diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LocalFunctions.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LocalFunctions.cs
index ca3ae943f..c4d784fd4 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LocalFunctions.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LocalFunctions.cs
@@ -134,6 +134,7 @@ namespace LocalFunctions
 						}
 					}
 				}
+				Console.WriteLine(t2);
 				return StaticInvokeAsFunc(MixedLocalFunction2Delegate<T1>) + StaticInvokeAsFunc(MixedLocalFunction2Delegate<T2>) + StaticInvokeAsFunc(StaticMethod1<decimal>) + StaticInvokeAsFunc(StaticMethod1<int>) + StaticInvokeAsFunc(NonStaticMethod3) + StaticInvokeAsFunc(StaticMethod5<T1>) + new Func<object, int>(StaticMethod4<object>)(null) + StaticInvokeAsFunc2<object>(StaticMethod4<object>) + new Func<Func<object, int>, int>(StaticInvokeAsFunc2<object>)(StaticMethod4<object>);
 				int NonStaticMethod3()
 				{
diff --git a/ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj b/ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj
index 571f107bf..0e7fcdcac 100644
--- a/ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj
+++ b/ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj
@@ -421,6 +421,7 @@
     <Compile Include="Util\GraphVizGraph.cs" />
     <Compile Include="Util\KeyComparer.cs" />
     <Compile Include="Util\LongDict.cs" />
+    <Compile Include="Util\NullAttributes.cs" />
     <Compile Include="Util\ResourcesFile.cs" />
     <Compile Include="Util\ResXResourceWriter.cs" />
     <Compile Include="Util\UnicodeNewline.cs" />
diff --git a/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs b/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs
index 241f0bdf4..f70e6ba0f 100644
--- a/ICSharpCode.Decompiler/Metadata/AssemblyReferences.cs
+++ b/ICSharpCode.Decompiler/Metadata/AssemblyReferences.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.IO;
 using System.Linq;
@@ -36,7 +38,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		{
 		}
 
-		public AssemblyResolutionException(IAssemblyReference reference, Exception innerException)
+		public AssemblyResolutionException(IAssemblyReference reference, Exception? innerException)
 			: base($"Failed to resolve assembly: '{reference}'", innerException)
 		{
 			this.Reference = reference;
@@ -46,10 +48,10 @@ namespace ICSharpCode.Decompiler.Metadata
 	public interface IAssemblyResolver
 	{
 #if !VSADDIN
-		PEFile Resolve(IAssemblyReference reference);
-		PEFile ResolveModule(PEFile mainModule, string moduleName);
-		Task<PEFile> ResolveAsync(IAssemblyReference reference);
-		Task<PEFile> ResolveModuleAsync(PEFile mainModule, string moduleName);
+		PEFile? Resolve(IAssemblyReference reference);
+		PEFile? ResolveModule(PEFile mainModule, string moduleName);
+		Task<PEFile?> ResolveAsync(IAssemblyReference reference);
+		Task<PEFile?> ResolveModuleAsync(PEFile mainModule, string moduleName);
 #endif
 	}
 
@@ -68,7 +70,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		/// For .NET Core framework references, the WholeProjectDecompiler will omit the
 		/// assembly reference if the runtimePack is already included as an SDK.
 		/// </summary>
-		public virtual bool IsSharedAssembly(IAssemblyReference reference, out string runtimePack)
+		public virtual bool IsSharedAssembly(IAssemblyReference reference, out string? runtimePack)
 		{
 			runtimePack = null;
 			return false;
@@ -79,9 +81,9 @@ namespace ICSharpCode.Decompiler.Metadata
 	{
 		string Name { get; }
 		string FullName { get; }
-		Version Version { get; }
-		string Culture { get; }
-		byte[] PublicKeyToken { get; }
+		Version? Version { get; }
+		string? Culture { get; }
+		byte[]? PublicKeyToken { get; }
 
 		bool IsWindowsRuntime { get; }
 		bool IsRetargetable { get; }
@@ -89,9 +91,9 @@ namespace ICSharpCode.Decompiler.Metadata
 
 	public class AssemblyNameReference : IAssemblyReference
 	{
-		string fullName;
+		string? fullName;
 
