Extend UseLambdaSyntax to emit statement lambdas

Under UseLambdaSyntax, anonymous functions became lambdas only when an
expression body was possible; statement-bodied ones kept C# 2 delegate
syntax. Now every anonymous function whose parameter shape a lambda can
express uses lambda syntax; delegate syntax remains for ref/out/in and
params parameters and for pre-C# 3 language profiles.

Two latent issues surfaced by the wider lambda coverage: DeclareVariables
assumed an insertion point directly under a LambdaExpression is an
expression body it must convert to a block, which block-bodied lambdas
now violate; and anonymous methods declared without a parameter list
carry compiler-generated parameter names like '<p0>' that are not valid
identifiers, so the lambda's mandatory parameter list regenerates such
names from the parameter type: (object obj, EventArgs e) => ...
A side effect visible in fixtures: an explicit parameter list can make
a delegate-creation cast redundant that bare 'delegate' syntax needed
for overload resolution, e.g. new Thread((ThreadStart)delegate { })
becomes new Thread(() => { }).

Assisted-by: Claude:claude-fable-5:Claude Code

ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs

@@ -51,6 +51,12 @@ namespace ICSharpCode.Decompiler.Tests
 			}
 		}
 
+		[Test]
+		public async Task AnonymousMethodEdgeCases()
+		{
+			await Run();
+		}
+
 		[Test, Ignore("Need to decide how to represent virtual methods without 'newslot' flag")]
 		public async Task Issue379()
 		{

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/AnonymousMethodEdgeCases.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
+{
+	public class AnonymousMethodEdgeCases
+	{
+		public Func<int, int> AssignmentIsTheLambdaBody()
+		{
+			return (int x) => {
+				int num;
+				return num = x;
+			};
+		}
+
+		public Action<object> UsedParameterWithInvalidName()
+		{
+			return (object value) => {
+				Console.WriteLine(value);
+			};
+		}
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/AnonymousMethodEdgeCases.il

@@ -0,0 +1,79 @@
+// Hand-written input for two anonymous-method shapes that C# cannot express directly:
+// an expression-bodied lambda whose body is the assignment declaring its own local, and
+// a parameter whose metadata name is not a valid C# identifier but IS used in the body.
+.assembly extern mscorlib
+{
+  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )
+  .ver 4:0:0:0
+}
+.assembly extern System.Core
+{
+  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )
+  .ver 4:0:0:0
+}
+.assembly AnonymousMethodEdgeCases
+{
+  .ver 1:0:0:0
+}
+.module AnonymousMethodEdgeCases.dll
+.imagebase 0x00400000
+.file alignment 0x00000200
+.stackreserve 0x00100000
+.subsystem 0x0003
+.corflags 0x00000001
+
+.class public auto ansi beforefieldinit ICSharpCode.Decompiler.Tests.TestCases.ILPretty.AnonymousMethodEdgeCases
+	extends [mscorlib]System.Object
+{
+	.method public hidebysig specialname rtspecialname instance void .ctor () cil managed
+	{
+		.maxstack 8
+		ldarg.0
+		call instance void [mscorlib]System.Object::.ctor()
+		ret
+	}
+
+	.method public hidebysig instance class [System.Core]System.Func`2<int32, int32> AssignmentIsTheLambdaBody () cil managed
+	{
+		.maxstack 8
+		ldarg.0
+		ldftn instance int32 ICSharpCode.Decompiler.Tests.TestCases.ILPretty.AnonymousMethodEdgeCases::'<AssignmentIsTheLambdaBody>b__1_0'(int32)
+		newobj instance void class [System.Core]System.Func`2<int32, int32>::.ctor(object, native int)
+		ret
+	}
+
+	// return num = x; -- the store's value is the return value, so the local has a store and
+	// no load, and the decompiled lambda body is the assignment itself.
+	.method private hidebysig instance int32 '<AssignmentIsTheLambdaBody>b__1_0' (int32 x) cil managed
+	{
+		.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 )
+		.maxstack 2
+		.locals init (
+			[0] int32 num
+		)
+		ldarg.1
+		dup
+		stloc.0
+		ret
+	}
+
+	.method public hidebysig instance class [System.Core]System.Action`1<object> UsedParameterWithInvalidName () cil managed
+	{
+		.maxstack 8
+		ldarg.0
+		ldftn instance void ICSharpCode.Decompiler.Tests.TestCases.ILPretty.AnonymousMethodEdgeCases::'<UsedParameterWithInvalidName>b__2_0'(object)
+		newobj instance void class [System.Core]System.Action`1<object>::.ctor(object, native int)
+		ret
+	}
+
+	// The parameter name '<p0>' is what C# emits for an anonymous method declared without a
+	// parameter list; used here, so the lambda's parameter list has to name it somehow.
+	.method private hidebysig instance void '<UsedParameterWithInvalidName>b__2_0' (object '<p0>') cil managed
+	{
+		.custom instance void [mscorlib]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 )
+		.maxstack 8
+		ldarg.1
+		call void [mscorlib]System.Console::WriteLine(object)
+		ret
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/Issue1038.cs

