Emit params and default values on lambda parameter lists

C# 12 allows both on the explicitly typed parameter list of a lambda, and
nowhere else: an anonymous method cannot declare either, and neither can a
lambda whose parameter list is about to be dropped. Guarded by a setting so
the output stays valid for earlier language versions.

Only what the anonymous function's own metadata declares is written. A lambda
may state a default the delegate does not have, a different one, or none where
the delegate has one, and reflection over the lambda's method reports what the
lambda declared - so filling either in from the delegate's Invoke would make
the recompiled assembly describe itself differently from the original. Call
sites are unaffected either way, because they bind against the delegate, which
still declares both.

Roslyn writes ParamArrayAttribute on the anonymous function's own method only
from version 5 on; before that it stands on the delegate type alone, where it
is not the lambda's to restate, so the fixture guards those cases on ROSLYN5.
The correctness test reads the metadata back through reflection, which is the
only place the difference is observable.

Assisted-by: Claude:claude-opus-5[1m]:Claude Code

ICSharpCode.Decompiler.Tests/CorrectnessTestRunner.cs

@@ -295,6 +295,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunCS(options: options);
 		}
 
+		[Test]
+		public async Task LambdaOptionalAndParamsParameters([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions options)
+		{
+			await RunCS(options: options);
+		}
+
 		[Test]
 		public async Task NullPropagation([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions options)
 		{

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -824,6 +824,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task LambdaOptionalAndParamsParameters([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task RefStructInterfaces([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Correctness/LambdaOptionalAndParamsParameters.cs

@@ -0,0 +1,68 @@
+#pragma warning disable CS9099, CS9100
+using System;
+using System.Reflection;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
+{
+	class LambdaOptionalAndParamsParameters
+	{
+		static void Main()
+		{
+#if CS120 && !NET40
+			ReportMetadata();
+			ReportCalls();
+#endif
+		}
+
+#if CS120 && !NET40
+		delegate int OptionalFunc(int x = 5);
+
+		delegate int PlainFunc(int x);
+
+		delegate void ParamsAction(params int[] xs);
+
+		delegate void PlainAction(int[] xs);
+
+		// A lambda's parameter list does not have to repeat what the delegate declares: it may
+		// state a different default, one the delegate does not have, or none where the delegate
+		// has one, and the same for the params modifier. Metadata records what the lambda itself
+		// declared, and reflection reports that rather than the delegate's, so the decompiled
+		// lambda has to carry the lambda's own list back.
+		static void Report(string name, Delegate d)
+		{
+			ParameterInfo p = d.Method.GetParameters()[0];
+			Console.WriteLine("{0}: hasDefault={1} default={2} params={3}",
+				name,
+				p.HasDefaultValue,
+				p.HasDefaultValue ? p.DefaultValue : "none",
+				p.IsDefined(typeof(ParamArrayAttribute), inherit: false));
+		}
+
+		static void ReportMetadata()
+		{
+			Report("DefaultOnlyInDelegate", (OptionalFunc)((int x) => x * 2));
+			Report("DefaultOnlyInLambda", (PlainFunc)((int x = 3) => x * 2));
+			Report("DefaultDiffersFromDelegate", (OptionalFunc)((int x = 7) => x * 2));
+			Report("DefaultAgreesWithDelegate", (OptionalFunc)((int x = 5) => x * 2));
+			Report("ParamsOnlyInDelegate", (ParamsAction)((int[] xs) => Console.WriteLine(xs.Length)));
+			Report("ParamsOnlyInLambda", (PlainAction)((params int[] xs) => Console.WriteLine(xs.Length)));
+		}
+
+		// The value a caller gets for an omitted argument comes from the delegate, never from the
+		// lambda, so these stay the same whatever the lambda declared.
+		static void ReportCalls()
+		{
+			OptionalFunc noDefault = (int x) => x * 2;
+			OptionalFunc otherDefault = (int x = 7) => x * 2;
+			Console.WriteLine(noDefault());
+			Console.WriteLine(otherDefault());
+			Console.WriteLine(otherDefault(1));
+
+			ParamsAction expandedByDelegate = (int[] xs) => Console.WriteLine(xs.Length);
+			expandedByDelegate(1, 2, 3);
+			PlainAction notExpanded = (params int[] xs) => Console.WriteLine(xs.Length);
+			notExpanded(new int[2]);
+		}
+#endif
+	}
+}