-		public string Name { get; private set; }
+		public string Name { get; private set; } = string.Empty;
 
 		public string FullName {
 			get {
@@ -132,11 +134,11 @@ namespace ICSharpCode.Decompiler.Metadata
 			}
 		}
 
-		public Version Version { get; private set; }
+		public Version? Version { get; private set; }
 
-		public string Culture { get; private set; }
+		public string? Culture { get; private set; }
 
-		public byte[] PublicKeyToken { get; private set; }
+		public byte[]? PublicKeyToken { get; private set; }
 
 		public bool IsWindowsRuntime { get; private set; }
 
@@ -234,11 +236,11 @@ namespace ICSharpCode.Decompiler.Metadata
 			}
 		}
 
-		public Version Version => entry.Version;
+		public Version? Version => entry.Version;
 		public string Culture => Metadata.GetString(entry.Culture);
-		byte[] IAssemblyReference.PublicKeyToken => GetPublicKeyToken();
+		byte[]? IAssemblyReference.PublicKeyToken => GetPublicKeyToken();
 
-		public byte[] GetPublicKeyToken()
+		public byte[]? GetPublicKeyToken()
 		{
 			if (entry.PublicKeyOrToken.IsNil)
 				return null;
diff --git a/ICSharpCode.Decompiler/Metadata/Dom.cs b/ICSharpCode.Decompiler/Metadata/Dom.cs
index de21e0ba3..bc9f06a9f 100644
--- a/ICSharpCode.Decompiler/Metadata/Dom.cs
+++ b/ICSharpCode.Decompiler/Metadata/Dom.cs
@@ -1,4 +1,6 @@
-using System;
+#nullable enable
+
+using System;
 using System.Collections.Immutable;
 using System.IO;
 using System.Reflection;
@@ -32,7 +34,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		public virtual ResourceType ResourceType => ResourceType.Embedded;
 		public virtual ManifestResourceAttributes Attributes => ManifestResourceAttributes.Public;
 		public abstract string Name { get; }
-		public abstract Stream TryOpenStream();
+		public abstract Stream? TryOpenStream();
 	}
 
 	public class ByteArrayResource : Resource
@@ -98,12 +100,16 @@ namespace ICSharpCode.Decompiler.Metadata
 			return ResourceType.Linked;
 		}
 
-		public override unsafe Stream TryOpenStream()
+		public override unsafe Stream? TryOpenStream()
 		{
 			if (ResourceType != ResourceType.Embedded)
 				return null;
 			var headers = Module.Reader.PEHeaders;
+			if (headers.CorHeader == null)
+				return null;
 			var resources = headers.CorHeader.ResourcesDirectory;
+			if (resources.RelativeVirtualAddress == 0)
+				return null;
 			var sectionData = Module.Reader.GetSectionData(resources.RelativeVirtualAddress);
 			if (sectionData.Length == 0)
 				throw new BadImageFormatException("RVA could not be found in any section!");
@@ -231,7 +237,7 @@ namespace ICSharpCode.Decompiler.Metadata
 
 	public class GenericContext
 	{
-		readonly MetadataReader metadata;
+		readonly MetadataReader? metadata;
 		readonly TypeDefinitionHandle declaringType;
 		readonly MethodDefinitionHandle method;
 