@@ -4,7 +4,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 {
 	public class Issue1038<TK, TR> where TR : class, new()
 	{
-		public event Action<TK, TR> TestEvent = delegate {
+		public event Action<TK, TR> TestEvent = (TK A_0, TR A_1) => {
 		};
 	}
 }

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Async.cs

@@ -524,7 +524,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static Func<Task<int>> AsyncDelegate()
 		{
-			return async delegate {
+			return async () => {
 				await Task.Delay(10);
 				return 2;
 			};

ICSharpCode.Decompiler.Tests/TestCases/Pretty/CustomTaskType.cs

@@ -119,7 +119,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public static Func<ValueTask<int>> AsyncDelegate()
 		{
-			return async delegate {
+			return async () => {
 				await Task.Delay(10);
 				return 2;
 			};

ICSharpCode.Decompiler.Tests/TestCases/Pretty/DeconstructionTests.cs

@@ -990,7 +990,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		{
 			int a = 0;
 			int b = 0;
-			await Task.Run(delegate {
+			await Task.Run(() => {
 				(a, b) = GetTuple<int, int>();
 			});
 			return a + b;

ICSharpCode.Decompiler.Tests/TestCases/Pretty/DelegateConstruction.cs

@@ -58,14 +58,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 			public Action CaptureOfThis()
 			{
-				return delegate {
+				return () => {
 					CaptureOfThis();
 				};
 			}
 
 			public Action CaptureOfThisAndParameter(int a)
 			{
-				return delegate {
+				return () => {
 					CaptureOfThisAndParameter(a);
 				};
 			}
@@ -76,7 +76,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 				{
 					if (item > 0)
 					{
-						return delegate {
+						return () => {
 							CaptureOfThisAndParameter(item + a);
 						};
 					}
@@ -91,7 +91,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 					int copyOfItem = item;
 					if (item > 0)
 					{
-						return delegate {
+						return () => {
 							CaptureOfThisAndParameter(item + a + copyOfItem);
 						};
 					}
@@ -118,18 +118,18 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 			private void Bug955()
 			{
-				new Thread((ThreadStart)delegate {
+				new Thread(() => {
 				});
 			}
 
 			public void Bug951(int amount)
 			{
-				DoAction(delegate {
+				DoAction(() => {
 					if (amount < 0)
 					{
 						amount = 0;
 					}
-					DoAction(delegate {
+					DoAction(() => {
 						NoOp(amount);
 					});
 				});
@@ -138,12 +138,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			public void Bug951b()
 			{
 				int amount = Foo();
-				DoAction(delegate {
+				DoAction(() => {
 					if (amount < 0)
 					{
 						amount = 0;
 					}
-					DoAction(delegate {
+					DoAction(() => {
 						NoOp(amount);
 					});
 				});
@@ -151,8 +151,8 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 			public void Bug951c(SomeData data)
 			{
-				DoAction(delegate {
-					DoAction(delegate {
+				DoAction(() => {
+					DoAction(() => {
 						DoSomething(data.Value);
 					});
 				});
@@ -165,7 +165,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 			public Action<object> Bug971_DelegateWithoutParameterList()
 			{
-				return delegate {
+				return (object obj) => {
 				};
 			}
 
