#nullable enable for ILAst (except for generated code)

ICSharpCode.Decompiler/CSharp/Resolver/CSharpOperators.cs

@@ -128,9 +128,7 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				get { return null; }
 			}
 
-			IType IEntity.DeclaringType {
+			public IType DeclaringType => SpecialType.UnknownType;
-				get { return null; }
-			}
 
 			IMember IMember.MemberDefinition {
 				get { return this; }

ICSharpCode.Decompiler/IL/ILAstWritingOptions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 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
@@ -84,7 +85,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+		protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
 		{
 			OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
 		}
@@ -94,6 +95,6 @@ namespace ICSharpCode.Decompiler.IL
 			PropertyChanged?.Invoke(this, e);
 		}
 
-		public event PropertyChangedEventHandler PropertyChanged;
+		public event PropertyChangedEventHandler? PropertyChanged;
 	}
 }

ICSharpCode.Decompiler/IL/ILInstructionExtensions.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Text;
 
@@ -20,7 +21,7 @@ namespace ICSharpCode.Decompiler.IL
 			return target;
 		}
 
-		public static ILInstruction GetNextSibling(this ILInstruction instruction)
+		public static ILInstruction? GetNextSibling(this ILInstruction? instruction)
 		{
 			if (instruction?.Parent == null)
 				return null;

ICSharpCode.Decompiler/IL/ILTypeExtensions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -145,12 +146,12 @@ namespace ICSharpCode.Decompiler.IL
 		/// 
 		/// Returns SpecialType.UnknownType for unsupported instructions.
 		/// </summary>
-		public static IType InferType(this ILInstruction inst, ICompilation compilation)
+		public static IType InferType(this ILInstruction inst, ICompilation? compilation)
 		{
 			switch (inst)
 			{
 				case NewObj newObj:
-					return newObj.Method.DeclaringType;
+					return newObj.Method.DeclaringType ?? SpecialType.UnknownType;
 				case NewArr newArr:
 					if (compilation != null)
 						return new ArrayType(compilation, newArr.Type, newArr.Indices.Count);

ICSharpCode.Decompiler/IL/ILVariable.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -19,7 +20,6 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
-using System.Linq;
 
 using ICSharpCode.Decompiler.TypeSystem;
 
@@ -192,7 +192,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		public string Name { get; set; }
+		public string? Name { get; set; }
 
 		public bool HasGeneratedName { get; set; }
 
@@ -202,7 +202,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <remarks>
 		/// This property is set automatically when the variable is added to the <c>ILFunction.Variables</c> collection.
 		/// </remarks>
-		public ILFunction Function { get; internal set; }
+		public ILFunction? Function { get; internal set; }
 
 		/// <summary>
 		/// Gets the block container in which this variable is captured.
@@ -212,7 +212,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <remarks>
 		/// This property returns null for variables that are not captured.
 		/// </remarks>
-		public BlockContainer CaptureScope { get; internal set; }
+		public BlockContainer? CaptureScope { get; internal set; }
 
 		/// <summary>
 		/// Gets the index of this variable within the <c>Function.Variables</c> collection.
@@ -309,7 +309,7 @@ namespace ICSharpCode.Decompiler.IL
 			return list.Count - 1;
 		}
 
-		void RemoveInstruction<T>(List<T> list, int index, T inst) where T : class, IInstructionWithVariableOperand
+		void RemoveInstruction<T>(List<T> list, int index, T? inst) where T : class, IInstructionWithVariableOperand
 		{
 			Debug.Assert(list[index] == inst);
 			int indexToMove = list.Count - 1;
@@ -370,7 +370,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// The field which was converted to a local variable.
 		/// Set when the variable is from a 'yield return' or 'async' state machine.
 		/// </summary>
-		public IField StateMachineField;
+		public IField? StateMachineField;
 
 		public ILVariable(VariableKind kind, IType type, int? index = null)
 		{
@@ -398,7 +398,7 @@ namespace ICSharpCode.Decompiler.IL
 			CheckInvariant();
 		}
 
-		public override string ToString()
+		public override string? ToString()
 		{
 			return Name;
 		}
@@ -472,7 +472,7 @@ namespace ICSharpCode.Decompiler.IL
 			if (CaptureScope != null)
 			{
 				output.Write(" captured in ");
-				output.WriteLocalReference(CaptureScope.EntryPoint.Label, CaptureScope);
+				output.WriteLocalReference(CaptureScope.EntryPoint?.Label, CaptureScope);
 			}
 			if (StateMachineField != null)
 			{
@@ -528,7 +528,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public static readonly ILVariableEqualityComparer Instance = new ILVariableEqualityComparer();
 
-		public bool Equals(ILVariable x, ILVariable y)
+		public bool Equals(ILVariable? x, ILVariable? y)
 		{
 			if (x == y)
 				return true;

ICSharpCode.Decompiler/IL/InstructionFlags.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/InstructionOutputExtensions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/Await.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 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
@@ -22,7 +23,7 @@ namespace ICSharpCode.Decompiler.IL
 {
 	partial class Await
 	{
-		public IMethod GetAwaiterMethod;
+		public IMethod? GetAwaiterMethod;
-		public IMethod GetResultMethod;
+		public IMethod? GetResultMethod;
 	}
 }

ICSharpCode.Decompiler/IL/Instructions/BinaryNumericInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -214,5 +215,3 @@ namespace ICSharpCode.Decompiler.IL
 		}
 	}
 }
-
-

ICSharpCode.Decompiler/IL/Instructions/Block.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014-2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014-2016 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
@@ -16,14 +17,13 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.TypeSystem;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	/// <summary>
@@ -48,7 +48,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		public readonly BlockKind Kind;
 		public readonly InstructionCollection<ILInstruction> Instructions;
-		ILInstruction finalInstruction;
+		ILInstruction finalInstruction = null!;
 
