Add ExtensionDeclaration syntax node

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -2417,6 +2417,36 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 			EndNode(enumMemberDeclaration);
 		}
 
+		public virtual void VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration)
+		{
+			StartNode(extensionDeclaration);
+			WriteAttributes(extensionDeclaration.Attributes);
+			WriteModifiers(extensionDeclaration.ModifierTokens);
+			WriteKeyword(ExtensionDeclaration.ExtensionKeywordRole);
+			WriteTypeParameters(extensionDeclaration.TypeParameters);
+			Space(policy.SpaceBeforeMethodDeclarationParentheses);
+			WriteCommaSeparatedListInParenthesis(extensionDeclaration.ReceiverParameters, policy.SpaceWithinMethodDeclarationParentheses);
+			foreach (Constraint constraint in extensionDeclaration.Constraints)
+			{
+				constraint.AcceptVisitor(this);
+			}
+			OpenBrace(policy.ClassBraceStyle);
+			bool first = true;
+			foreach (var member in extensionDeclaration.Members)
+			{
+				if (!first)
+				{
+					for (int i = 0; i < policy.MinimumBlankLinesBetweenMembers; i++)
+						NewLine();
+				}
+				first = false;
+				member.AcceptVisitor(this);
+			}
+			CloseBrace(policy.ClassBraceStyle);
+			NewLine();
+			EndNode(extensionDeclaration);
+		}
+
 		public virtual void VisitEventDeclaration(EventDeclaration eventDeclaration)
 		{
 			StartNode(eventDeclaration);

ICSharpCode.Decompiler/CSharp/Syntax/DepthFirstAstVisitor.cs

@@ -157,6 +157,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			VisitChildren(enumMemberDeclaration);
 		}
 
+		public virtual void VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration)
+		{
+			VisitChildren(extensionDeclaration);
+		}
+
 		public virtual void VisitUsingDeclaration(UsingDeclaration usingDeclaration)
 		{
 			VisitChildren(usingDeclaration);
@@ -840,6 +845,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return VisitChildren(enumMemberDeclaration);
 		}
 
+		public virtual T VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration)
+		{
+			return VisitChildren(extensionDeclaration);
+		}
+
 		public virtual T VisitUsingDeclaration(UsingDeclaration usingDeclaration)
 		{
 			return VisitChildren(usingDeclaration);
@@ -1523,6 +1533,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			return VisitChildren(enumMemberDeclaration, data);
 		}
 
+		public virtual S VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration, T data)
+		{
+			return VisitChildren(extensionDeclaration, data);
+		}
+
 		public virtual S VisitUsingDeclaration(UsingDeclaration usingDeclaration, T data)
 		{
 			return VisitChildren(usingDeclaration, data);

ICSharpCode.Decompiler/CSharp/Syntax/IAstVisitor.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.
 
-
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	/// <summary>
@@ -136,6 +135,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		void VisitVariableInitializer(VariableInitializer variableInitializer);
 		void VisitFixedFieldDeclaration(FixedFieldDeclaration fixedFieldDeclaration);
 		void VisitFixedVariableInitializer(FixedVariableInitializer fixedVariableInitializer);
+		void VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration);
 
 		void VisitSyntaxTree(SyntaxTree syntaxTree);
 		void VisitSimpleType(SimpleType simpleType);
@@ -285,6 +285,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		S VisitVariableInitializer(VariableInitializer variableInitializer);
 		S VisitFixedFieldDeclaration(FixedFieldDeclaration fixedFieldDeclaration);
 		S VisitFixedVariableInitializer(FixedVariableInitializer fixedVariableInitializer);
+		S VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration);
 
 		S VisitSyntaxTree(SyntaxTree syntaxTree);
 		S VisitSimpleType(SimpleType simpleType);
@@ -434,6 +435,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		S VisitVariableInitializer(VariableInitializer variableInitializer, T data);
 		S VisitFixedFieldDeclaration(FixedFieldDeclaration fixedFieldDeclaration, T data);
 		S VisitFixedVariableInitializer(FixedVariableInitializer fixedVariableInitializer, T data);
+		S VisitExtensionDeclaration(ExtensionDeclaration extensionDeclaration, T data);
 
 		S VisitSyntaxTree(SyntaxTree syntaxTree, T data);
 		S VisitSimpleType(SimpleType simpleType, T data);

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/ExtensionDeclaration.cs

@@ -0,0 +1,70 @@
+// Copyright (c) 2025 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
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using ICSharpCode.Decompiler.TypeSystem;
+
+namespace ICSharpCode.Decompiler.CSharp.Syntax
+{
+	public class ExtensionDeclaration : EntityDeclaration
+	{
+		public readonly static TokenRole ExtensionKeywordRole = new TokenRole("extension");
+
+		public override SymbolKind SymbolKind => throw new System.NotImplementedException();
+
+		public AstNodeCollection<TypeParameterDeclaration> TypeParameters {
+			get { return GetChildrenByRole(Roles.TypeParameter); }
+		}
+
+		public AstNodeCollection<ParameterDeclaration> ReceiverParameters {
+			get { return GetChildrenByRole(Roles.Parameter); }
+		}
+
+		public AstNodeCollection<Constraint> Constraints {
+			get { return GetChildrenByRole(Roles.Constraint); }
+		}
+
+		public AstNodeCollection<EntityDeclaration> Members {
+			get { return GetChildrenByRole(Roles.TypeMemberRole); }
+		}
+
+		public ExtensionDeclaration()
+		{
+		}
+
+		public override void AcceptVisitor(IAstVisitor visitor)
+		{
+			visitor.VisitExtensionDeclaration(this);
+		}
+
+		public override T AcceptVisitor<T>(IAstVisitor<T> visitor)
+		{
+			return visitor.VisitExtensionDeclaration(this);
+		}
+
+		public override S AcceptVisitor<T, S>(IAstVisitor<T, S> visitor, T data)
+		{
+			return visitor.VisitExtensionDeclaration(this, data);
+		}
+
+		protected internal override bool DoMatch(AstNode other, PatternMatching.Match match)
+		{
+			var o = other as ExtensionDeclaration;
+			return o != null;
+		}
+	}
+}

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -104,6 +104,7 @@
     <Compile Include="CSharp\CallBuilder.cs" />
     <Compile Include="CSharp\CSharpLanguageVersion.cs" />
     <Compile Include="CSharp\Syntax\Expressions\RecursivePatternExpression.cs" />
+    <Compile Include="CSharp\Syntax\TypeMembers\ExtensionDeclaration.cs" />
     <Compile Include="DecompilationProgress.cs" />
     <Compile Include="Disassembler\IEntityProcessor.cs" />
     <Compile Include="Disassembler\SortByNameProcessor.cs" />

ILSpy/Languages/CSharpHighlightingTokenWriter.cs

@@ -242,6 +242,7 @@ namespace ICSharpCode.ILSpy
 				case "class":
 				case "interface":
 				case "delegate":
+				case "extension":
 					color = referenceTypeKeywordsColor;
 					break;
 				case "record":