@@ -256,7 +256,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			public Func<TCaptured> GetFunc(Func<TNonCaptured, TCaptured> f)
 			{
 				TCaptured captured = f(default(TNonCaptured));
-				return delegate {
+				return () => {
 					Console.WriteLine(captured.GetType().FullName);
 					return captured;
 				};
@@ -265,7 +265,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			public Func<TNonCaptured, TNonCapturedMP, TCaptured> GetFunc<TNonCapturedMP>(Func<TCaptured> f)
 			{
 				TCaptured captured = f();
-				return delegate (TNonCaptured a, TNonCapturedMP d) {
+				return (TNonCaptured a, TNonCapturedMP d) => {
 					Console.WriteLine(a.GetHashCode());
 					Console.WriteLine(captured.GetType().FullName);
 					return captured;
@@ -343,7 +343,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			for (int i = 0; i < 10; i++)
 			{
 				int counter;
-				list.Add(delegate (int x) {
+				list.Add((int x) => {
 					counter = x;
 				});
 			}
@@ -356,7 +356,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			int counter;
 			for (int i = 0; i < 10; i++)
 			{
-				list.Add(delegate (int x) {
+				list.Add((int x) => {
 					counter = x;
 				});
 			}
@@ -365,7 +365,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 		public static Action StaticAnonymousMethodNoClosure()
 		{
-			return delegate {
+			return () => {
 				Console.WriteLine();
 			};
 		}
@@ -383,7 +383,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 				int j;
 				for (j = 0; j < 10; j++)
 				{
-					list.Add(delegate (int k) {
+					list.Add((int k) => {
 						for (int l = 0; l < j; l += k)
 						{
 							Console.WriteLine();
@@ -398,7 +398,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			List<Action<int>> list = new List<Action<int>>();
 			for (int i = 0; i < 10; i++)
 			{
-				list.Add(delegate (int k) {
+				list.Add((int k) => {
 					Console.WriteLine(k);
 				});
 			}
@@ -406,7 +406,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 
 		public static Action<int> NameConflict3(int i)
 		{
-			return delegate (int j) {
+			return (int j) => {
 				for (int k = 0; k < j; k++)
 				{
 					Console.WriteLine(k);
@@ -427,7 +427,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 		public static Func<TCaptured> CapturedTypeParameter1<TNonCaptured, TCaptured>(TNonCaptured a, Func<TNonCaptured, TCaptured> f)
 		{
 			TCaptured captured = f(a);
-			return delegate {
+			return () => {
 				Console.WriteLine(captured.GetType().FullName);
 				return captured;
 			};
@@ -436,7 +436,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 		public static Func<TCaptured> CapturedTypeParameter2<TNonCaptured, TCaptured>(TNonCaptured a, Func<TNonCaptured, List<TCaptured>> f)
 		{
 			List<TCaptured> captured = f(a);
-			return delegate {
+			return () => {
 				Console.WriteLine(captured.GetType().FullName);
 				return captured.FirstOrDefault();
 			};
@@ -624,14 +624,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 	{
 		public void M()
 		{
-			Run(delegate (object o) {
+			Run((object o) => {
 				try
 				{
 					List<int> list = o as List<int>;
-					Action action = delegate {
+					Action action = () => {
 						list.Select((int x) => x * 2);
 					};
-					Action action2 = delegate {
+					Action action2 = () => {
 						list.Select((int x) => x * 2);
 					};
 					Console.WriteLine();
@@ -655,6 +655,20 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 			del(x);
 		}
 
+		public void AnonymousMethodWithByRefParameters()
+		{
+			RefAction refAction = delegate (ref int reference) {
+				reference++;
+			};
+			OutAction outAction = delegate (out int reference) {
+				reference = 1;
+			};
+			int value = 0;
+			refAction(ref value);
+			outAction(out value);
+			Console.WriteLine(value);
+		}
+
 		public void Issue1572(DelegateConstruction.Dummy dum)
 		{
 #if EXPECTED_OUTPUT
@@ -675,4 +689,8 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.DelegateConstruction
 	internal class MyAttribute : Attribute
 	{
 	}
+
+	public delegate void OutAction(out int value);
+
+	public delegate void RefAction(ref int value);
 }