 		/// <summary>
 		/// For blocks in a block container, this field holds
@@ -125,7 +125,7 @@ namespace ICSharpCode.Decompiler.IL
 					{
 						var stloc = inst as StLoc;
 						Debug.Assert(stloc != null, "Instructions in CallWithNamedArgs must be assignments");
-						Debug.Assert(stloc.Variable.Kind == VariableKind.NamedArgument);
+						Debug.Assert(stloc!.Variable.Kind == VariableKind.NamedArgument);
 						Debug.Assert(stloc.Variable.IsSingleDefinition && stloc.Variable.LoadCount == 1);
 						Debug.Assert(stloc.Variable.LoadInstructions.Single().Parent == finalInstruction);
 					}
@@ -140,22 +140,22 @@ namespace ICSharpCode.Decompiler.IL
 				case BlockKind.ArrayInitializer:
 					var final = finalInstruction as LdLoc;
 					Debug.Assert(final != null && final.Variable.IsSingleDefinition && final.Variable.Kind == VariableKind.InitializerTarget);
-					IType type = null;
+					IType? type = null;
-					Debug.Assert(Instructions[0].MatchStLoc(final.Variable, out var init) && init.MatchNewArr(out type));
+					Debug.Assert(Instructions[0].MatchStLoc(final!.Variable, out var init) && init.MatchNewArr(out type));
 					for (int i = 1; i < Instructions.Count; i++)
 					{
-						Debug.Assert(Instructions[i].MatchStObj(out ILInstruction target, out _, out var t) && type != null && type.Equals(t));
+						Debug.Assert(Instructions[i].MatchStObj(out ILInstruction? target, out _, out var t) && type != null && type.Equals(t));
-						Debug.Assert(target.MatchLdElema(out t, out ILInstruction array) && type.Equals(t));
+						Debug.Assert(target.MatchLdElema(out t, out ILInstruction? array) && type!.Equals(t));
-						Debug.Assert(array.MatchLdLoc(out ILVariable v) && v == final.Variable);
+						Debug.Assert(array.MatchLdLoc(out ILVariable? v) && v == final.Variable);
 					}
 					break;
 				case BlockKind.CollectionInitializer:
 				case BlockKind.ObjectInitializer:
 					var final2 = finalInstruction as LdLoc;
 					Debug.Assert(final2 != null);
-					var initVar2 = final2.Variable;
+					var initVar2 = final2!.Variable;
 					Debug.Assert(initVar2.StoreCount == 1 && initVar2.Kind == VariableKind.InitializerTarget);
-					IType type2 = null;
+					IType? type2 = null;
 					bool condition = Instructions[0].MatchStLoc(final2.Variable, out var init2);
 					Debug.Assert(condition);
 					Debug.Assert(init2 is NewObj
@@ -300,7 +300,7 @@ namespace ICSharpCode.Decompiler.IL
 		public void Remove()
 		{
 			Debug.Assert(ChildIndex > 0);
-			var container = (BlockContainer)Parent;
+			var container = (BlockContainer)Parent!;
 			Debug.Assert(container.Blocks[ChildIndex] == this);
 			container.Blocks.SwapRemoveAt(ChildIndex);
 		}
@@ -325,7 +325,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the predecessor of the given instruction.
 		/// Returns null if inst.Parent is not a block.
 		/// </summary>
-		public static ILInstruction GetPredecessor(ILInstruction inst)
+		public static ILInstruction? GetPredecessor(ILInstruction inst)
 		{
 			if (inst.Parent is Block block && inst.ChildIndex > 0)
 			{
@@ -341,7 +341,8 @@ namespace ICSharpCode.Decompiler.IL
 		/// If inst is a block consisting of a single instruction, returns that instruction.
 		/// Otherwise, returns the input instruction.
 		/// </summary>
-		public static ILInstruction Unwrap(ILInstruction inst)
+		[return: NotNullIfNotNull("inst")]
+		public static ILInstruction? Unwrap(ILInstruction? inst)
 		{
 			if (inst is Block block)
 			{
@@ -355,19 +356,19 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the closest parent Block.
 		/// Returns null, if the instruction is not a descendant of a Block.
 		/// </summary>
-		public static Block FindClosestBlock(ILInstruction inst)
+		public static Block? FindClosestBlock(ILInstruction? inst)
 		{
 			var curr = inst;
 			while (curr != null)
 			{
-				if (curr is Block)
+				if (curr is Block b)
-					return (Block)curr;
+					return b;
 				curr = curr.Parent;
 			}
 			return null;
 		}
 
-		public bool MatchInlineAssignBlock(out CallInstruction call, out ILInstruction value)
+		public bool MatchInlineAssignBlock([NotNullWhen(true)] out CallInstruction? call, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			call = null;
 			value = null;
@@ -385,7 +386,7 @@ namespace ICSharpCode.Decompiler.IL
 			return this.FinalInstruction.MatchLdLoc(tmp);
 		}
 
-		public bool MatchIfAtEndOfBlock(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfAtEndOfBlock([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
 			condition = null;
 			trueInst = null;

ICSharpCode.Decompiler/IL/Instructions/BlockContainer.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -19,11 +20,11 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.Util;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	/// <summary>
@@ -57,13 +58,18 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		Block entryPoint;
+		Block? entryPoint;
 
 		/// <summary>
 		/// Gets the container's entry point. This is the first block in the Blocks collection.
 		/// </summary>
 		public Block EntryPoint {
-			get { return entryPoint; }
+			get {
+				// HACK: While it's possible to have BlockContainers without entry point,
+				// normally every container must have an entry point according to its invariant.
+				// Thus it's easier on the transforms if this property returns a non-nullable EntryPoint.
+				return entryPoint!;
+			}
 			private set {
 				if (entryPoint != null && IsConnected)
 					entryPoint.IncomingEdgeCount--;
@@ -102,7 +108,7 @@ namespace ICSharpCode.Decompiler.IL
 		protected internal override void InstructionCollectionUpdateComplete()
 		{
 			base.InstructionCollectionUpdateComplete();
-			this.EntryPoint = this.Blocks.FirstOrDefault();
+			this.EntryPoint = this.Blocks.FirstOrDefault()!;
 		}
 
 		protected override void Connected()
@@ -174,7 +180,7 @@ namespace ICSharpCode.Decompiler.IL
 			return Blocks[index];
 		}
 
-		protected override void SetChild(int index, ILInstruction value)
+		protected override void SetChild(int index, ILInstruction? value)
 		{
 			if (Blocks[index] != value)
 				throw new InvalidOperationException("Cannot replace blocks in BlockContainer");
@@ -189,12 +195,12 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			base.CheckInvariant(phase);
 			Debug.Assert(Blocks.Count > 0 && EntryPoint == Blocks[0]);
-			Debug.Assert(!IsConnected || EntryPoint?.IncomingEdgeCount >= 1);
+			Debug.Assert(!IsConnected || EntryPoint.IncomingEdgeCount >= 1);
-			Debug.Assert(EntryPoint == null || Parent is ILFunction || !ILRangeIsEmpty);
+			Debug.Assert(Parent is ILFunction || !ILRangeIsEmpty);
 			Debug.Assert(Blocks.All(b => b.HasFlag(InstructionFlags.EndPointUnreachable)));
 			Debug.Assert(Blocks.All(b => b.Kind == BlockKind.ControlFlow)); // this also implies that the blocks don't use FinalInstruction
 			Debug.Assert(TopologicalSort(deleteUnreachableBlocks: true).Count == Blocks.Count, "Container should not have any unreachable blocks");
-			Block bodyStartBlock;
+			Block? bodyStartBlock;
 			switch (Kind)
 			{
 				case ContainerKind.Normal:
@@ -319,7 +325,7 @@ namespace ICSharpCode.Decompiler.IL
 			Blocks.ReplaceList(newOrder);
 		}
 
-		public static BlockContainer FindClosestContainer(ILInstruction inst)
+		public static BlockContainer? FindClosestContainer(ILInstruction? inst)
 		{
 			while (inst != null)
 			{
@@ -330,18 +336,18 @@ namespace ICSharpCode.Decompiler.IL
 			return null;
 		}
 
-		public static BlockContainer FindClosestSwitchContainer(ILInstruction inst)
+		public static BlockContainer? FindClosestSwitchContainer(ILInstruction? inst)
 		{
 			while (inst != null)
 			{
-				if (inst is BlockContainer bc && bc.entryPoint.Instructions.FirstOrDefault() is SwitchInstruction)
+				if (inst is BlockContainer { Kind: ContainerKind.Switch } bc)
 					return bc;
 				inst = inst.Parent;
 			}
 			return null;
 		}
 
-		public bool MatchConditionBlock(Block block, out ILInstruction condition, out Block bodyStartBlock)
+		public bool MatchConditionBlock(Block block, [NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out Block? bodyStartBlock)
 		{
 			condition = null;
 			bodyStartBlock = null;

