WIP

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -327,6 +327,8 @@ namespace ICSharpCode.Decompiler.CSharp
 							return true;
 						if (settings.Dynamic && type.IsDelegate(metadata) && (name.StartsWith("<>A", StringComparison.Ordinal) || name.StartsWith("<>F", StringComparison.Ordinal)))
 							return true;
+						if (settings.AnonymousMethods && type.IsAnonymousDelegate(metadata))
+							return true;
 					}
 					if (settings.ArrayInitializers && settings.SwitchStatementOnString && name.StartsWith("<PrivateImplementationDetails>", StringComparison.Ordinal))
 						return true;

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2409,10 +2409,10 @@ namespace ICSharpCode.Decompiler.CSharp
 				// C# 10 lambdas can have attributes, but anonymous methods cannot
 				isLambda = true;
 			}
-			else if (settings.UseLambdaSyntax && ame.Parameters.All(p => p.ParameterModifier == ParameterModifier.None))
+			else if (settings.UseLambdaSyntax)
 			{
 				// otherwise use lambda only if an expression lambda is possible
-				isLambda = (body.Statements.Count == 1 && body.Statements.Single() is ReturnStatement);
+				isLambda = body.Statements.Count == 1 && body.Statements.Single() is ReturnStatement;
 			}
 			// 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

@@ -590,6 +590,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						type = new SimpleType("var");
 					}
+					else if (context.Settings.NaturalTypeForLambdaAndMethodGroup && v.Type.ContainsAnonymousDelegate())
+					{
+						type = new SimpleType("var");
+					}
 					else
 					{
 						type = context.TypeSystemAstBuilder.ConvertType(v.Type);
@@ -622,6 +626,10 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 					{
 						type = new SimpleType("var");
 					}
+					else if (context.Settings.NaturalTypeForLambdaAndMethodGroup && v.Type.ContainsAnonymousDelegate())
+					{
+						type = new SimpleType("var");
+					}
 					else if (dirExpr.Annotation<UseImplicitlyTypedOutAnnotation>() != null)
 					{
 						type = new SimpleType("var");

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -149,6 +149,7 @@ namespace ICSharpCode.Decompiler
 			{
 				fileScopedNamespaces = false;
 				recordStructs = false;
+				naturalTypeForLambdaAndMethodGroup = false;
 			}
 			if (languageVersion < CSharp.LanguageVersion.CSharp11_0)
 			{
@@ -166,7 +167,7 @@ namespace ICSharpCode.Decompiler
 		{
 			if (parameterNullCheck || scopedRef || requiredMembers || numericIntPtr || utf8StringLiterals || unsignedRightShift || checkedOperators)
 				return CSharp.LanguageVersion.CSharp11_0;
-			if (fileScopedNamespaces || recordStructs)
+			if (fileScopedNamespaces || recordStructs || naturalTypeForLambdaAndMethodGroup)
 				return CSharp.LanguageVersion.CSharp10_0;
 			if (nativeIntegers || initAccessors || functionPointers || forEachWithGetEnumeratorExtension
 				|| recordClasses || withExpressions || usePrimaryConstructorSyntax || covariantReturns
@@ -443,6 +444,24 @@ namespace ICSharpCode.Decompiler
 			}
 		}
 
+		bool naturalTypeForLambdaAndMethodGroup = true;
+
+		/// <summary>
+		/// Use C# 10 natural types for delegates and lambdas.
+		/// </summary>
+		[Category("C# 10.0 / VS 2022")]
+		[Description("DecompilerSettings.NaturalTypeForLambdaAndMethodGroup")]
+		public bool NaturalTypeForLambdaAndMethodGroup {
+			get { return naturalTypeForLambdaAndMethodGroup; }
+			set {
+				if (naturalTypeForLambdaAndMethodGroup != value)
+				{
+					naturalTypeForLambdaAndMethodGroup = value;
+					OnPropertyChanged();
+				}
+			}
+		}
+
 		bool parameterNullCheck = false;
 
 		/// <summary>

ICSharpCode.Decompiler/IL/Transforms/AssignVariableNames.cs

@@ -594,7 +594,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			{
 				return name;
 			}
-			if (type.IsAnonymousType())
+			if (type.IsAnonymousType() || type.IsAnonymousDelegate())
 			{
 				name = "anon";
 			}

ICSharpCode.Decompiler/IL/Transforms/DelegateConstruction.cs

@@ -127,11 +127,11 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		static bool ContainsAnonymousType(IMethod method)
 		{
-			if (method.ReturnType.ContainsAnonymousType())
+			if (method.ReturnType.ContainsAnonymousTypeOrDelegate())
 				return true;
 			foreach (var p in method.Parameters)
 			{
-				if (p.Type.ContainsAnonymousType())
+				if (p.Type.ContainsAnonymousTypeOrDelegate())
 					return true;
 			}
 			return false;