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ExpressionTrees.cs

@@ -818,15 +818,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			Test<Func<int, string>>((int a) => a.ToString(), (int a) => a.ToString());
 			Test<Func<string, char[]>>((string a) => a.ToArray(), (string a) => a.ToArray());
 			Test<Func<bool>>(() => 'a'.CompareTo('b') < 0, () => 'a'.CompareTo('b') < 0);
-			Test<Action<object, bool>>(delegate (object lockObj, bool lockTaken) {
+			Test<Action<object, bool>>((object lockObj, bool lockTaken) => {
 				Monitor.Enter(lockObj, ref lockTaken);
 			}, (object lockObj, bool lockTaken) => Monitor.Enter(lockObj, ref lockTaken));
 			Test<Func<string, int, bool>>((string str, int num) => int.TryParse(str, out num), (string str, int num) => int.TryParse(str, out num));
 			Test<Func<string, SimpleType, bool>>((string str, SimpleType t) => int.TryParse(str, out t.Field), (string str, SimpleType t) => int.TryParse(str, out t.Field));
-			Test<Action<object>>(delegate (object o) {
+			Test<Action<object>>((object o) => {
 				TestCall(o);
 			}, (object o) => TestCall(o));
-			Test<Action<object>>(delegate (object o) {
+			Test<Action<object>>((object o) => {
 				TestCall(ref o);
 			}, (object o) => TestCall(ref o));
 		}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/FixProxyCalls.cs

@@ -96,7 +96,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 
 		protected internal override void Test(string test)
 		{
-			action = delegate (string a) {
+			action = (string a) => {
 				base.Test(a);
 			};
 			if (test.Equals(1))
@@ -119,7 +119,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 	{
 		public Action<object> M(object state)
 		{
-			return delegate (object x) {
+			return (object x) => {
 				base.BaseCall(x, state, () => (object)null);
 			};
 		}
@@ -136,7 +136,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.ILPretty
 	{
 		protected internal override void Test(int a)
 		{
-			Action action = delegate {
+			Action action = () => {
 				base.Test(a);
 			};
 			if (a.Equals(1))

ICSharpCode.Decompiler.Tests/TestCases/Pretty/InitializerTests.cs

@@ -916,7 +916,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.InitializerTests
 		public static void NotAnObjectInitializerWithEvent()
 		{
 			Data data = new Data();
-			data.TestEvent += delegate {
+#if NET50
+			data.TestEvent += (object? obj, EventArgs e) => {
+#else
+			data.TestEvent += (object obj, EventArgs e) => {
+#endif
 				Console.WriteLine();
 			};
 			X(Y(), data);

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3439.cs

@@ -12,7 +12,7 @@ internal class VariableScopeTest
 
 	private void Test(List<string> list1)
 	{
-		AddAction(delegate (List<Item> list2) {
+		AddAction((List<Item> list2) => {
 			long num2 = 1L;
 			foreach (string item in list1)
 			{
@@ -28,7 +28,7 @@ internal class VariableScopeTest
 		{
 			int preservedName = num;
 			num++;
-			AddAction(item2, delegate (object x) {
+			AddAction(item2, (object x) => {
 				SetValue(x, preservedName);
 			});
 		}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3751.cs

@@ -13,7 +13,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public object Trigger()
 		{
-			return Infer(delegate {
+			return Infer(() => {
 				if (Cond)
 				{
 					Console.WriteLine();

ICSharpCode.Decompiler.Tests/TestCases/Pretty/LocalFunctions.cs

@@ -329,7 +329,7 @@ namespace LocalFunctions
 
 		private int field;
 