ICSharpCode.Decompiler/IL/Instructions/Branch.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -30,7 +31,7 @@ namespace ICSharpCode.Decompiler.IL
 	partial class Branch : SimpleInstruction, IBranchOrLeaveInstruction
 	{
 		readonly int targetILOffset;
-		Block targetBlock;
+		Block? targetBlock;
 
 		public Branch(int targetILOffset) : base(OpCode.Branch)
 		{
@@ -48,7 +49,11 @@ namespace ICSharpCode.Decompiler.IL
 		}
 
 		public Block TargetBlock {
-			get { return targetBlock; }
+			get {
+				// HACK: We treat TargetBlock as non-nullable publicly, because it's only null inside
+				// the ILReader, and becomes non-null once the BlockBuilder has run.
+				return targetBlock!;
+			}
 			set {
 				if (targetBlock != null && IsConnected)
 					targetBlock.IncomingEdgeCount--;
@@ -62,7 +67,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the BlockContainer that contains the target block.
 		/// </summary>
 		public BlockContainer TargetContainer {
-			get { return (BlockContainer)targetBlock?.Parent; }
+			get { return (BlockContainer)targetBlock?.Parent!; }
 		}
 
 		protected override void Connected()
@@ -92,9 +97,9 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		internal static bool GetExecutesFinallyBlock(ILInstruction inst, BlockContainer container)
+		internal static bool GetExecutesFinallyBlock(ILInstruction? inst, BlockContainer? container)
 		{
-			for (; inst != container; inst = inst.Parent)
+			for (; inst != container && inst != null; inst = inst.Parent)
 			{
 				if (inst.Parent is TryFinally && inst.SlotInfo == TryFinally.TryBlockSlot)
 					return true;
@@ -107,9 +112,9 @@ namespace ICSharpCode.Decompiler.IL
 			base.CheckInvariant(phase);
 			if (phase > ILPhase.InILReader)
 			{
-				Debug.Assert(targetBlock.Parent is BlockContainer);
+				Debug.Assert(targetBlock?.Parent is BlockContainer);
-				Debug.Assert(this.IsDescendantOf(targetBlock.Parent));
+				Debug.Assert(this.IsDescendantOf(targetBlock!.Parent!));
-				Debug.Assert(targetBlock.Parent.Children[targetBlock.ChildIndex] == targetBlock);
+				Debug.Assert(targetBlock!.Parent!.Children[targetBlock.ChildIndex] == targetBlock);
 			}
 		}
 
@@ -118,7 +123,7 @@ namespace ICSharpCode.Decompiler.IL
 			WriteILRange(output, options);
 			output.Write(OpCode);
 			output.Write(' ');
-			output.WriteLocalReference(TargetLabel, (object)targetBlock ?? TargetILOffset);
+			output.WriteLocalReference(TargetLabel, (object?)targetBlock ?? TargetILOffset);
 		}
 	}
 

ICSharpCode.Decompiler/IL/Instructions/CallIndirect.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 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
@@ -31,7 +32,7 @@ namespace ICSharpCode.Decompiler.IL
 		public static readonly SlotInfo FunctionPointerSlot = new SlotInfo("FunctionPointer", canInlineInto: true);
 		public static readonly SlotInfo ArgumentSlot = new SlotInfo("Argument", canInlineInto: true, isCollection: true);
 
-		ILInstruction functionPointer;
+		ILInstruction functionPointer = null!;
 		public readonly InstructionCollection<ILInstruction> Arguments;
 		public bool IsInstance { get; }
 		public bool HasExplicitThis { get; }

ICSharpCode.Decompiler/IL/Instructions/CallInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -19,7 +20,6 @@
 using System;
 using System.Diagnostics;
 
-using ICSharpCode.Decompiler.Disassembler;
 using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.IL
@@ -52,7 +52,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets/Sets the type specified in the 'constrained.' prefix.
 		/// Returns null if no 'constrained.' prefix exists for this call.
 		/// </summary>
-		public IType ConstrainedTo;
+		public IType? ConstrainedTo;
 
 		/// <summary>
 		/// Gets whether the IL stack was empty at the point of this call.
@@ -62,8 +62,7 @@ namespace ICSharpCode.Decompiler.IL
 
 		protected CallInstruction(OpCode opCode, IMethod method) : base(opCode)
 		{
-			Debug.Assert(method != null);
+			this.Method = method ?? throw new ArgumentNullException(nameof(method));
-			this.Method = method;
 			this.Arguments = new InstructionCollection<ILInstruction>(this, 0);
 		}
 
@@ -78,7 +77,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the parameter for the argument with the specified index.
 		/// Returns null for the <c>this</c> parameter.
 		/// </summary>
-		public IParameter GetParameter(int argumentIndex)
+		public IParameter? GetParameter(int argumentIndex)
 		{
 			int firstParamIndex = (Method.IsStatic || OpCode == OpCode.NewObj) ? 0 : 1;
 			if (argumentIndex < firstParamIndex)
@@ -160,9 +159,9 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		protected internal sealed override bool PerformMatch(ILInstruction other, ref Patterns.Match match)
+		protected internal sealed override bool PerformMatch(ILInstruction? other, ref Patterns.Match match)
 		{
-			CallInstruction o = other as CallInstruction;
+			CallInstruction? o = other as CallInstruction;
 			return o != null && this.OpCode == o.OpCode && this.Method.Equals(o.Method) && this.IsTail == o.IsTail
 				&& object.Equals(this.ConstrainedTo, o.ConstrainedTo)
 				&& Patterns.ListMatch.DoMatch(this.Arguments, o.Arguments, ref match);

ICSharpCode.Decompiler/IL/Instructions/Comp.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -21,7 +22,6 @@ using System.Diagnostics;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.TypeSystem;
-using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.IL
 {
@@ -231,5 +231,3 @@ namespace ICSharpCode.Decompiler.IL
 		}
 	}
 }
-
-

ICSharpCode.Decompiler/IL/Instructions/CompoundAssignmentInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2016 Siegfried Pammer
 // 
 // 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
@@ -172,7 +173,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Gets whether the specific binary instruction is compatible with a compound operation on the specified type.
 		/// </summary>
-		internal static bool IsBinaryCompatibleWithType(BinaryNumericInstruction binary, IType type, DecompilerSettings settings)
+		internal static bool IsBinaryCompatibleWithType(BinaryNumericInstruction binary, IType type, DecompilerSettings? settings)
 		{
 			if (binary.IsLifted)
 			{

ICSharpCode.Decompiler/IL/Instructions/Conv.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/DeconstructResultInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2020 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2020 Siegfried Pammer
 // 
 // 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
@@ -47,9 +48,9 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		MatchInstruction FindMatch()
+		MatchInstruction? FindMatch()
 		{
-			for (ILInstruction inst = this; inst != null; inst = inst.Parent)
+			for (ILInstruction? inst = this; inst != null; inst = inst.Parent)
 			{
 				if (inst.Parent is MatchInstruction match && inst != match.TestedOperand)
 					return match;
@@ -61,7 +62,7 @@ namespace ICSharpCode.Decompiler.IL
 		{
 			var matchInst = FindMatch();
 			Debug.Assert(matchInst != null && (matchInst.IsDeconstructCall || matchInst.IsDeconstructTuple));
-			Debug.Assert(Argument.MatchLdLoc(matchInst.Variable));
+			Debug.Assert(Argument.MatchLdLoc(matchInst!.Variable));
 			var outParamType = matchInst.GetDeconstructResultType(this.Index);
 			Debug.Assert(outParamType.GetStackType() == ResultType);
 		}

ICSharpCode.Decompiler/IL/Instructions/DefaultValue.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // 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