ICSharpCode.Decompiler/NRExtensions.cs

@@ -16,7 +16,6 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 using System;
-using System.Collections.Generic;
 
 using ICSharpCode.Decompiler.Documentation;
 using ICSharpCode.Decompiler.TypeSystem;
@@ -66,6 +65,20 @@ namespace ICSharpCode.Decompiler
 			return false;
 		}
 
+		public static bool IsAnonymousDelegate(this IType type)
+		{
+			if (type == null)
+				return false;
+			if (string.IsNullOrEmpty(type.Namespace) && type.HasGeneratedName()
+				&& type.Kind == TypeKind.Delegate
+				&& (type.Name.Contains("AnonymousDelegate") || type.Name.StartsWith("<>F{")))
+			{
+				ITypeDefinition td = type.GetDefinition();
+				return td != null && td.IsCompilerGenerated();
+			}
+			return false;
+		}
+
 		public static bool ContainsAnonymousType(this IType type)
 		{
 			var visitor = new ContainsAnonTypeVisitor();
@@ -73,14 +86,31 @@ namespace ICSharpCode.Decompiler
 			return visitor.ContainsAnonType;
 		}
 
+		public static bool ContainsAnonymousDelegate(this IType type)
+		{
+			var visitor = new ContainsAnonTypeVisitor();
+			type.AcceptVisitor(visitor);
+			return visitor.ContainsAnonDelegate;
+		}
+
+		public static bool ContainsAnonymousTypeOrDelegate(this IType type)
+		{
+			var visitor = new ContainsAnonTypeVisitor();
+			type.AcceptVisitor(visitor);
+			return visitor.ContainsAnonType || visitor.ContainsAnonDelegate;
+		}
+
 		class ContainsAnonTypeVisitor : TypeVisitor
 		{
 			public bool ContainsAnonType;
+			public bool ContainsAnonDelegate;
 
 			public override IType VisitOtherType(IType type)
 			{
 				if (IsAnonymousType(type))
 					ContainsAnonType = true;
+				if (IsAnonymousDelegate(type))
+					ContainsAnonDelegate = true;
 				return base.VisitOtherType(type);
 			}
 
@@ -88,6 +118,8 @@ namespace ICSharpCode.Decompiler
 			{
 				if (IsAnonymousType(type))
 					ContainsAnonType = true;
+				if (IsAnonymousDelegate(type))
+					ContainsAnonDelegate = true;
 				return base.VisitTypeDefinition(type);
 			}
 		}

ICSharpCode.Decompiler/SRMExtensions.cs

@@ -464,6 +464,18 @@ namespace ICSharpCode.Decompiler
 			return false;
 		}
 
+		public static bool IsAnonymousDelegate(this TypeDefinition type, MetadataReader metadata)
+		{
+			string name = metadata.GetString(type.Name);
+			if (type.Namespace.IsNil && type.HasGeneratedName(metadata)
+				&& type.IsDelegate(metadata)
+				&& (name.Contains("AnonymousDelegate") || name.StartsWith("<>F{")))
+			{
+				return type.IsCompilerGenerated(metadata);
+			}
+			return false;
+		}
+
 		#region HasGeneratedName
 
 		public static bool IsGeneratedName(this StringHandle handle, MetadataReader metadata)

ILSpy/Properties/Resources.Designer.cs

@@ -1109,6 +1109,15 @@ namespace ICSharpCode.ILSpy.Properties {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Use natural delegate types for lambdas and method groups.
+        /// </summary>
+        public static string DecompilerSettings_NaturalTypeForLambdaAndMethodGroup {
+            get {
+                return ResourceManager.GetString("DecompilerSettings.NaturalTypeForLambdaAndMethodGroup", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Nullable reference types.
         /// </summary>

ILSpy/Properties/Resources.resx

@@ -393,6 +393,9 @@ Are you sure you want to continue?</value>
   <data name="DecompilerSettings.NativeIntegers" xml:space="preserve">
     <value>Use nint/nuint types</value>
   </data>
+  <data name="DecompilerSettings.NaturalTypeForLambdaAndMethodGroup" xml:space="preserve">
+    <value>Use natural delegate types for lambdas and method groups</value>
+  </data>
   <data name="DecompilerSettings.NullPropagation" xml:space="preserve">
     <value>Decompile ?. and ?[] operators</value>
   </data>