will be frozen.
 		/// </summary>
-		public abstract ISupportsInterning Intern(ISupportsInterning obj);
+		[return: NotNullIfNotNull("obj")]
+		public abstract ISupportsInterning? Intern(ISupportsInterning? obj);
 
 		/// <summary>
 		/// Interns the specified object.
 		/// 
 		/// If the object is freezable, it will be frozen.
 		/// </summary>
-		public T Intern<T>(T obj) where T : class, ISupportsInterning
+		[return: NotNullIfNotNull("obj")]
+		public T? Intern<T>(T? obj) where T : class, ISupportsInterning
 		{
-			ISupportsInterning input = obj;
-			return (T)Intern(input);
+			ISupportsInterning? input = obj;
+			return (T?)Intern(input);
 		}
 
 		/// <summary>
 		/// Interns the specified string.
 		/// </summary>
-		public abstract string Intern(string text);
+		[return: NotNullIfNotNull("text")]
+		public abstract string? Intern(string? text);
 
 		/// <summary>
 		/// Inters a boxed value type.
 		/// </summary>
-		public abstract object InternValue(object obj);
+		[return: NotNullIfNotNull("obj")]
+		public abstract object? InternValue(object? obj);
 
 		/// <summary>
 		/// Interns the given list. Uses reference equality to compare the list elements.
 		/// </summary>
-		public abstract IList<T> InternList<T>(IList<T> list) where T : class;
+		[return: NotNullIfNotNull("list")]
+		public abstract IList<T>? InternList<T>(IList<T>? list) where T : class;
 
 		sealed class DummyInterningProvider : InterningProvider
 		{
-			public override ISupportsInterning Intern(ISupportsInterning obj)
+			public override ISupportsInterning? Intern(ISupportsInterning? obj)
 			{
 				return obj;
 			}
 
-			public override string Intern(string text)
+			public override string? Intern(string? text)
 			{
 				return text;
 			}
 
-			public override object InternValue(object obj)
+			public override object? InternValue(object? obj)
 			{
 				return obj;
 			}
 
-			public override IList<T> InternList<T>(IList<T> list)
+			public override IList<T>? InternList<T>(IList<T>? list)
 			{
 				return list;
 			}
diff --git a/ICSharpCode.Decompiler/TypeSystem/IMember.cs b/ICSharpCode.Decompiler/TypeSystem/IMember.cs
index 47718c4d1..d5f359072 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IMember.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IMember.cs
@@ -16,7 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -40,7 +41,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <returns>
 		/// Returns the resolved member, or <c>null</c> if the member could not be found.
 		/// </returns>
-		IMember Resolve(ITypeResolveContext context);
+		IMember? Resolve(ITypeResolveContext context);
 	}
 
 	/// <summary>
@@ -119,6 +120,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Gets whether the members are considered equal when applying the specified type normalization.
 		/// </summary>
-		bool Equals(IMember obj, TypeVisitor typeNormalization);
+		bool Equals(IMember? obj, TypeVisitor typeNormalization);
 	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/IMethod.cs b/ICSharpCode.Decompiler/TypeSystem/IMethod.cs
index d82dc5491..26e4626c8 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IMethod.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IMethod.cs
@@ -16,7 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
+#nullable enable
+
 using System.Collections.Generic;
 using System.Reflection;
 
@@ -86,7 +87,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// If this method is an accessor, returns the corresponding property/event.
 		/// Otherwise, returns null.
 		/// </summary>
-		IMember AccessorOwner { get; }
+		IMember? AccessorOwner { get; }
 
 		/// <summary>
 		/// Gets the kind of accessor this is.
@@ -98,7 +99,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// A reduced method doesn't contain the extension method parameter. That means that it has one parameter less than its definition.
 		/// A local function doesn't contain compiler-generated method parameters at the end.
 		/// </summary>
-		IMethod ReducedFrom { get; }
+		IMethod? ReducedFrom { get; }
 
 		/// <summary>
 		/// Specializes this method with the given substitution.
diff --git a/ICSharpCode.Decompiler/TypeSystem/INamespace.cs b/ICSharpCode.Decompiler/TypeSystem/INamespace.cs
index 353fd633b..81c6fe967 100644
--- a/ICSharpCode.Decompiler/TypeSystem/INamespace.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/INamespace.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -49,7 +51,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the parent namespace.
 		/// Returns null if this is the root namespace.
 		/// </summary>
-		INamespace ParentNamespace { get; }
+		INamespace? ParentNamespace { get; }
 