ICSharpCode.Decompiler/IL/Instructions/DynamicInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2018 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2018 Siegfried Pammer
 // 
 // 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
@@ -56,7 +57,7 @@ namespace ICSharpCode.Decompiler.IL
 
 	public struct CSharpArgumentInfo
 	{
-		public string Name { get; set; }
+		public string? Name { get; set; }
 		public CSharpArgumentInfoFlags Flags { get; set; }
 		public IType CompileTimeType { get; set; }
 
@@ -66,9 +67,9 @@ namespace ICSharpCode.Decompiler.IL
 	partial class DynamicInstruction
 	{
 		public CSharpBinderFlags BinderFlags { get; }
-		public IType CallingContext { get; }
+		public IType? CallingContext { get; }
 
-		protected DynamicInstruction(OpCode opCode, CSharpBinderFlags binderFlags, IType context)
+		protected DynamicInstruction(OpCode opCode, CSharpBinderFlags binderFlags, IType? context)
 			: base(opCode)
 		{
 			BinderFlags = binderFlags;
@@ -141,7 +142,7 @@ namespace ICSharpCode.Decompiler.IL
 			output.Write(')');
 		}
 
-		public DynamicConvertInstruction(CSharpBinderFlags binderFlags, IType type, IType context, ILInstruction argument)
+		public DynamicConvertInstruction(CSharpBinderFlags binderFlags, IType type, IType? context, ILInstruction argument)
 			: base(OpCode.DynamicConvertInstruction, binderFlags, context)
 		{
 			Type = type;
@@ -171,7 +172,7 @@ namespace ICSharpCode.Decompiler.IL
 		public IReadOnlyList<IType> TypeArguments { get; }
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeMemberInstruction(CSharpBinderFlags binderFlags, string name, IType[] typeArguments, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeMemberInstruction(CSharpBinderFlags binderFlags, string name, IType[]? typeArguments, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -216,10 +217,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicGetMemberInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 		public CSharpArgumentInfo TargetArgumentInfo { get; }
 
-		public DynamicGetMemberInstruction(CSharpBinderFlags binderFlags, string name, IType context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target)
+		public DynamicGetMemberInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target)
 			: base(OpCode.DynamicGetMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -249,11 +250,11 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicSetMemberInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 		public CSharpArgumentInfo TargetArgumentInfo { get; }
 		public CSharpArgumentInfo ValueArgumentInfo { get; }
 
-		public DynamicSetMemberInstruction(CSharpBinderFlags binderFlags, string name, IType context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target, CSharpArgumentInfo valueArgumentInfo, ILInstruction value)
+		public DynamicSetMemberInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, CSharpArgumentInfo targetArgumentInfo, ILInstruction target, CSharpArgumentInfo valueArgumentInfo, ILInstruction value)
 			: base(OpCode.DynamicSetMemberInstruction, binderFlags, context)
 		{
 			Name = name;
@@ -293,7 +294,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicGetIndexInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicGetIndexInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicGetIndexInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -325,7 +326,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicSetIndexInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicSetIndexInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicSetIndexInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -355,11 +356,11 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicInvokeConstructorInstruction
 	{
-		readonly IType resultType;
+		readonly IType? resultType;
 
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeConstructorInstruction(CSharpBinderFlags binderFlags, IType type, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeConstructorInstruction(CSharpBinderFlags binderFlags, IType? type, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeConstructorInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -395,7 +396,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo RightArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicBinaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
+		public DynamicBinaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
 			: base(OpCode.DynamicBinaryOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -437,7 +438,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo RightArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicLogicOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
+		public DynamicLogicOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo leftArgumentInfo, ILInstruction left, CSharpArgumentInfo rightArgumentInfo, ILInstruction right)
 			: base(OpCode.DynamicLogicOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -486,7 +487,7 @@ namespace ICSharpCode.Decompiler.IL
 		public CSharpArgumentInfo OperandArgumentInfo { get; }
 		public ExpressionType Operation { get; }
 
-		public DynamicUnaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType context, CSharpArgumentInfo operandArgumentInfo, ILInstruction operand)
+		public DynamicUnaryOperatorInstruction(CSharpBinderFlags binderFlags, ExpressionType operation, IType? context, CSharpArgumentInfo operandArgumentInfo, ILInstruction operand)
 			: base(OpCode.DynamicUnaryOperatorInstruction, binderFlags, context)
 		{
 			Operation = operation;
@@ -533,7 +534,7 @@ namespace ICSharpCode.Decompiler.IL
 	{
 		public IReadOnlyList<CSharpArgumentInfo> ArgumentInfo { get; }
 
-		public DynamicInvokeInstruction(CSharpBinderFlags binderFlags, IType context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
+		public DynamicInvokeInstruction(CSharpBinderFlags binderFlags, IType? context, CSharpArgumentInfo[] argumentInfo, ILInstruction[] arguments)
 			: base(OpCode.DynamicInvokeInstruction, binderFlags, context)
 		{
 			ArgumentInfo = argumentInfo;
@@ -562,9 +563,9 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class DynamicIsEventInstruction
 	{
-		public string Name { get; }
+		public string? Name { get; }
 
-		public DynamicIsEventInstruction(CSharpBinderFlags binderFlags, string name, IType context, ILInstruction argument)
+		public DynamicIsEventInstruction(CSharpBinderFlags binderFlags, string? name, IType? context, ILInstruction argument)
 			: base(OpCode.DynamicIsEventInstruction, binderFlags, context)
 		{
 			Name = name;

ICSharpCode.Decompiler/IL/Instructions/ExpressionTreeCast.cs

@@ -1,4 +1,5 @@
-using System;
+#nullable enable
+using System;
 using System.Collections.Generic;
 using System.Text;
 

ICSharpCode.Decompiler/IL/Instructions/ILFunction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -18,7 +19,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.Immutable;
 using System.Diagnostics;
 using System.Linq;
 
@@ -35,7 +35,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// May be null for functions that were not constructed from metadata,
 		/// e.g., expression trees.
 		/// </summary>
-		public readonly IMethod Method;
+		public readonly IMethod? Method;
 
 		/// <summary>
 		/// Gets the generic context of this function.
@@ -54,7 +54,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// has no effect, as the name should not be used in the final AST construction.
 		/// </para>
 		/// </summary>
-		public string Name;
+		public string? Name;
 
 		/// <summary>
 		/// Size of the IL code in this function.
@@ -71,7 +71,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Gets the scope in which the local function is declared.
 		/// Returns null, if this is not a local function.
 		/// </summary>
-		public BlockContainer DeclarationScope { get; internal set; }
+		public BlockContainer? DeclarationScope { get; internal set; }
 
 		/// <summary>
 		/// Gets the set of captured variables by this ILFunction.
@@ -108,17 +108,17 @@ namespace ICSharpCode.Decompiler.IL
 		/// Return element type -- if the async method returns Task{T}, this field stores T.
 		/// If the async method returns Task or void, this field stores void.
 		/// </summary>
-		public IType AsyncReturnType;
+		public IType? AsyncReturnType;
 
 		/// <summary>
 		/// If this function is an iterator/async, this field stores the compiler-generated MoveNext() method.
 		/// </summary>
-		public IMethod MoveNextMethod;
+		public IMethod? MoveNextMethod;
 
 		/// <summary>
 		/// If this function is a local function, this field stores the reduced version of the function.
 		/// </summary>
-		internal TypeSystem.Implementation.LocalFunctionMethod ReducedMethod;
+		internal TypeSystem.Implementation.LocalFunctionMethod? ReducedMethod;
 
 		public DebugInfo.AsyncDebugInfo AsyncDebugInfo;
 