@@ -268,7 +274,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		public string GetGenericTypeParameterName(int index)
 		{
 			GenericParameterHandle genericParameter = GetGenericTypeParameterHandleOrNull(index);
-			if (genericParameter.IsNil)
+			if (genericParameter.IsNil || metadata == null)
 				return index.ToString();
 			return metadata.GetString(metadata.GetGenericParameter(genericParameter).Name);
 		}
@@ -276,23 +282,27 @@ namespace ICSharpCode.Decompiler.Metadata
 		public string GetGenericMethodTypeParameterName(int index)
 		{
 			GenericParameterHandle genericParameter = GetGenericMethodTypeParameterHandleOrNull(index);
-			if (genericParameter.IsNil)
+			if (genericParameter.IsNil || metadata == null)
 				return index.ToString();
 			return metadata.GetString(metadata.GetGenericParameter(genericParameter).Name);
 		}
 
 		public GenericParameterHandle GetGenericTypeParameterHandleOrNull(int index)
 		{
-			GenericParameterHandleCollection genericParameters;
-			if (declaringType.IsNil || index < 0 || index >= (genericParameters = metadata.GetTypeDefinition(declaringType).GetGenericParameters()).Count)
+			if (declaringType.IsNil || index < 0 || metadata == null)
+				return MetadataTokens.GenericParameterHandle(0);
+			var genericParameters = metadata.GetTypeDefinition(declaringType).GetGenericParameters();
+			if (index >= genericParameters.Count)
 				return MetadataTokens.GenericParameterHandle(0);
 			return genericParameters[index];
 		}
 
 		public GenericParameterHandle GetGenericMethodTypeParameterHandleOrNull(int index)
 		{
-			GenericParameterHandleCollection genericParameters;
-			if (method.IsNil || index < 0 || index >= (genericParameters = metadata.GetMethodDefinition(method).GetGenericParameters()).Count)
+			if (method.IsNil || index < 0 || metadata == null)
+				return MetadataTokens.GenericParameterHandle(0);
+			var genericParameters = metadata.GetMethodDefinition(method).GetGenericParameters();
+			if (index >= genericParameters.Count)
 				return MetadataTokens.GenericParameterHandle(0);
 			return genericParameters[index];
 		}
diff --git a/ICSharpCode.Decompiler/Metadata/PEFile.cs b/ICSharpCode.Decompiler/Metadata/PEFile.cs
index 2b8192a90..f2a57ab0f 100644
--- a/ICSharpCode.Decompiler/Metadata/PEFile.cs
+++ b/ICSharpCode.Decompiler/Metadata/PEFile.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.Collections.Immutable;
@@ -121,7 +123,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			Reader.Dispose();
 		}
 
-		Dictionary<TopLevelTypeName, TypeDefinitionHandle> typeLookup;
+		Dictionary<TopLevelTypeName, TypeDefinitionHandle>? typeLookup;
 
 		/// <summary>
 		/// Finds the top-level-type with the specified name.
@@ -152,7 +154,7 @@ namespace ICSharpCode.Decompiler.Metadata
 				return default;
 		}
 
-		Dictionary<FullTypeName, ExportedTypeHandle> typeForwarderLookup;
+		Dictionary<FullTypeName, ExportedTypeHandle>? typeForwarderLookup;
 
 		/// <summary>
 		/// Finds the type forwarder with the specified name.
@@ -176,7 +178,7 @@ namespace ICSharpCode.Decompiler.Metadata
 				return default;
 		}
 
-		MethodSemanticsLookup methodSemanticsLookup;
+		MethodSemanticsLookup? methodSemanticsLookup;
 
 		internal MethodSemanticsLookup MethodSemanticsLookup {
 			get {
diff --git a/ICSharpCode.Decompiler/Util/LazyInit.cs b/ICSharpCode.Decompiler/Util/LazyInit.cs
index 6f95ca130..d19344cd4 100644
--- a/ICSharpCode.Decompiler/Util/LazyInit.cs
+++ b/ICSharpCode.Decompiler/Util/LazyInit.cs
@@ -16,13 +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.Diagnostics.CodeAnalysis;
 using System.Threading;
 