 		/// <summary>
 		/// Gets the child namespaces in this namespace.
@@ -73,7 +75,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <remarks>
 		/// This method uses the compilation's current string comparer.
 		/// </remarks>
-		INamespace GetChildNamespace(string name);
+		INamespace? GetChildNamespace(string name);
 
 		/// <summary>
 		/// Gets the type with the specified short name and type parameter count.
@@ -82,6 +84,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <remarks>
 		/// This method uses the compilation's current string comparer.
 		/// </remarks>
-		ITypeDefinition GetTypeDefinition(string name, int typeParameterCount);
+		ITypeDefinition? GetTypeDefinition(string name, int typeParameterCount);
 	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/IParameter.cs b/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
index ff7fdea3b..b28b4db09 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IParameter.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
@@ -90,6 +92,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the owner of this parameter.
 		/// May return null; for example when parameters belong to lambdas or anonymous methods.
 		/// </summary>
-		IParameterizedMember Owner { get; }
+		IParameterizedMember? Owner { get; }
 	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/IParameterizedMember.cs b/ICSharpCode.Decompiler/TypeSystem/IParameterizedMember.cs
index 7f7e055ed..aba9a8f1f 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IParameterizedMember.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IParameterizedMember.cs
@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System.Collections.Generic;
 
diff --git a/ICSharpCode.Decompiler/TypeSystem/IProperty.cs b/ICSharpCode.Decompiler/TypeSystem/IProperty.cs
index 56673e52b..f8b6203d1 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IProperty.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IProperty.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
@@ -26,8 +28,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		bool CanGet { get; }
 		bool CanSet { get; }
 
-		IMethod Getter { get; }
-		IMethod Setter { get; }
+		IMethod? Getter { get; }
+		IMethod? Setter { get; }
 
 		bool IsIndexer { get; }
 
diff --git a/ICSharpCode.Decompiler/TypeSystem/ISupportsInterning.cs b/ICSharpCode.Decompiler/TypeSystem/ISupportsInterning.cs
index dfab4ea57..e3517fce7 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ISupportsInterning.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ISupportsInterning.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
diff --git a/ICSharpCode.Decompiler/TypeSystem/ISymbol.cs b/ICSharpCode.Decompiler/TypeSystem/ISymbol.cs
index 6bfcd5b71..430479efb 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ISymbol.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ISymbol.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	public enum SymbolKind : byte
diff --git a/ICSharpCode.Decompiler/TypeSystem/IType.cs b/ICSharpCode.Decompiler/TypeSystem/IType.cs
index c9725d04e..a5e38fc6a 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IType.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IType.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 
@@ -83,13 +85,13 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the underlying type definition.
 		/// Can return null for types which do not have a type definition (for example arrays, pointers, type parameters).
 		/// </summary>
-		ITypeDefinition GetDefinition();
+		ITypeDefinition? GetDefinition();
 
 		/// <summary>
 		/// Gets the parent type, if this is a nested type.
 		/// Returns null for top-level types.
 		/// </summary>
-		IType DeclaringType { get; }
+		IType? DeclaringType { get; }
 
 		/// <summary>
 		/// Gets the number of type parameters.
@@ -168,7 +170,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Base.GetNestedTypes() = { Base`1+Nested`1[`0, unbound] }
 		/// </code>
 		/// </example>
-		IEnumerable<IType> GetNestedTypes(Predicate<ITypeDefinition> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IType> GetNestedTypes(Predicate<ITypeDefinition>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		// Note that we cannot 'leak' the additional type parameter as we leak the normal type parameters, because
 		// the index might collide. For example,
@@ -193,7 +195,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// and thus 'leaked' to the caller in the same way the GetMembers() method does not specialize members
 		/// from an <see cref="ITypeDefinition"/> and 'leaks' type parameters in member signatures.
 		/// </remarks>
-		IEnumerable<IType> GetNestedTypes(IReadOnlyList<IType> typeArguments, Predicate<ITypeDefinition> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IType> GetNestedTypes(IReadOnlyList<IType> typeArguments, Predicate<ITypeDefinition>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all instance constructors for this type.
@@ -209,7 +211,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// and the appropriate <see cref="Implementation.SpecializedMethod"/> will be returned.
 		/// </para>
 		/// </remarks>
-		IEnumerable<IMethod> GetConstructors(Predicate<IMethod> filter = null, GetMemberOptions options = GetMemberOptions.IgnoreInheritedMembers);
+		IEnumerable<IMethod> GetConstructors(Predicate<IMethod>? filter = null, GetMemberOptions options = GetMemberOptions.IgnoreInheritedMembers);
 