-		private Lazy<object> nonCapturinglocalFunctionInLambda = new Lazy<object>(delegate {
+		private Lazy<object> nonCapturinglocalFunctionInLambda = new Lazy<object>(() => {
 			return CreateValue();
 
 #if CS80
@@ -342,7 +342,7 @@ namespace LocalFunctions
 			}
 		});
 
-		private Lazy<object> capturinglocalFunctionInLambda = new Lazy<object>(delegate {
+		private Lazy<object> capturinglocalFunctionInLambda = new Lazy<object>(() => {
 			int x = 42;
 			return Do();
 
@@ -648,7 +648,7 @@ namespace LocalFunctions
 
 		public static int LocalFunctionInLambda(IEnumerable<int> xs)
 		{
-			return xs.First(delegate (int x) {
+			return xs.First((int x) => {
 				return Do();
 
 				bool Do()
@@ -797,20 +797,20 @@ namespace LocalFunctions
 				{
 					t0 = 0;
 					int t2 = t0;
-					return ((Func<int>)delegate {
+					return ((Func<int>)(() => {
 						t0 = 0;
 						t2 = 0;
 						return ZZZ2();
-					})();
+					}))();
 				}
 				int ZZZ2()
 				{
 					t0 = 0;
 					int t3 = t0;
 #if !OPT
-					Func<int> func = delegate {
+					Func<int> func = () => {
 #else
-					return ((Func<int>)delegate {
+					return ((Func<int>)(() => {
 #endif
 						t0 = 0;
 						t3 = 0;
@@ -819,7 +819,7 @@ namespace LocalFunctions
 					};
 					return func();
 #else
-					})();
+					}))();
 #endif
 				}
 			}
@@ -840,20 +840,20 @@ namespace LocalFunctions
 				{
 					t0 = 0;
 					int t2 = t0;
-					return ((Func<int>)delegate {
+					return ((Func<int>)(() => {
 						t0 = 0;
 						t2 = 0;
 						return ZZZ2();
-					})();
+					}))();
 				}
 				int ZZZ2()
 				{
 					t0 = 0;
 					int t3 = t0;
 #if !OPT
-					Func<int> func = delegate {
+					Func<int> func = () => {
 #else
-					return ((Func<int>)delegate {
+					return ((Func<int>)(() => {
 #endif
 						t0 = 0;
 						t3 = 0;
@@ -862,7 +862,7 @@ namespace LocalFunctions
 					};
 					return func();
 #else
-					})();
+					}))();
 #endif
 				}
 			}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/OutVariables.cs

@@ -37,7 +37,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			// to ensure that the value is initialized when the delegate is declared.
 			if (d.Count > 2 && d.TryGetValue(42, out var value))
 			{
-				return delegate {
+				return () => {
 					Console.WriteLine(value);
 				};
 			}

ICSharpCode.Decompiler.Tests/TestCases/Pretty/PropertiesAndEvents.cs

@@ -169,7 +169,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		public event EventHandler AutomaticEvent;
 
 		[field: NonSerialized]
-		public event EventHandler AutomaticEventWithInitializer = delegate {
+#if NET50
+		public event EventHandler AutomaticEventWithInitializer = (object? obj, EventArgs e) => {
+#else
+		public event EventHandler AutomaticEventWithInitializer = (object obj, EventArgs e) => {
+#endif
 		};
 
 #if ROSLYN

ICSharpCode.Decompiler.Tests/TestCases/Pretty/QualifierTests.cs

@@ -280,7 +280,7 @@ namespace ICSharpCode.Decompiler.Tests.Pretty
 		{
 			int fieldConflict = 5;
 			Capturer(() => this.fieldConflict + fieldConflict);
-			Capturer(delegate {
+			Capturer(() => {
 				int innerConflict = 5;
 				return this.fieldConflict + fieldConflict + Capturer2(() => this.innerConflict + innerConflict + this.fieldConflict + fieldConflict);
 			});

ICSharpCode.Decompiler.Tests/TestCases/Pretty/TupleTests.cs

@@ -100,14 +100,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public (int, int) AccessRest => (1, 2, 3, 4, 5, 6, 7, 8, 9).Rest;
 