 namespace ICSharpCode.Decompiler.Util
 {
 	public static class LazyInit
 	{
-		public static T VolatileRead<T>(ref T location) where T : class
+		public static T VolatileRead<T>(ref T location) where T : class?
 		{
 			return Volatile.Read(ref location);
 		}
@@ -32,9 +34,10 @@ namespace ICSharpCode.Decompiler.Util
 		/// - If target is null: stores newValue in target and returns newValue.
 		/// - If target is not null: returns target.
 		/// </summary>
-		public static T GetOrSet<T>(ref T target, T newValue) where T : class
+		[return: NotNullIfNotNull("newValue")]
+		public static T? GetOrSet<T>(ref T? target, T? newValue) where T : class
 		{
-			T oldValue = Interlocked.CompareExchange(ref target, newValue, null);
+			T? oldValue = Interlocked.CompareExchange(ref target, newValue, null);
 			return oldValue ?? newValue;
 		}
 	}
diff --git a/ICSharpCode.Decompiler/Util/NullAttributes.cs b/ICSharpCode.Decompiler/Util/NullAttributes.cs
new file mode 100644
index 000000000..cf3c9249b
--- /dev/null
+++ b/ICSharpCode.Decompiler/Util/NullAttributes.cs
@@ -0,0 +1,18 @@
+#if !NETCORE
+
+#nullable enable
+
+namespace System.Diagnostics.CodeAnalysis
+{
+	[AttributeUsage(AttributeTargets.Property | AttributeTargets.Parameter | AttributeTargets.ReturnValue, AllowMultiple = true)]
+	internal sealed class NotNullIfNotNullAttribute : Attribute
+	{
+		public string ParameterName { get; }
+
+		public NotNullIfNotNullAttribute(string parameterName)
+		{
+			ParameterName = parameterName;
+		}
+	}
+}
+#endif
diff --git a/ILSpy.AddIn/ILSpy.AddIn.csproj b/ILSpy.AddIn/ILSpy.AddIn.csproj
index 31fc45fc9..0e5e53060 100644
--- a/ILSpy.AddIn/ILSpy.AddIn.csproj
+++ b/ILSpy.AddIn/ILSpy.AddIn.csproj
@@ -14,6 +14,7 @@
     <Description>ILSpy</Description>
     <Version>1.7.1.0</Version>
     <FileVersion>1.7.1.0</FileVersion>
+    <LangVersion>9.0</LangVersion>
 
     <EnableDefaultItems>False</EnableDefaultItems>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
diff --git a/ILSpy/ILSpy.csproj b/ILSpy/ILSpy.csproj
index 29f2e2085..c150d99b5 100644
--- a/ILSpy/ILSpy.csproj
+++ b/ILSpy/ILSpy.csproj
@@ -4,7 +4,7 @@
   <PropertyGroup>
     <TargetFramework>net472</TargetFramework>
     <OutputType>WinExe</OutputType>
-    <LangVersion>8.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <GenerateAssemblyInfo>False</GenerateAssemblyInfo>
     <AutoGenerateBindingRedirects>false</AutoGenerateBindingRedirects>
     <GenerateSupportedRuntime>false</GenerateSupportedRuntime>
diff --git a/ILSpy/LoadedAssembly.cs b/ILSpy/LoadedAssembly.cs
index 2a7ed4e53..cc23499dc 100644
--- a/ILSpy/LoadedAssembly.cs
+++ b/ILSpy/LoadedAssembly.cs
@@ -35,6 +35,8 @@ using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 using ICSharpCode.ILSpy.Options;
 