 		/// <summary>
 		/// Gets all methods that can be called on this type.
@@ -236,7 +238,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// the ambiguity can be avoided.
 		/// </para>
 		/// </remarks>
-		IEnumerable<IMethod> GetMethods(Predicate<IMethod> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IMethod> GetMethods(Predicate<IMethod>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all generic methods that can be called on this type with the specified type arguments.
@@ -257,7 +259,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// and the other overload's remarks about ambiguous signatures apply here as well.
 		/// </para>
 		/// </remarks>
-		IEnumerable<IMethod> GetMethods(IReadOnlyList<IType> typeArguments, Predicate<IMethod> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IMethod> GetMethods(IReadOnlyList<IType> typeArguments, Predicate<IMethod>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all properties that can be called on this type.
@@ -269,7 +271,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// For properties on parameterized types, type substitution will be performed on the property signature,
 		/// and the appropriate <see cref="Implementation.SpecializedProperty"/> will be returned.
 		/// </remarks>
-		IEnumerable<IProperty> GetProperties(Predicate<IProperty> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IProperty> GetProperties(Predicate<IProperty>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all fields that can be accessed on this type.
@@ -281,7 +283,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// For fields on parameterized types, type substitution will be performed on the field's return type,
 		/// and the appropriate <see cref="Implementation.SpecializedField"/> will be returned.
 		/// </remarks>
-		IEnumerable<IField> GetFields(Predicate<IField> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IField> GetFields(Predicate<IField>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all events that can be accessed on this type.
@@ -293,7 +295,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// For fields on parameterized types, type substitution will be performed on the event's return type,
 		/// and the appropriate <see cref="Implementation.SpecializedEvent"/> will be returned.
 		/// </remarks>
-		IEnumerable<IEvent> GetEvents(Predicate<IEvent> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IEvent> GetEvents(Predicate<IEvent>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all members that can be called on this type.
@@ -312,7 +314,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <see cref="GetMethods(Predicate{IMethod}, GetMemberOptions)"/> method apply here as well.
 		/// </para>
 		/// </remarks>
-		IEnumerable<IMember> GetMembers(Predicate<IMember> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IMember> GetMembers(Predicate<IMember>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 
 		/// <summary>
 		/// Gets all accessors belonging to properties or events on this type.
@@ -323,7 +325,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <remarks>
 		/// Accessors are not returned by GetMembers() or GetMethods().
 		/// </remarks>
-		IEnumerable<IMethod> GetAccessors(Predicate<IMethod> filter = null, GetMemberOptions options = GetMemberOptions.None);
+		IEnumerable<IMethod> GetAccessors(Predicate<IMethod>? filter = null, GetMemberOptions options = GetMemberOptions.None);
 	}
 
 	[Flags]
diff --git a/ICSharpCode.Decompiler/TypeSystem/ITypeDefinition.cs b/ICSharpCode.Decompiler/TypeSystem/ITypeDefinition.cs
index 4eb6b922c..bd7bb1b3c 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ITypeDefinition.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ITypeDefinition.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System.Collections.Generic;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -60,7 +62,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets/Sets the declaring type (incl. type arguments, if any).
 		/// This property will return null for top-level types.
 		/// </summary>
-		new IType DeclaringType { get; } // solves ambiguity between IType.DeclaringType and IEntity.DeclaringType
+		new IType? DeclaringType { get; } // solves ambiguity between IType.DeclaringType and IEntity.DeclaringType
 
 		/// <summary>
 		/// Gets whether this type contains extension methods.
diff --git a/ICSharpCode.Decompiler/TypeSystem/ITypeParameter.cs b/ICSharpCode.Decompiler/TypeSystem/ITypeParameter.cs
index 4079d274a..09bed7da3 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ITypeParameter.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ITypeParameter.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 
@@ -43,7 +45,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// inner.TypeParameters[0].Owner will be the outer class, because the same
 		/// ITypeParameter instance is used both on Outer`1 and Outer`1+Inner.
 		/// </remarks>
-		IEntity Owner { get; }
+		IEntity? Owner { get; }
 