@@ -131,8 +131,10 @@ namespace ICSharpCode.Decompiler.IL
 			get {
 				if (ctorCallStart == int.MinValue)
 				{
-					if (!this.Method.IsConstructor || this.Method.IsStatic)
+					if (this.Method == null || !this.Method.IsConstructor || this.Method.IsStatic)
+					{
 						ctorCallStart = -1;
+					}
 					else
 					{
 						ctorCallStart = this.Descendants.FirstOrDefault(d => d is CallInstruction call && !(call is NewObj)
@@ -150,7 +152,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// T is the delegate type that matches the signature of this method.
 		/// Otherwise this must be null.
 		/// </summary>
-		public IType DelegateType;
+		public IType? DelegateType;
 
 		ILFunctionKind kind;
 
@@ -181,7 +183,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// where the stack is empty, nop instructions, and the instruction following
 		/// a call instruction
 		/// </summary>
-		public List<int> SequencePointCandidates { get; set; }
+		public List<int>? SequencePointCandidates { get; set; }
 
 		/// <summary>
 		/// Constructs a new ILFunction from the given metadata and with the given ILAst body.
@@ -408,18 +410,18 @@ namespace ICSharpCode.Decompiler.IL
 
 		int helperVariableCount;
 
-		public ILVariable RegisterVariable(VariableKind kind, IType type, string name = null)
+		public ILVariable RegisterVariable(VariableKind kind, IType type, string? name = null)
 		{
 			return RegisterVariable(kind, type, type.GetStackType(), name);
 		}
 
-		public ILVariable RegisterVariable(VariableKind kind, StackType stackType, string name = null)
+		public ILVariable RegisterVariable(VariableKind kind, StackType stackType, string? name = null)
 		{
 			var type = Method.Compilation.FindType(stackType.ToKnownTypeCode());
 			return RegisterVariable(kind, type, stackType, name);
 		}
 
-		ILVariable RegisterVariable(VariableKind kind, IType type, StackType stackType, string name = null)
+		ILVariable RegisterVariable(VariableKind kind, IType type, StackType stackType, string? name = null)
 		{
 			var variable = new ILVariable(kind, type, stackType);
 			if (string.IsNullOrWhiteSpace(name))

ICSharpCode.Decompiler/IL/Instructions/ILInstruction.cs

@@ -60,7 +60,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.OpCode = opCode;
 		}
 
-		protected void ValidateChild(ILInstruction inst)
+		protected void ValidateChild(ILInstruction? inst)
 		{
 			if (inst == null)
 				throw new ArgumentNullException(nameof(inst));

ICSharpCode.Decompiler/IL/Instructions/ILVariableCollection.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 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
@@ -128,7 +129,7 @@ namespace ICSharpCode.Decompiler.IL
 				{
 					// However, remove unused this-parameters of delegates, expression trees, etc.
 					// These will be replaced with the top-level function's this-parameter.
-					if (v.Index == -1 && v.Function.Kind != ILFunctionKind.TopLevelFunction)
+					if (v.Index == -1 && v.Function!.Kind != ILFunctionKind.TopLevelFunction)
 						return true;
 					return false;
 				}

ICSharpCode.Decompiler/IL/Instructions/IfInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -35,7 +36,7 @@ namespace ICSharpCode.Decompiler.IL
 	/// </remarks>
 	partial class IfInstruction : ILInstruction
 	{
-		public IfInstruction(ILInstruction condition, ILInstruction trueInst, ILInstruction falseInst = null) : base(OpCode.IfInstruction)
+		public IfInstruction(ILInstruction condition, ILInstruction trueInst, ILInstruction? falseInst = null) : base(OpCode.IfInstruction)
 		{
 			this.Condition = condition;
 			this.TrueInst = trueInst;
@@ -47,7 +48,7 @@ namespace ICSharpCode.Decompiler.IL
 			return new IfInstruction(lhs, rhs, new LdcI4(0));
 		}
 
-		public static IfInstruction LogicOr(ILInstruction lhs, ILInstruction rhs)
+		public static IfInstruction LogicOr(ILInstruction lhs, ILInstruction? rhs)
 		{
 			return new IfInstruction(lhs, new LdcI4(1), rhs);
 		}
@@ -126,7 +127,7 @@ namespace ICSharpCode.Decompiler.IL
 			if (slot == IfInstruction.ConditionSlot)
 				return true;
 			if (slot == IfInstruction.TrueInstSlot || slot == IfInstruction.FalseInstSlot || slot == NullCoalescingInstruction.FallbackInstSlot)
-				return IsInConditionSlot(inst.Parent);
+				return IsInConditionSlot(inst.Parent!);
 			if (inst.Parent is Comp comp)
 			{
 				if (comp.Left == inst && comp.Right.MatchLdcI4(0))

ICSharpCode.Decompiler/IL/Instructions/LdFlda.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/LdLen.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/Leave.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -32,9 +33,9 @@ namespace ICSharpCode.Decompiler.IL
 	/// </remarks>
 	partial class Leave : ILInstruction, IBranchOrLeaveInstruction
 	{
-		BlockContainer targetContainer;
+		BlockContainer? targetContainer;
 
-		public Leave(BlockContainer targetContainer, ILInstruction value = null) : base(OpCode.Leave)
+		public Leave(BlockContainer? targetContainer, ILInstruction? value = null) : base(OpCode.Leave)
 		{
 			// Note: ILReader will create Leave instructions with targetContainer==null to represent 'endfinally',
 			// the targetContainer will then be filled in by BlockBuilder
@@ -54,7 +55,7 @@ namespace ICSharpCode.Decompiler.IL
 		}
 
 		public BlockContainer TargetContainer {
-			get { return targetContainer; }
+			get { return targetContainer!; }
 			set {
 				if (targetContainer != null && IsConnected)
 					targetContainer.LeaveCount--;
@@ -109,8 +110,8 @@ namespace ICSharpCode.Decompiler.IL
 		internal override void CheckInvariant(ILPhase phase)
 		{
 			base.CheckInvariant(phase);
-			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(targetContainer));
+			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(targetContainer!));
-			Debug.Assert(phase <= ILPhase.InILReader || phase == ILPhase.InAsyncAwait || value.ResultType == targetContainer.ResultType);
+			Debug.Assert(phase <= ILPhase.InILReader || phase == ILPhase.InAsyncAwait || value.ResultType == targetContainer!.ResultType);
 		}
 
 		public override void WriteTo(ITextOutput output, ILAstWritingOptions options)

ICSharpCode.Decompiler/IL/Instructions/LockInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // 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

ICSharpCode.Decompiler/IL/Instructions/LogicInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2017 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

ICSharpCode.Decompiler/IL/Instructions/MatchInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2020 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2020 Siegfried Pammer
 // 
 // 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
@@ -18,10 +19,9 @@
 
 using System;
 using System.Diagnostics;
-using System.Linq;
+using System.Diagnostics.CodeAnalysis;
 
 using ICSharpCode.Decompiler.TypeSystem;
-
 namespace ICSharpCode.Decompiler.IL
 {
 	partial class MatchInstruction : ILInstruction
@@ -104,7 +104,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		public MatchInstruction(ILVariable variable, ILInstruction testedOperand)
+		public MatchInstruction(ILVariable variable, ILInstruction? testedOperand)
 			: this(variable, method: null, testedOperand)
 		{
 		}
@@ -118,7 +118,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// (even if the pattern fails to match!).
 		/// The pattern matching instruction evaluates to 1 (as I4) if the pattern matches, or 0 otherwise.
 		/// </summary>
-		public static bool IsPatternMatch(ILInstruction inst, out ILInstruction testedOperand)
+		public static bool IsPatternMatch(ILInstruction? inst, [NotNullWhen(true)] out ILInstruction? testedOperand)
 		{
 			switch (inst)
 			{
@@ -185,8 +185,8 @@ namespace ICSharpCode.Decompiler.IL
 			Debug.Assert(SubPatterns.Count >= NumPositionalPatterns);
 			foreach (var subPattern in SubPatterns)
 			{
-				if (!IsPatternMatch(subPattern, out ILInstruction operand))
+				if (!IsPatternMatch(subPattern, out ILInstruction? operand))
-					Debug.Fail("Sub-Pattern must be a valid pattern");
+					throw new InvalidOperationException("Sub-Pattern must be a valid pattern");
 				// the first child is TestedOperand
 				int subPatternIndex = subPattern.ChildIndex - 1;
 				if (subPatternIndex < NumPositionalPatterns)