+#nullable enable
+
 namespace ICSharpCode.ILSpy
 {
 	/// <summary>
@@ -61,9 +63,9 @@ namespace ICSharpCode.ILSpy
 
 		public sealed class LoadResult
 		{
-			public PEFile PEFile { get; }
-			public Exception PEFileLoadException { get; }
-			public LoadedPackage Package { get; }
+			public PEFile? PEFile { get; }
+			public Exception? PEFileLoadException { get; }
+			public LoadedPackage? Package { get; }
 
 			public LoadResult(PEFile peFile)
 			{
@@ -80,12 +82,12 @@ namespace ICSharpCode.ILSpy
 		readonly AssemblyList assemblyList;
 		readonly string fileName;
 		readonly string shortName;
-		readonly IAssemblyResolver providedAssemblyResolver;
+		readonly IAssemblyResolver? providedAssemblyResolver;
 
-		public LoadedAssembly ParentBundle { get; }
+		public LoadedAssembly? ParentBundle { get; }
 
 		public LoadedAssembly(AssemblyList assemblyList, string fileName,
-			Task<Stream> stream = null, IAssemblyResolver assemblyResolver = null, string pdbFileName = null)
+			Task<Stream?>? stream = null, IAssemblyResolver? assemblyResolver = null, string? pdbFileName = null)
 		{
 			this.assemblyList = assemblyList ?? throw new ArgumentNullException(nameof(assemblyList));
 			this.fileName = fileName ?? throw new ArgumentNullException(nameof(fileName));
@@ -96,7 +98,7 @@ namespace ICSharpCode.ILSpy
 			this.shortName = Path.GetFileNameWithoutExtension(fileName);
 		}
 
-		public LoadedAssembly(LoadedAssembly bundle, string fileName, Task<Stream> stream, IAssemblyResolver assemblyResolver = null)
+		public LoadedAssembly(LoadedAssembly bundle, string fileName, Task<Stream?>? stream, IAssemblyResolver assemblyResolver = null)
 			: this(bundle.assemblyList, fileName, stream, assemblyResolver)
 		{
 			this.ParentBundle = bundle;
@@ -116,7 +118,7 @@ namespace ICSharpCode.ILSpy
 
 		public ReferenceLoadInfo LoadedAssemblyReferencesInfo { get; } = new ReferenceLoadInfo();
 
-		IDebugInfoProvider debugInfoProvider;
+		IDebugInfoProvider? debugInfoProvider;
 
 		/// <summary>
 		/// Gets the <see cref="LoadResult"/>.
@@ -135,14 +137,14 @@ namespace ICSharpCode.ILSpy
 			if (loadResult.PEFile != null)
 				return loadResult.PEFile;
 			else
-				throw loadResult.PEFileLoadException;
+				throw loadResult.PEFileLoadException!;
 		}
 
 		/// <summary>
 		/// Gets the <see cref="PEFile"/>.
 		/// Returns null in case of load errors.
 		/// </summary>
-		public PEFile GetPEFileOrNull()
+		public PEFile? GetPEFileOrNull()
 		{
 			try
 			{
@@ -160,7 +162,7 @@ namespace ICSharpCode.ILSpy
 		/// Gets the <see cref="PEFile"/>.
 		/// Returns null in case of load errors.
 		/// </summary>
-		public async Task<PEFile> GetPEFileOrNullAsync()
+		public async Task<PEFile?> GetPEFileOrNullAsync()
 		{
 			try
 			{
@@ -174,7 +176,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		ICompilation typeSystem;
+		ICompilation? typeSystem;
 
 		/// <summary>
 		/// Gets a type system containing all types from this assembly + primitive types from mscorlib.
@@ -183,7 +185,7 @@ namespace ICSharpCode.ILSpy
 		/// <remarks>
 		/// This is an uncached type system.
 		/// </remarks>
-		public ICompilation GetTypeSystemOrNull()
+		public ICompilation? GetTypeSystemOrNull()
 		{
 			return LazyInitializer.EnsureInitialized(ref this.typeSystem, () => {
 				var module = GetPEFileOrNull();
@@ -196,10 +198,10 @@ namespace ICSharpCode.ILSpy
 		}
 
 		readonly object typeSystemWithOptionsLockObj = new object();
-		ICompilation typeSystemWithOptions;
-		TypeSystemOptions currentTypeSystemOptions;
+		ICompilation? typeSystemWithOptions;
+		TypeSystemOptions? currentTypeSystemOptions;
 