 		/// <summary>
 		/// Gets the index of the type parameter in the type parameter list of the owning method/class.
@@ -112,7 +114,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		public IType Type { get; }
 		public IReadOnlyList<IAttribute> Attributes { get; }
 
-		public TypeConstraint(IType type, IReadOnlyList<IAttribute> attributes = null)
+		public TypeConstraint(IType type, IReadOnlyList<IAttribute>? attributes = null)
 		{
 			this.Type = type ?? throw new ArgumentNullException(nameof(type));
 			this.Attributes = attributes ?? EmptyList<IAttribute>.Instance;
diff --git a/ICSharpCode.Decompiler/TypeSystem/ITypeReference.cs b/ICSharpCode.Decompiler/TypeSystem/ITypeReference.cs
index 92ef08450..0716a4807 100644
--- a/ICSharpCode.Decompiler/TypeSystem/ITypeReference.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/ITypeReference.cs
@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
@@ -53,19 +55,19 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the current module.
 		/// This property may return null if this context does not specify any module.
 		/// </summary>
-		IModule CurrentModule { get; }
+		IModule? CurrentModule { get; }
 
 		/// <summary>
 		/// Gets the current type definition.
 		/// </summary>
-		ITypeDefinition CurrentTypeDefinition { get; }
+		ITypeDefinition? CurrentTypeDefinition { get; }
 
 		/// <summary>
 		/// Gets the current member.
 		/// </summary>
-		IMember CurrentMember { get; }
+		IMember? CurrentMember { get; }
 
-		ITypeResolveContext WithCurrentTypeDefinition(ITypeDefinition typeDefinition);
-		ITypeResolveContext WithCurrentMember(IMember member);
+		ITypeResolveContext WithCurrentTypeDefinition(ITypeDefinition? typeDefinition);
+		ITypeResolveContext WithCurrentMember(IMember? member);
 	}
-}
\ No newline at end of file
+}
diff --git a/ICSharpCode.Decompiler/TypeSystem/IVariable.cs b/ICSharpCode.Decompiler/TypeSystem/IVariable.cs
index 13e1c270e..1e537cb88 100644
--- a/ICSharpCode.Decompiler/TypeSystem/IVariable.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/IVariable.cs
@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 namespace ICSharpCode.Decompiler.TypeSystem
 {
@@ -42,6 +43,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// If this field is a constant, retrieves the value.
 		/// For parameters, this is the default value.
 		/// </summary>
-		object GetConstantValue(bool throwOnInvalidMetadata = false);
+		object? GetConstantValue(bool throwOnInvalidMetadata = false);
 	}
 }
diff --git a/ICSharpCode.Decompiler/TypeSystem/KnownTypeReference.cs b/ICSharpCode.Decompiler/TypeSystem/KnownTypeReference.cs
index 03b2b1267..06ba9d6cd 100644
--- a/ICSharpCode.Decompiler/TypeSystem/KnownTypeReference.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/KnownTypeReference.cs
@@ -15,6 +15,7 @@
 // FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
+#nullable enable
 
 using System;
 using System.Collections.Generic;
@@ -162,7 +163,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 	{
 		internal const int KnownTypeCodeCount = (int)KnownTypeCode.Range + 1;
 
-		static readonly KnownTypeReference[] knownTypeReferences = new KnownTypeReference[KnownTypeCodeCount] {
+		static readonly KnownTypeReference?[] knownTypeReferences = new KnownTypeReference?[KnownTypeCodeCount] {
 			null, // None
 			new KnownTypeReference(KnownTypeCode.Object,   TypeKind.Class, "System", "Object", baseType: KnownTypeCode.None),
 			new KnownTypeReference(KnownTypeCode.DBNull,   TypeKind.Class, "System", "DBNull"),
@@ -231,7 +232,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the known type reference for the specified type code.
 		/// Returns null for KnownTypeCode.None.
 		/// </summary>
-		public static KnownTypeReference Get(KnownTypeCode typeCode)
+		public static KnownTypeReference? Get(KnownTypeCode typeCode)
 		{
 			return knownTypeReferences[(int)typeCode];
 		}
@@ -299,7 +300,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// Gets the C# primitive type name from the known type code.
 		/// Returns null if there is no primitive name for the specified type.
 		/// </summary>
-		public static string GetCSharpNameByTypeCode(KnownTypeCode knownTypeCode)
+		public static string? GetCSharpNameByTypeCode(KnownTypeCode knownTypeCode)
 		{
 			switch (knownTypeCode)
 			{