ICSharpCode.Decompiler/IL/Instructions/MemoryInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/NullCoalescingInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // 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

ICSharpCode.Decompiler/IL/Instructions/NullableInstructions.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2018 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2018 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

ICSharpCode.Decompiler/IL/Instructions/PatternMatching.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -16,8 +17,9 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using ICSharpCode.Decompiler.TypeSystem;
+using System.Diagnostics.CodeAnalysis;
 
+using ICSharpCode.Decompiler.TypeSystem;
 namespace ICSharpCode.Decompiler.IL
 {
 	partial class ILInstruction
@@ -73,13 +75,13 @@ namespace ICSharpCode.Decompiler.IL
 			return MatchLdcI(out long v) && v == val;
 		}
 
-		public bool MatchLdLoc(ILVariable variable)
+		public bool MatchLdLoc(ILVariable? variable)
 		{
 			var inst = this as LdLoc;
 			return inst != null && inst.Variable == variable;
 		}
 
-		public bool MatchLdLoca(ILVariable variable)
+		public bool MatchLdLoca(ILVariable? variable)
 		{
 			var inst = this as LdLoca;
 			return inst != null && inst.Variable == variable;
@@ -88,7 +90,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches either ldloc (if the variable is a reference type), or ldloca (otherwise).
 		/// </summary>
-		public bool MatchLdLocRef(ILVariable variable)
+		public bool MatchLdLocRef(ILVariable? variable)
 		{
 			return MatchLdLocRef(out var v) && v == variable;
 		}
@@ -96,7 +98,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches either ldloc (if the variable is a reference type), or ldloca (otherwise).
 		/// </summary>
-		public bool MatchLdLocRef(out ILVariable variable)
+		public bool MatchLdLocRef([NotNullWhen(true)] out ILVariable? variable)
 		{
 			switch (this)
 			{
@@ -118,7 +120,7 @@ namespace ICSharpCode.Decompiler.IL
 			return inst != null && inst.Variable.Kind == VariableKind.Parameter && inst.Variable.Index < 0;
 		}
 
-		public bool MatchStLoc(out ILVariable variable)
+		public bool MatchStLoc([NotNullWhen(true)] out ILVariable? variable)
 		{
 			var inst = this as StLoc;
 			if (inst != null)
@@ -130,7 +132,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchStLoc(ILVariable variable, out ILInstruction value)
+		public bool MatchStLoc(ILVariable? variable, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as StLoc;
 			if (inst != null && inst.Variable == variable)
@@ -142,7 +144,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdLen(StackType type, out ILInstruction array)
+		public bool MatchLdLen(StackType type, [NotNullWhen(true)] out ILInstruction? array)
 		{
 			var inst = this as LdLen;
 			if (inst != null && inst.ResultType == type)
@@ -154,7 +156,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchReturn(out ILInstruction value)
+		public bool MatchReturn([NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null && inst.IsLeavingFunction)
@@ -166,7 +168,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBranch(out Block targetBlock)
+		public bool MatchBranch([NotNullWhen(true)] out Block? targetBlock)
 		{
 			var inst = this as Branch;
 			if (inst != null)
@@ -178,13 +180,13 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBranch(Block targetBlock)
+		public bool MatchBranch(Block? targetBlock)
 		{
 			var inst = this as Branch;
 			return inst != null && inst.TargetBlock == targetBlock;
 		}
 
-		public bool MatchLeave(out BlockContainer targetContainer, out ILInstruction value)
+		public bool MatchLeave([NotNullWhen(true)] out BlockContainer? targetContainer, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null)
@@ -198,7 +200,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(BlockContainer targetContainer, out ILInstruction value)
+		public bool MatchLeave(BlockContainer? targetContainer, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			var inst = this as Leave;
 			if (inst != null && targetContainer == inst.TargetContainer)
@@ -210,7 +212,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(out BlockContainer targetContainer)
+		public bool MatchLeave([NotNullWhen(true)] out BlockContainer? targetContainer)
 		{
 			var inst = this as Leave;
 			if (inst != null && inst.Value.MatchNop())
@@ -222,16 +224,15 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLeave(BlockContainer targetContainer)
+		public bool MatchLeave(BlockContainer? targetContainer)
 		{
 			var inst = this as Leave;
 			return inst != null && inst.TargetContainer == targetContainer && inst.Value.MatchNop();
 		}
 