-		public ICompilation GetTypeSystemOrNull(TypeSystemOptions options)
+		public ICompilation? GetTypeSystemOrNull(TypeSystemOptions options)
 		{
 			lock (typeSystemWithOptionsLockObj)
 			{
@@ -225,9 +227,9 @@ namespace ICSharpCode.ILSpy
 			get {
 				if (IsLoaded && !HasLoadError)
 				{
-					PEFile module = GetPEFileOrNull();
+					PEFile? module = GetPEFileOrNull();
 					var metadata = module?.Metadata;
-					string versionOrInfo = null;
+					string? versionOrInfo = null;
 					if (metadata != null)
 					{
 						if (metadata.IsAssembly)
@@ -278,14 +280,14 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Gets the PDB file name or null, if no PDB was found or it's embedded.
 		/// </summary>
-		public string PdbFileName { get; private set; }
+		public string? PdbFileName { get; private set; }
 
-		async Task<LoadResult> LoadAsync(Task<Stream> streamTask)
+		async Task<LoadResult> LoadAsync(Task<Stream?>? streamTask)
 		{
 			// runs on background thread
-			if (streamTask != null)
+			var stream = streamTask != null ? await streamTask.ConfigureAwait(false) : null;
+			if (stream != null)
 			{
-				var stream = await streamTask;
 				// Read the module from a precrafted stream
 				if (!stream.CanSeek)
 				{
@@ -357,7 +359,7 @@ namespace ICSharpCode.ILSpy
 			return new LoadResult(module);
 		}
 
-		IDebugInfoProvider LoadDebugInfo(PEFile module)
+		IDebugInfoProvider? LoadDebugInfo(PEFile module)
 		{
 			if (DecompilerSettingsPanel.CurrentDecompilerSettings.UseDebugSymbols)
 			{
@@ -380,7 +382,7 @@ namespace ICSharpCode.ILSpy
 			return null;
 		}
 
-		public async Task<IDebugInfoProvider> LoadDebugInfo(string fileName)
+		public async Task<IDebugInfoProvider?> LoadDebugInfo(string fileName)
 		{
 			this.PdbFileName = fileName;
 			var assembly = await GetPEFileAsync().ConfigureAwait(false);
@@ -393,7 +395,7 @@ namespace ICSharpCode.ILSpy
 			readonly LoadedAssembly parent;
 			readonly bool loadOnDemand;
 
-			readonly IAssemblyResolver providedAssemblyResolver;
+			readonly IAssemblyResolver? providedAssemblyResolver;
 			readonly AssemblyList assemblyList;
 			readonly LoadedAssembly[] alreadyLoadedAssemblies;
 			readonly Task<string> tfmTask;
@@ -416,13 +418,13 @@ namespace ICSharpCode.ILSpy
 				this.referenceLoadInfo = parent.LoadedAssemblyReferencesInfo;
 			}
 
-			public PEFile Resolve(IAssemblyReference reference)
+			public PEFile? Resolve(IAssemblyReference reference)
 			{
 				return ResolveAsync(reference).GetAwaiter().GetResult();
 			}
 
-			Dictionary<string, PEFile> asmLookupByFullName;
-			Dictionary<string, PEFile> asmLookupByShortName;
+			Dictionary<string, PEFile>? asmLookupByFullName;
+			Dictionary<string, PEFile>? asmLookupByShortName;
 
