diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LambdaOptionalAndParamsParameters.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LambdaOptionalAndParamsParameters.cs
index b9fa0b055..110db46f9 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LambdaOptionalAndParamsParameters.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/LambdaOptionalAndParamsParameters.cs
@@ -10,9 +10,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		private int total;
 
+		// Roslyn 4.14 puts no ParamArrayAttribute on the lambda's own method, so for a named
+		// delegate type the 'params' cannot be recovered there; newer compilers record it.
 		public ParamsAction ParamsStatementBody()
 		{
+#if ROSLYN5 || !EXPECTED_OUTPUT
 			return (params int[] xs) => {
+#else
+			return (int[] xs) => {
+#endif
 				total += xs.Length;
 				Console.WriteLine(xs.Length);
 			};
@@ -20,7 +26,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 
 		public ParamsAction ParamsExpressionBody()
 		{
+#if ROSLYN5 || !EXPECTED_OUTPUT
 			return (params int[] xs) => Console.WriteLine(xs.Length);
+#else
+			return (int[] xs) => Console.WriteLine(xs.Length);
+#endif
 		}
 
 		public OptionalFunc OptionalStatementBody()
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoLambdaOptionalAndParamsParameters.Expected.cs b/ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoLambdaOptionalAndParamsParameters.Expected.cs
index 988cf2535..c2394285a 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoLambdaOptionalAndParamsParameters.Expected.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Ugly/NoLambdaOptionalAndParamsParameters.Expected.cs
@@ -1,5 +1,4 @@
 using System;
-using System.Runtime.InteropServices;
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 {
@@ -13,7 +12,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 
 		public ParamsAction ParamsLambda()
 		{
-			return ([ParamArray] int[] xs) => {
+			return (int[] xs) => {
 				total += xs.Length;
 				Console.WriteLine(xs.Length);
 			};
@@ -21,7 +20,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Ugly
 
 		public OptionalFunc OptionalLambda()
 		{
-			return ([Optional][DefaultParameterValue(5)] int x) => x * 2;
+			return (int x) => x * 2;
 		}
 	}
 }
diff --git a/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs b/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
index 6cae69619..18f1208cf 100644
--- a/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
+++ b/ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs
@@ -2543,13 +2543,16 @@ namespace ICSharpCode.Decompiler.CSharp
 			AnonymousMethodExpression ame = new AnonymousMethodExpression();
 			ame.IsAsync = function.IsAsync;
 			ame.Parameters.AddRange(MakeParameters(function.Parameters, function));
-			// 'params' and parameter default values are part of the delegate's signature, and it
-			// is the delegate's Invoke method that call sites bind against. An anonymous function's
-			// own method does not reliably carry them - Roslyn 4.14 emits no ParamArrayAttribute
-			// there - so take them from Invoke, keeping declaration and call sites consistent.
-			var invokeParameters = delegateType.GetDelegateInvokeMethod()?.Parameters;
-			if (invokeParameters?.Count == ame.Parameters.Count)
-			{
+			if (delegateType.IsAnonymousDelegate()
+				&& delegateType.GetDelegateInvokeMethod()?.Parameters is { } invokeParameters
+				&& invokeParameters.Count == ame.Parameters.Count)
+			{
+				// A compiler-synthesized delegate type exists only because the lambda declared
+				// 'params' or a parameter default value, and the lambda's own declaration is the
+				// only way to spell that type. Roslyn 4.14 does not put a ParamArrayAttribute on
+				// the lambda's method, so take both from the delegate's Invoke method, which
+				// always carries them. Named delegate types are left alone: there the lambda's
+				// own metadata decides, and a plain parameter list is valid either way.
 				int parameterIndex = 0;
 				foreach (var pd in ame.Parameters)
 				{
@@ -2611,29 +2614,14 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (!settings.LambdaOptionalAndParamsParameters || ame.Parameters.Any(p => p.Type is null))
 			{
 				// 'params' and parameter default values are only legal on the explicitly typed
-				// parameter list of a lambda, and only since C# 12. Elsewhere, downgrade them to
-				// their underlying metadata attributes so the information is not lost.
+				// parameter list of a lambda, and only since C# 12. There is no other spelling:
+				// [ParamArray] cannot be written explicitly in any language version, and attributes
+				// on lambda parameters need C# 10. Drop them; the delegate's Invoke method still
+				// carries them at every call site.
 				foreach (var p in ame.Parameters)
 				{
-					if (p.IsParams)
-					{
-						p.IsParams = false;
-						p.Attributes.Add(new AttributeSection(new Syntax.Attribute {
-							Type = astBuilder.ConvertAttributeType(compilation.FindType(KnownAttribute.ParamArray))
-						}));
-					}
-					if (p.DefaultExpression is { } defaultValue)
-					{
-						defaultValue.Detach();
-						p.Attributes.Add(new AttributeSection(new Syntax.Attribute {
-							Type = astBuilder.ConvertAttributeType(compilation.FindType(KnownAttribute.Optional))
-						}));
-						var defaultParameterValue = new Syntax.Attribute {
-							Type = astBuilder.ConvertAttributeType(compilation.FindType(KnownAttribute.DefaultParameterValue))
-						};
-						defaultParameterValue.Arguments.Add(defaultValue);
-						p.Attributes.Add(new AttributeSection(defaultParameterValue));
-					}
+					p.IsParams = false;
+					p.DefaultExpression = null;
 				}
 			}
 			bool isLambda = false;
diff --git a/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs b/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
index edf305c5a..753f317db 100644
--- a/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
+++ b/ICSharpCode.Decompiler/CSharp/RequiredNamespaceCollector.cs
@@ -144,13 +144,6 @@ namespace ICSharpCode.Decompiler.CSharp
 						{
 							HandleAttributes(param.GetAttributes());
 							CollectNamespacesForTypeReference(param.Type);
-							if (param.IsOptional)
-							{
-								// ExpressionBuilder.TranslateFunction may downgrade a default value
-								// on an anonymous-function parameter to [Optional] and
-								// [DefaultParameterValue(...)]; keep their namespace in the superset.
-								namespaces.Add(KnownAttribute.DefaultParameterValue.GetTypeName().Namespace);
-							}
 						}
 						HandleTypeParameters(partMethod.TypeParameters);
 						HandleOverrides(part.GetMethodImplementations(module.metadata), module);