-		public bool MatchIfInstruction(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfInstruction([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
-			var inst = this as IfInstruction;
+			if (this is IfInstruction inst)
-			if (inst != null)
 			{
 				condition = inst.Condition;
 				trueInst = inst.TrueInst;
@@ -244,7 +245,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchIfInstructionPositiveCondition(out ILInstruction condition, out ILInstruction trueInst, out ILInstruction falseInst)
+		public bool MatchIfInstructionPositiveCondition([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst, [NotNullWhen(true)] out ILInstruction? falseInst)
 		{
 			if (MatchIfInstruction(out condition, out trueInst, out falseInst))
 			{
@@ -252,7 +253,7 @@ namespace ICSharpCode.Decompiler.IL
 				while (condition.MatchLogicNot(out var arg))
 				{
 					condition = arg;
-					ILInstruction tmp = trueInst;
+					ILInstruction? tmp = trueInst;
 					trueInst = falseInst;
 					falseInst = tmp;
 				}
@@ -264,7 +265,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches an if instruction where the false instruction is a nop.
 		/// </summary>
-		public bool MatchIfInstruction(out ILInstruction condition, out ILInstruction trueInst)
+		public bool MatchIfInstruction([NotNullWhen(true)] out ILInstruction? condition, [NotNullWhen(true)] out ILInstruction? trueInst)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.FalseInst.MatchNop())
@@ -283,7 +284,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Note: unlike C# '&amp;&amp;', this instruction is not limited to booleans,
 		/// but allows passing through arbitrary I4 values on the rhs (but not on the lhs).
 		/// </summary>
-		public bool MatchLogicAnd(out ILInstruction lhs, out ILInstruction rhs)
+		public bool MatchLogicAnd([NotNullWhen(true)] out ILInstruction? lhs, [NotNullWhen(true)] out ILInstruction? rhs)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.FalseInst.MatchLdcI4(0))
@@ -302,7 +303,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// Note: unlike C# '||', this instruction is not limited to booleans,
 		/// but allows passing through arbitrary I4 values on the rhs (but not on the lhs).
 		/// </summary>
-		public bool MatchLogicOr(out ILInstruction lhs, out ILInstruction rhs)
+		public bool MatchLogicOr([NotNullWhen(true)] out ILInstruction? lhs, [NotNullWhen(true)] out ILInstruction? rhs)
 		{
 			var inst = this as IfInstruction;
 			if (inst != null && inst.TrueInst.MatchLdcI4(1))
@@ -319,7 +320,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches an logical negation.
 		/// </summary>
-		public bool MatchLogicNot(out ILInstruction arg)
+		public bool MatchLogicNot([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (this is Comp comp && comp.Kind == ComparisonKind.Equality
 				&& comp.LiftingKind == ComparisonLiftingKind.None
@@ -332,7 +333,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchTryCatchHandler(out ILVariable variable)
+		public bool MatchTryCatchHandler([NotNullWhen(true)] out ILVariable? variable)
 		{
 			var inst = this as TryCatchHandler;
 			if (inst != null)
@@ -347,7 +348,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches comp(left == right) or logic.not(comp(left != right)).
 		/// </summary>
-		public bool MatchCompEquals(out ILInstruction left, out ILInstruction right)
+		public bool MatchCompEquals([NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			ILInstruction thisInst = this;
 			var compKind = ComparisonKind.Equality;
@@ -376,7 +377,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches 'comp(arg == ldnull)'
 		/// </summary>
-		public bool MatchCompEqualsNull(out ILInstruction arg)
+		public bool MatchCompEqualsNull([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (!MatchCompEquals(out var left, out var right))
 			{
@@ -403,7 +404,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches 'comp(arg != ldnull)'
 		/// </summary>
-		public bool MatchCompNotEqualsNull(out ILInstruction arg)
+		public bool MatchCompNotEqualsNull([NotNullWhen(true)] out ILInstruction? arg)
 		{
 			if (!MatchCompNotEquals(out var left, out var right))
 			{
@@ -430,7 +431,7 @@ namespace ICSharpCode.Decompiler.IL
 		/// <summary>
 		/// Matches comp(left != right) or logic.not(comp(left == right)).
 		/// </summary>
-		public bool MatchCompNotEquals(out ILInstruction left, out ILInstruction right)
+		public bool MatchCompNotEquals([NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			ILInstruction thisInst = this;
 			var compKind = ComparisonKind.Inequality;
@@ -456,7 +457,7 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
-		public bool MatchLdFld(out ILInstruction target, out IField field)
+		public bool MatchLdFld([NotNullWhen(true)] out ILInstruction? target, [NotNullWhen(true)] out IField? field)
 		{
 			if (this is LdObj ldobj && ldobj.Target is LdFlda ldflda && ldobj.UnalignedPrefix == 0 && !ldobj.IsVolatile)
 			{
@@ -472,7 +473,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdsFld(out IField field)
+		public bool MatchLdsFld([NotNullWhen(true)] out IField? field)
 		{
 			if (this is LdObj ldobj && ldobj.Target is LdsFlda ldsflda && ldobj.UnalignedPrefix == 0 && !ldobj.IsVolatile)
 			{
@@ -483,12 +484,12 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchLdsFld(IField field)
+		public bool MatchLdsFld(IField? field)
 		{
 			return MatchLdsFld(out var f) && f.Equals(field);
 		}
 
-		public bool MatchStsFld(out IField field, out ILInstruction value)
+		public bool MatchStsFld([NotNullWhen(true)] out IField? field, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			if (this is StObj stobj && stobj.Target is LdsFlda ldsflda && stobj.UnalignedPrefix == 0 && !stobj.IsVolatile)
 			{
@@ -501,7 +502,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchStFld(out ILInstruction target, out IField field, out ILInstruction value)
+		public bool MatchStFld([NotNullWhen(true)] out ILInstruction? target, [NotNullWhen(true)] out IField? field, [NotNullWhen(true)] out ILInstruction? value)
 		{
 			if (this is StObj stobj && stobj.Target is LdFlda ldflda && stobj.UnalignedPrefix == 0 && !stobj.IsVolatile)
 			{
@@ -522,7 +523,7 @@ namespace ICSharpCode.Decompiler.IL
 			return op != null && op.Operator == @operator;
 		}
 
-		public bool MatchBinaryNumericInstruction(BinaryNumericOperator @operator, out ILInstruction left, out ILInstruction right)
+		public bool MatchBinaryNumericInstruction(BinaryNumericOperator @operator, [NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			var op = this as BinaryNumericInstruction;
 			if (op != null && op.Operator == @operator)
@@ -536,7 +537,7 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 
-		public bool MatchBinaryNumericInstruction(out BinaryNumericOperator @operator, out ILInstruction left, out ILInstruction right)
+		public bool MatchBinaryNumericInstruction(out BinaryNumericOperator @operator, [NotNullWhen(true)] out ILInstruction? left, [NotNullWhen(true)] out ILInstruction? right)
 		{
 			var op = this as BinaryNumericInstruction;
 			if (op != null)

ICSharpCode.Decompiler/IL/Instructions/SimpleInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -40,7 +41,7 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class Nop
 	{
-		public string Comment;
+		public string? Comment;
 
 		public NopKind Kind;
 
@@ -61,10 +62,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class InvalidBranch : SimpleInstruction
 	{
-		public string Message;
+		public string? Message;
 		public StackType ExpectedResultType = StackType.Void;
 
-		public InvalidBranch(string message) : this()
+		public InvalidBranch(string? message) : this()
 		{
 			this.Message = message;
 		}
@@ -88,10 +89,10 @@ namespace ICSharpCode.Decompiler.IL
 
 	partial class InvalidExpression : SimpleInstruction
 	{
-		public string Message;
+		public string? Message;
 		public StackType ExpectedResultType = StackType.Unknown;
 
-		public InvalidExpression(string message) : this()
+		public InvalidExpression(string? message) : this()
 		{
 			this.Message = message;
 		}

ICSharpCode.Decompiler/IL/Instructions/StLoc.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -38,8 +39,8 @@ namespace ICSharpCode.Decompiler.IL
 		internal override void CheckInvariant(ILPhase phase)
 		{
 			base.CheckInvariant(phase);
-			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function));
+			Debug.Assert(phase <= ILPhase.InILReader || this.IsDescendantOf(variable.Function!));
-			Debug.Assert(phase <= ILPhase.InILReader || variable.Function.Variables[variable.IndexInFunction] == variable);
+			Debug.Assert(phase <= ILPhase.InILReader || variable.Function!.Variables[variable.IndexInFunction] == variable);
 			Debug.Assert(value.ResultType == variable.StackType);
 		}
 	}

ICSharpCode.Decompiler/IL/Instructions/StringToInt.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // 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
@@ -23,23 +24,23 @@ namespace ICSharpCode.Decompiler.IL
 {
 	partial class StringToInt
 	{
-		public List<(string Key, int Value)> Map { get; }
+		public List<(string? Key, int Value)> Map { get; }
 
-		public StringToInt(ILInstruction argument, List<(string Key, int Value)> map)
+		public StringToInt(ILInstruction? argument, List<(string? Key, int Value)> map)
 			: base(OpCode.StringToInt)
 		{
 			this.Argument = argument;
 			this.Map = map;
 		}
 
-		public StringToInt(ILInstruction argument, string[] map)
+		public StringToInt(ILInstruction? argument, string?[] map)
 			: this(argument, ArrayToDictionary(map))
 		{
 		}
 
-		static List<(string Key, int Value)> ArrayToDictionary(string[] map)
+		static List<(string? Key, int Value)> ArrayToDictionary(string?[] map)
 		{
-			var dict = new List<(string Key, int Value)>();
+			var dict = new List<(string? Key, int Value)>();
 			for (int i = 0; i < map.Length; i++)
 			{
 				dict.Add((map[i], i));