 			/// <summary>
 			/// 0) if we're inside a package, look for filename.dll in parent directories
@@ -436,9 +438,9 @@ namespace ICSharpCode.ILSpy
 			/// 8) search C:\Windows\Microsoft.NET\Framework64\v4.0.30319
 			/// 9) try to find match by asm name (no tfm/version) in loaded assemblies
 			/// </summary>
-			public async Task<PEFile> ResolveAsync(IAssemblyReference reference)
+			public async Task<PEFile?> ResolveAsync(IAssemblyReference reference)
 			{
-				PEFile module;
+				PEFile? module;
 				// 0) if we're inside a package, look for filename.dll in parent directories
 				if (providedAssemblyResolver != null)
 				{
@@ -527,7 +529,9 @@ namespace ICSharpCode.ILSpy
 					try
 					{
 						var module = await loaded.GetPEFileOrNullAsync().ConfigureAwait(false);
-						var reader = module?.Metadata;
+						if (module == null)
+							continue;
+						var reader = module.Metadata;
 						if (reader == null || !reader.IsAssembly)
 							continue;
 						var asmDef = reader.GetAssemblyDefinition();
@@ -547,12 +551,12 @@ namespace ICSharpCode.ILSpy
 				return result;
 			}
 
-			public PEFile ResolveModule(PEFile mainModule, string moduleName)
+			public PEFile? ResolveModule(PEFile mainModule, string moduleName)
 			{
 				return ResolveModuleAsync(mainModule, moduleName).GetAwaiter().GetResult();
 			}
 
-			public async Task<PEFile> ResolveModuleAsync(PEFile mainModule, string moduleName)
+			public async Task<PEFile?> ResolveModuleAsync(PEFile mainModule, string moduleName)
 			{
 				if (providedAssemblyResolver != null)
 				{
@@ -608,7 +612,7 @@ namespace ICSharpCode.ILSpy
 
 		private MyUniversalResolver GetUniversalResolver()
 		{
-			return LazyInitializer.EnsureInitialized(ref this.universalResolver, () => new MyUniversalResolver(this));
+			return LazyInitializer.EnsureInitialized(ref this.universalResolver, () => new MyUniversalResolver(this))!;
 		}
 
 		public AssemblyReferenceClassifier GetAssemblyReferenceClassifier()
@@ -619,7 +623,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Returns the debug info for this assembly. Returns null in case of load errors or no debug info is available.
 		/// </summary>
-		public IDebugInfoProvider GetDebugInfoOrNull()
+		public IDebugInfoProvider? GetDebugInfoOrNull()
 		{
 			if (GetPEFileOrNull() == null)
 				return null;
@@ -634,7 +638,7 @@ namespace ICSharpCode.ILSpy
 			}
 		}
 
-		MyUniversalResolver universalResolver;
+		MyUniversalResolver? universalResolver;
 
 		/// <summary>
 		/// Wait until the assembly is loaded.
diff --git a/ILSpy/LoadedPackage.cs b/ILSpy/LoadedPackage.cs
index 5c889124f..9a72d987c 100644
--- a/ILSpy/LoadedPackage.cs
+++ b/ILSpy/LoadedPackage.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;
@@ -45,7 +47,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Gets the LoadedAssembly instance representing this bundle.
 		/// </summary>
-		internal LoadedAssembly LoadedAssembly { get; set; }
+		internal LoadedAssembly? LoadedAssembly { get; set; }
 
 		public PackageKind Kind { get; }
 
@@ -106,7 +108,7 @@ namespace ICSharpCode.ILSpy
 		/// <summary>
 		/// Load a .NET single-file bundle.
 		/// </summary>
-		public static LoadedPackage FromBundle(string fileName)
+		public static LoadedPackage? FromBundle(string fileName)
 		{
 			using var memoryMappedFile = MemoryMappedFile.CreateFromFile(fileName, FileMode.Open, null, 0, MemoryMappedFileAccess.Read);
 			var view = memoryMappedFile.CreateViewAccessor(0, 0, MemoryMappedFileAccess.Read);
@@ -144,7 +146,7 @@ namespace ICSharpCode.ILSpy
 			public override ManifestResourceAttributes Attributes => originalEntry.Attributes;
 			public override string FullName => originalEntry.FullName;
 			public override ResourceType ResourceType => originalEntry.ResourceType;
-			public override Stream TryOpenStream() => originalEntry.TryOpenStream();
+			public override Stream? TryOpenStream() => originalEntry.TryOpenStream();
 		}
 
 		sealed class ZipFileEntry : PackageEntry
@@ -159,7 +161,7 @@ namespace ICSharpCode.ILSpy
 				this.Name = entry.FullName;
 			}
 