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

@@ -0,0 +1,99 @@
+#pragma warning disable CS9099, CS9100
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal class LambdaOptionalAndParamsParameters
+	{
+		public delegate void ParamsAction(params int[] xs);
+
+		public delegate int OptionalFunc(int x = 5);
+
+		public delegate int PlainFunc(int x);
+
+		// A lambda states 'params' and defaults on its own account: what the delegate declares
+		// is the delegate's, and reflection over the lambda's method reports only the lambda's.
+		// Roslyn records the modifier on the anonymous function's method from version 5 on, so
+		// the cases that need it back are guarded on ROSLYN5.
+
+		private int total;
+
+#if ROSLYN5
+		public ParamsAction ParamsStatementBody()
+		{
+			return (params int[] xs) => {
+				total += xs.Length;
+				Console.WriteLine(xs.Length);
+			};
+		}
+#endif
+
+#if ROSLYN5
+		public ParamsAction ParamsSingleStatementBody()
+		{
+			return (params int[] xs) => {
+				Console.WriteLine(xs.Length);
+			};
+		}
+#endif
+
+		public OptionalFunc OptionalStatementBody()
+		{
+			return (int x = 5) => {
+				total += x;
+				return x * 2;
+			};
+		}
+
+		public OptionalFunc OptionalExpressionBody()
+		{
+			return (int x = 5) => x * 2;
+		}
+
+		public ParamsAction ParamsWithoutParameterList()
+		{
+			return delegate {
+			};
+		}
+
+		public OptionalFunc OptionalWithoutParameterList()
+		{
+			return delegate {
+				return 1;
+			};
+		}
+
+		// The lambda's own defaults and params modifier can differ from the delegate's; the metadata
+		// records the lambda's, so that is what must round-trip.
+		public OptionalFunc OptionalDifferentDefault()
+		{
+			return (int x = 7) => x * 2;
+		}
+
+		public OptionalFunc OptionalOnlyInDelegate()
+		{
+			return (int x) => x * 2;
+		}
+
+		public PlainFunc OptionalOnlyInLambda()
+		{
+			return (int x = 3) => x * 2;
+		}
+
+		public ParamsAction ParamsOnlyInDelegate()
+		{
+			return (int[] xs) => {
+				Console.WriteLine(xs.Length);
+			};
+		}
+
+#if ROSLYN5
+		public Action<int[]> ParamsOnlyInLambda()
+		{
+			return (params int[] xs) => {
+				Console.WriteLine(xs.Length);
+			};
+		}
+#endif
+	}
+}

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2627,7 +2627,6 @@ namespace ICSharpCode.Decompiler.CSharp
 				let v = ident.GetILVariable()
 				where v != null && v.Function == function && v.Kind == VariableKind.Parameter
 				select ident).Any();
-
 			bool isLambda = false;
 			if (ame.Parameters.Any(p => p.Type is null))
 			{
@@ -2650,6 +2649,33 @@ namespace ICSharpCode.Decompiler.CSharp
 				// form is compatible with any delegate signature, so it is always legal there.
 				isLambda = true;
 			}
+			// 'params' and parameter default values are only legal on the explicitly typed
+			// parameter list of a lambda, and only since C# 12; and a list that is about to be
+			// dropped cannot carry them at all. Everywhere else they are decorative - the
+			// delegate type still declares both, and that is what call sites bind against.
+			if (settings.LambdaOptionalAndParamsParameters
+				&& (isLambda || parametersAreUsed)
+				&& ame.Parameters.All(p => p.Type is not null))
+			{
+				// Only what the anonymous function's own metadata declares is written. A lambda
+				// may state a different default than its target delegate, or none where the
+				// delegate has one, and reflection over the lambda's method reports what the
+				// lambda declared - so taking either from the delegate's Invoke would change
+				// what the recompiled assembly says. The delegate type keeps declaring both,
+				// and call sites bind against it, so nothing is lost by leaving them out here.
+
+				// An anonymous method cannot declare either, in any language version.
+				if (ame.Parameters.Any(p => p.IsParams || p.DefaultExpression is not null))
+					isLambda = true;
+			}
+			else
+			{
+				foreach (var p in ame.Parameters)
+				{
+					p.IsParams = false;
+					p.DefaultExpression?.Detach();
+				}
+			}
 			// Remove the parameter list from an AnonymousMethodExpression if the parameters are not used in the method body
 			if (!isLambda && !parametersAreUsed)
 			{

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -848,6 +848,14 @@ namespace ICSharpCode.Decompiler
 		[DecompilerSetting(CSharp.LanguageVersion.CSharp12_0)]
 		public partial bool InlineArrays { get; set; }
 
+		/// <summary>
+		/// Gets/sets whether lambda parameter lists may declare 'params' and parameter default
+		/// values. When disabled, these modifiers are dropped from anonymous functions instead.
+		/// </summary>
+		[Description("DecompilerSettings.LambdaOptionalAndParamsParameters")]
+		[DecompilerSetting(CSharp.LanguageVersion.CSharp12_0)]
+		public partial bool LambdaOptionalAndParamsParameters { get; set; }
+
 		/// <summary>
 		/// Gets/Sets whether C# 14.0 extension members should be transformed.
 		/// </summary>

ILSpy/Properties/Resources.Designer.cs

@@ -1361,6 +1361,15 @@ namespace ICSharpCode.ILSpy.Properties {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to &apos;params&apos; and optional parameters in lambdas.
+        /// </summary>
+        public static string DecompilerSettings_LambdaOptionalAndParamsParameters {
+            get {
+                return ResourceManager.GetString("DecompilerSettings.LambdaOptionalAndParamsParameters", resourceCulture);
+            }
+        }
+
         /// <summary>
         ///   Looks up a localized string similar to Use nint/nuint types.
         /// </summary>

ILSpy/Properties/Resources.resx

@@ -486,6 +486,9 @@ Are you sure you want to continue?</value>
   <data name="DecompilerSettings.IsUnmanagedAttributeOnTypeParametersShouldBeReplacedWithUnmanagedConstraints" xml:space="preserve">
     <value>IsUnmanagedAttribute on type parameters should be replaced with 'unmanaged' constraints</value>
   </data>
+  <data name="DecompilerSettings.LambdaOptionalAndParamsParameters" xml:space="preserve">
+    <value>'params' and optional parameters in lambdas</value>
+  </data>
   <data name="DecompilerSettings.NativeIntegers" xml:space="preserve">
     <value>Use nint/nuint types</value>
   </data>