ICSharpCode.Decompiler/IL/Instructions/SwitchInstruction.cs

@@ -1,3 +1,4 @@
+#nullable enable
 // Copyright (c) 2014 AlphaSierraPapa for the SharpDevelop Team
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
@@ -16,7 +17,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-using System;
 using System.Diagnostics;
 using System.Linq;
 
@@ -49,7 +49,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.Sections = new InstructionCollection<SwitchSection>(this, 1);
 		}
 
-		ILInstruction value;
+		ILInstruction value = null!;
 		public ILInstruction Value {
 			get { return this.value; }
 			set {

ICSharpCode.Decompiler/IL/Instructions/TryInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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
@@ -33,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.TryBlock = tryBlock;
 		}
 
-		ILInstruction tryBlock;
+		ILInstruction tryBlock = null!;
 		public ILInstruction TryBlock {
 			get { return this.tryBlock; }
 			set {
@@ -144,7 +145,7 @@ namespace ICSharpCode.Decompiler.IL
 			base.CheckInvariant(phase);
 			Debug.Assert(Parent is TryCatch);
 			Debug.Assert(filter.ResultType == StackType.I4);
-			Debug.Assert(this.IsDescendantOf(variable.Function));
+			Debug.Assert(this.IsDescendantOf(variable.Function!));
 		}
 
 		public override StackType ResultType {
@@ -202,7 +203,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.FinallyBlock = finallyBlock;
 		}
 
-		ILInstruction finallyBlock;
+		ILInstruction finallyBlock = null!;
 		public ILInstruction FinallyBlock {
 			get { return this.finallyBlock; }
 			set {
@@ -299,7 +300,7 @@ namespace ICSharpCode.Decompiler.IL
 			this.FaultBlock = faultBlock;
 		}
 
-		ILInstruction faultBlock;
+		ILInstruction faultBlock = null!;
 		public ILInstruction FaultBlock {
 			get { return this.faultBlock; }
 			set {

ICSharpCode.Decompiler/IL/Instructions/UnaryInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/Instructions/UsingInstruction.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2017 Siegfried Pammer
+#nullable enable
+// Copyright (c) 2017 Siegfried Pammer
 // 
 // 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

ICSharpCode.Decompiler/IL/Patterns/AnyNode.cs

@@ -1,8 +1,5 @@
-using System;
+#nullable enable
-using System.Collections.Generic;
+using System;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace ICSharpCode.Decompiler.IL.Patterns
 {
@@ -37,19 +34,20 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 
 	partial class AnyNode : PatternInstruction
 	{
-		CaptureGroup group;
+		CaptureGroup? group;
 
-		public AnyNode(CaptureGroup group = null)
+		public AnyNode(CaptureGroup? group = null)
 			: base(OpCode.AnyNode)
 		{
 			this.group = group;
 		}
 
-		protected internal override bool PerformMatch(ILInstruction other, ref Match match)
+		protected internal override bool PerformMatch(ILInstruction? other, ref Match match)
 		{
 			if (other == null)
 				return false;
-			match.Add(group, other);
+			if (group != null)
+				match.Add(group, other);
 			return true;
 		}
 	}

ICSharpCode.Decompiler/IL/Patterns/ListMatch.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 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
@@ -44,7 +45,7 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 		/// If the method returns true, it adds the capture groups (if any) to the match.
 		/// If the method returns false, the match object remains in a partially-updated state and needs to be restored
 		/// before it can be reused.</returns>
-		internal static bool DoMatch(IReadOnlyList<ILInstruction> patterns, IReadOnlyList<ILInstruction> syntaxList, ref Match match)
+		internal static bool DoMatch(IReadOnlyList<ILInstruction> patterns, IReadOnlyList<ILInstruction?> syntaxList, ref Match match)
 		{
 			ListMatch listMatch = new ListMatch(syntaxList);
 			do
@@ -107,14 +108,14 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 		/// <summary>
 		/// The syntax list we are matching against.
 		/// </summary>
-		internal readonly IReadOnlyList<ILInstruction> SyntaxList;
+		internal readonly IReadOnlyList<ILInstruction?> SyntaxList;
 
 		/// <summary>
 		/// The current index in the syntax list.
 		/// </summary>
 		internal int SyntaxIndex;
 
-		ListMatch(IReadOnlyList<ILInstruction> syntaxList)
+		ListMatch(IReadOnlyList<ILInstruction?> syntaxList)
 		{
 			this.SyntaxList = syntaxList;
 			this.SyntaxIndex = 0;
@@ -122,8 +123,8 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 			this.restoreStack = null;
 		}
 
-		List<SavePoint> backtrackingStack;
+		List<SavePoint>? backtrackingStack;
-		Stack<int> restoreStack;
+		Stack<int>? restoreStack;
 
 		void AddSavePoint(SavePoint savepoint)
 		{

ICSharpCode.Decompiler/IL/Patterns/Match.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2016 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2016 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
@@ -33,7 +34,7 @@ namespace ICSharpCode.Decompiler.IL.Patterns
 	{
 		static readonly List<KeyValuePair<CaptureGroup, ILInstruction>> emptyResults = new List<KeyValuePair<CaptureGroup, ILInstruction>>();
 
-		List<KeyValuePair<CaptureGroup, ILInstruction>> results;
+		List<KeyValuePair<CaptureGroup, ILInstruction>>? results;
 
 		/// <summary>
 		/// Gets whether the match was successful.

ICSharpCode.Decompiler/IL/PrimitiveType.cs

@@ -1,4 +1,5 @@
-// Copyright (c) 2014 Daniel Grunwald
+#nullable enable
+// Copyright (c) 2014 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

ICSharpCode.Decompiler/IL/SemanticHelper.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.Linq;
 
 using ICSharpCode.Decompiler.Util;
@@ -89,4 +91,4 @@ namespace ICSharpCode.Decompiler.IL
 			return false;
 		}
 	}
-}
+}

ICSharpCode.Decompiler/IL/SlotInfo.cs

@@ -16,6 +16,7 @@
 // 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.IL
 {

ICSharpCode.Decompiler/IL/StackType.cs

@@ -16,6 +16,7 @@
 // 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.IL
 {

ICSharpCode.Decompiler/TypeSystem/IMember.cs

@@ -62,6 +62,12 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		/// </summary>
 		IType ReturnType { get; }
 
+		/// <summary>
+		/// Gets/Sets the declaring type (incl. type arguments, if any).
+		/// If this is not a specialized member, the value returned is equal to <see cref="IEntity.DeclaringTypeDefinition"/>.
+		/// </summary>
+		new IType DeclaringType { get; }
+
 		/// <summary>
 		/// Gets the interface members explicitly implemented by this member.
 		/// </summary>

ICSharpCode.Decompiler/TypeSystem/Implementation/SyntheticRangeIndexer.cs

@@ -92,7 +92,7 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 		TypeParameterSubstitution IMember.Substitution => underlyingMethod.Substitution;
 		EntityHandle IEntity.MetadataToken => underlyingMethod.MetadataToken;
 		public string Name => underlyingMethod.Name;
-		IType IEntity.DeclaringType => underlyingMethod.DeclaringType;
+		public IType DeclaringType => underlyingMethod.DeclaringType;
 		ITypeDefinition IEntity.DeclaringTypeDefinition => underlyingMethod.DeclaringTypeDefinition;
 		IModule IEntity.ParentModule => underlyingMethod.ParentModule;
 		Accessibility IEntity.Accessibility => underlyingMethod.Accessibility;