Add MemberNotNullWhenAttribute to IMethod, IProperty and IEvent. Make sure that it's guaranteed that AccessorOwner is non-null, when IsAccessor is true.

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -90,6 +90,7 @@
     <Compile Include="CSharp\Annotations.cs" />
     <Compile Include="CSharp\CallBuilder.cs" />
     <Compile Include="CSharp\CSharpLanguageVersion.cs" />
+    <Compile Include="NRTAttributes.cs" />
     <Compile Include="PartialTypeInfo.cs" />
     <Compile Include="CSharp\ProjectDecompiler\IProjectFileWriter.cs" />
     <Compile Include="CSharp\OutputVisitor\GenericGrammarAmbiguityVisitor.cs" />

ICSharpCode.Decompiler/NRTAttributes.cs

@@ -0,0 +1,25 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Diagnostics.CodeAnalysis
+{
+	[AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+	internal sealed class MemberNotNullWhenAttribute : Attribute
+	{
+		public bool ReturnValue { get; }
+
+		public string[] Members { get; }
+
+		public MemberNotNullWhenAttribute(bool returnValue, string member)
+		{
+			ReturnValue = returnValue;
+			Members = new string[1] { member };
+		}
+
+		public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+		{
+			ReturnValue = returnValue;
+			Members = members;
+		}
+	}
+}

ICSharpCode.Decompiler/TypeSystem/IEvent.cs

@@ -18,12 +18,17 @@
 
 #nullable enable
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	public interface IEvent : IMember
 	{
+		[MemberNotNullWhen(true, nameof(AddAccessor))]
 		bool CanAdd { get; }
+		[MemberNotNullWhen(true, nameof(RemoveAccessor))]
 		bool CanRemove { get; }
+		[MemberNotNullWhen(true, nameof(InvokeAccessor))]
 		bool CanInvoke { get; }
 
 		IMethod? AddAccessor { get; }

ICSharpCode.Decompiler/TypeSystem/IMethod.cs

@@ -19,6 +19,7 @@
 #nullable enable
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 
 namespace ICSharpCode.Decompiler.TypeSystem
@@ -81,6 +82,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// <summary>
 		/// Gets whether the method is a property/event accessor.
 		/// </summary>
+		[MemberNotNullWhen(true, nameof(AccessorOwner))]
 		bool IsAccessor { get; }
 
 		/// <summary>

ICSharpCode.Decompiler/TypeSystem/IProperty.cs

@@ -18,6 +18,8 @@
 
 #nullable enable
 
+using System.Diagnostics.CodeAnalysis;
+
 namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
@@ -25,7 +27,9 @@ namespace ICSharpCode.Decompiler.TypeSystem
 	/// </summary>
 	public interface IProperty : IParameterizedMember
 	{
+		[MemberNotNullWhen(true, nameof(Getter))]
 		bool CanGet { get; }
+		[MemberNotNullWhen(true, nameof(Setter))]
 		bool CanSet { get; }
 
 		IMethod? Getter { get; }

ICSharpCode.Decompiler/TypeSystem/Implementation/MetadataMethod.cs

@@ -67,7 +67,8 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			var (accessorOwner, semanticsAttribute) = module.PEFile.MethodSemanticsLookup.GetSemantics(handle);
 			const MethodAttributes finalizerAttributes = (MethodAttributes.Virtual | MethodAttributes.Family | MethodAttributes.HideBySig);
 			this.typeParameters = MetadataTypeParameter.Create(module, this, def.GetGenericParameters());
-			if (semanticsAttribute != 0)
+			if (semanticsAttribute != 0 && !accessorOwner.IsNil
+				&& accessorOwner.Kind is HandleKind.PropertyDefinition or HandleKind.EventDefinition)
 			{
 				this.symbolKind = SymbolKind.Accessor;
 				this.accessorOwner = accessorOwner;
@@ -129,7 +130,6 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public bool HasBody => module.metadata.GetMethodDefinition(handle).HasBody();
 
-
 		public IMember AccessorOwner {
 			get {
 				if (accessorOwner.IsNil)

ICSharpCode.Decompiler/TypeSystem/NormalizeTypeVisitor.cs

@@ -1,6 +1,22 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
+// Copyright (c) 2019 Daniel Grunwald
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// 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 ICSharpCode.Decompiler.TypeSystem.Implementation;
 

ICSharpCode.Decompiler/TypeSystem/Nullability.cs

@@ -1,6 +1,20 @@
-using System;
-using System.Collections.Generic;
-using System.Text;
+// Copyright (c) 2019 Daniel Grunwald
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// 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.
 
 namespace ICSharpCode.Decompiler.TypeSystem
 {

ICSharpCode.Decompiler/TypeSystem/TypeVisitor.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 ICSharpCode.Decompiler.TypeSystem.Implementation;
 
 namespace ICSharpCode.Decompiler.TypeSystem