-			public override Stream TryOpenStream()
+			public override Stream? TryOpenStream()
 			{
 				Debug.WriteLine("Decompress " + Name);
 				using var archive = ZipFile.OpenRead(zipFile);
@@ -221,9 +223,9 @@ namespace ICSharpCode.ILSpy
 		public string Name { get; }
 
 		readonly LoadedPackage package;
-		readonly PackageFolder parent;
+		readonly PackageFolder? parent;
 
-		internal PackageFolder(LoadedPackage package, PackageFolder parent, string name)
+		internal PackageFolder(LoadedPackage package, PackageFolder? parent, string name)
 		{
 			this.package = package;
 			this.parent = parent;
@@ -233,7 +235,7 @@ namespace ICSharpCode.ILSpy
 		public List<PackageFolder> Folders { get; } = new List<PackageFolder>();
 		public List<PackageEntry> Entries { get; } = new List<PackageEntry>();
 
-		public PEFile Resolve(IAssemblyReference reference)
+		public PEFile? Resolve(IAssemblyReference reference)
 		{
 			var asm = ResolveFileName(reference.Name + ".dll");
 			if (asm != null)
@@ -243,7 +245,7 @@ namespace ICSharpCode.ILSpy
 			return parent?.Resolve(reference);
 		}
 
-		public Task<PEFile> ResolveAsync(IAssemblyReference reference)
+		public Task<PEFile?> ResolveAsync(IAssemblyReference reference)
 		{
 			var asm = ResolveFileName(reference.Name + ".dll");
 			if (asm != null)
@@ -254,10 +256,10 @@ namespace ICSharpCode.ILSpy
 			{
 				return parent.ResolveAsync(reference);
 			}
-			return Task.FromResult<PEFile>(null);
+			return Task.FromResult<PEFile?>(null);
 		}
 
-		public PEFile ResolveModule(PEFile mainModule, string moduleName)
+		public PEFile? ResolveModule(PEFile mainModule, string moduleName)
 		{
 			var asm = ResolveFileName(moduleName + ".dll");
 			if (asm != null)
@@ -267,7 +269,7 @@ namespace ICSharpCode.ILSpy
 			return parent?.ResolveModule(mainModule, moduleName);
 		}
 
-		public Task<PEFile> ResolveModuleAsync(PEFile mainModule, string moduleName)
+		public Task<PEFile?> ResolveModuleAsync(PEFile mainModule, string moduleName)
 		{
 			var asm = ResolveFileName(moduleName + ".dll");
 			if (asm != null)
@@ -278,12 +280,12 @@ namespace ICSharpCode.ILSpy
 			{
 				return parent.ResolveModuleAsync(mainModule, moduleName);
 			}
-			return Task.FromResult<PEFile>(null);
+			return Task.FromResult<PEFile?>(null);
 		}
 
-		readonly Dictionary<string, LoadedAssembly> assemblies = new Dictionary<string, LoadedAssembly>(StringComparer.OrdinalIgnoreCase);
+		readonly Dictionary<string, LoadedAssembly?> assemblies = new Dictionary<string, LoadedAssembly?>(StringComparer.OrdinalIgnoreCase);
 
-		internal LoadedAssembly ResolveFileName(string name)
+		internal LoadedAssembly? ResolveFileName(string name)
 		{
 			if (package.LoadedAssembly == null)
 				return null;