-		public (string, object, Action) TargetTyping => (null, 1, delegate {
+		public (string, object, Action) TargetTyping => (null, 1, () => {
 #pragma warning disable format
 		});
 #pragma warning restore format
 
-		public object NotTargetTyping => ((string)null, (object)1, (Action)delegate {
+		public object NotTargetTyping => ((string)null, (object)1, (Action)(() => {
 #pragma warning disable format
-		});
+		}));
 #pragma warning restore format
 
 		public void UnnamedTupleOut(out (int, string, Action, dynamic) tuple)

ICSharpCode.Decompiler.Tests/TestCases/Ugly/AggressiveScalarReplacementOfAggregates.Expected.cs

@@ -207,7 +207,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 				field1 = 1,
 				field2 = "Hello World!"
 			};
-			Invoke(delegate {
+			Invoke(() => {
 				displayClass.thisField = new Program();
 			});
 			Console.WriteLine("{0} {1}", this, displayClass.thisField);

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2550,8 +2550,9 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 			else if (settings.UseLambdaSyntax && ame.Parameters.All(p => p.ParameterModifier == ReferenceKind.None && !p.IsParams))
 			{
-				// otherwise use lambda only if an expression lambda is possible
-				isLambda = (body.Statements.Count == 1 && body.Statements.Single() is ReturnStatement);
+				// Lambdas cover statement bodies too; anonymous method syntax remains only for
+				// parameter shapes a lambda cannot express (ref/out/in and params modifiers).
+				isLambda = true;
 			}
 			// Remove the parameter list from an AnonymousMethodExpression if the parameters are not used in the method body
 			var parameterReferencingIdentifiers =

ICSharpCode.Decompiler/CSharp/Transforms/DeclareVariables.cs

@@ -472,8 +472,11 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			{
 				// We can only insert variable declarations in blocks, but FindInsertionPoints() didn't
 				// guarantee that it finds only blocks.
-				// Fix that up now.
-				while (!(v.InsertionPoint.nextNode.Parent is BlockStatement or LambdaExpression))
+				// Fix that up now. A lambda is a valid stop only for its expression body (insertion
+				// will convert that body to a block); a point at a statement-bodied lambda's block
+				// itself must keep moving up into the enclosing scope.
+				while (!(v.InsertionPoint.nextNode.Parent is BlockStatement
+					|| (v.InsertionPoint.nextNode.Parent is LambdaExpression && v.InsertionPoint.nextNode is Expression)))
 				{
 					if (v.InsertionPoint.nextNode.Parent is ForStatement f && v.InsertionPoint.nextNode == f.Initializers.FirstOrDefault() && IsMatchingAssignment(v, out _))
 					{
@@ -593,6 +596,14 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			if (v.Type.IsByRefLike)
 				return true; // by-ref-like variables always must be initialized at their declaration.
 
+			if (v.InsertionPoint.nextNode.Parent is LambdaExpression)
+			{
+				// The insertion point is an expression-bodied lambda's body. Combining would put a
+				// declaration statement in expression position ("x => int num = x;"); the separate
+				// declaration path turns the body into a block first, which stays valid C#.
+				return false;
+			}
+
 			if (v.InsertionPoint.nextNode.Slot?.Kind == Slots.ForInitializer)
 				return true; // for-statement initializers always should combine declaration and initialization.
 

ICSharpCode.Decompiler/IL/Transforms/AssignVariableNames.cs

@@ -234,6 +234,14 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 							if (variables.TryGetValue(i, out var v))
 								variableMapping[v] = name;
 						}
+						else if (!IsValidName(name))
+						{
+							// Compiler-generated parameter names (e.g. "<p0>" on an anonymous method
+							// declared without a parameter list) are not valid C# identifiers. Skipping
+							// the reservation and the mapping leaves the parameter to AssignName, which
+							// generates a fresh name from the type for any name that fails IsValidName.
+							continue;
+						}
 						string nameWithoutNumber = SplitName(name, out int newIndex);
 						if (!parentScope.IsReservedVariableName(nameWithoutNumber, out _))
 						{