ExtensionTreeNode for C# 14 extension blocks

Each `extension<T>(ReceiverType source) { ... }` block inside a static class
now appears as its own tree node sibling to the regular methods/properties.
Expanding the node lists the block's declared methods + properties as
PropertyTreeNode / MethodTreeNode children sorted by name.

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy/Languages/CSharpLanguage.cs

@@ -198,6 +198,49 @@ namespace ILSpy.Languages
 			}
 		}
 
+		/// <summary>
+		/// Decompiles a C# 14 explicit-extension declaration block back to source. Used by
+		/// <see cref="TreeNodes.ExtensionTreeNode"/> when the user activates an extension
+		/// container; the type-level overload is what the node actually calls. The method
+		/// and property overloads exist so individual members can also be decompiled in
+		/// isolation (e.g. when navigated to by analyzer results in a future commit).
+		/// </summary>
+		public void DecompileExtension(ITypeDefinition extension, ITextOutput output, DecompilationOptions options)
+		{
+			MetadataFile assembly = extension.ParentModule!.MetadataFile!;
+			CSharpDecompiler decompiler = CreateDecompiler(assembly, options);
+			AddReferenceAssemblyWarningMessage(assembly, output);
+			AddReferenceWarningMessage(assembly, output);
+			WriteCommentLine(output, assembly.FullName);
+			WriteCommentLine(output, TypeToString(extension,
+				ConversionFlags.UseFullyQualifiedTypeNames | ConversionFlags.UseFullyQualifiedEntityNames | ConversionFlags.SupportExtensionDeclarations));
+			WriteCode(output, options.DecompilerSettings, decompiler.DecompileExtension(extension.MetadataToken), decompiler.TypeSystem);
+		}
+
+		public void DecompileExtension(IMethod extension, ITextOutput output, DecompilationOptions options)
+		{
+			MetadataFile assembly = extension.ParentModule!.MetadataFile!;
+			CSharpDecompiler decompiler = CreateDecompiler(assembly, options);
+			AddReferenceAssemblyWarningMessage(assembly, output);
+			AddReferenceWarningMessage(assembly, output);
+			WriteCommentLine(output, assembly.FullName);
+			WriteCommentLine(output, TypeToString(extension.DeclaringType,
+				ConversionFlags.UseFullyQualifiedTypeNames | ConversionFlags.UseFullyQualifiedEntityNames | ConversionFlags.SupportExtensionDeclarations));
+			WriteCode(output, options.DecompilerSettings, decompiler.DecompileExtension(extension.MetadataToken), decompiler.TypeSystem);
+		}
+
+		public void DecompileExtension(IProperty extension, ITextOutput output, DecompilationOptions options)
+		{
+			MetadataFile assembly = extension.ParentModule!.MetadataFile!;
+			CSharpDecompiler decompiler = CreateDecompiler(assembly, options);
+			AddReferenceAssemblyWarningMessage(assembly, output);
+			AddReferenceWarningMessage(assembly, output);
+			WriteCommentLine(output, assembly.FullName);
+			WriteCommentLine(output, TypeToString(extension.DeclaringType,
+				ConversionFlags.UseFullyQualifiedTypeNames | ConversionFlags.UseFullyQualifiedEntityNames | ConversionFlags.SupportExtensionDeclarations));
+			WriteCode(output, options.DecompilerSettings, decompiler.DecompileExtension(extension.MetadataToken), decompiler.TypeSystem);
+		}
+
 		public override void DecompileEvent(IEvent ev, ITextOutput output, DecompilationOptions options)
 		{
 			MetadataFile assembly = ev.ParentModule!.MetadataFile!;

ILSpy/TreeNodes/ExtensionTreeNode.cs

@@ -0,0 +1,124 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// 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 System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+
+using ICSharpCode.Decompiler;
+using ICSharpCode.Decompiler.Output;
+using ICSharpCode.Decompiler.TypeSystem;
+
+using ILSpy.AppEnv;
+using ILSpy.Images;
+using ILSpy.Languages;
+
+using SRM = System.Reflection.Metadata;
+
+namespace ILSpy.TreeNodes
+{
+	/// <summary>
+	/// Tree-node container for a single C# 14 explicit-extension declaration block
+	/// — the new <c>extension&lt;T&gt;(ReceiverType source) { ... }</c> syntax. Each
+	/// block surfaces under its containing static class as a sibling of the regular
+	/// methods/properties; the block's own members (methods + properties) appear as
+	/// children of this node when expanded.
+	/// </summary>
+	internal sealed class ExtensionTreeNode : ILSpyTreeNode
+	{
+		public ExtensionTreeNode(
+			ITypeDefinition typeDefinition,
+			(IMethod Marker, IReadOnlyList<ITypeParameter> TypeParameters) extensionGroup,
+			AssemblyTreeNode parentAssemblyNode)
+		{
+			this.ParentAssemblyNode = parentAssemblyNode ?? throw new ArgumentNullException(nameof(parentAssemblyNode));
+			this.ContainerTypeDefinition = typeDefinition ?? throw new ArgumentNullException(nameof(typeDefinition));
+			this.MarkerMethod = extensionGroup.Marker ?? throw new ArgumentNullException(nameof(extensionGroup.Marker));
+			this.TypeParameters = extensionGroup.TypeParameters ?? throw new ArgumentNullException(nameof(extensionGroup.TypeParameters));
+			this.LazyLoading = true;
+		}
+
+		public ITypeDefinition ContainerTypeDefinition { get; }
+		public IMethod MarkerMethod { get; }
+		public IReadOnlyList<ITypeParameter> TypeParameters { get; }
+		public AssemblyTreeNode ParentAssemblyNode { get; }
+
+		public override object Icon => Images.Images.GetIcon(Images.Images.Class, AccessOverlayIcon.Public, isStatic: false, isExtension: true);
+
+		public override object Text
+			=> Language.TypeToString(GetTypeDefinition(), ConversionFlags.SupportExtensionDeclarations);
+
+		public override object NavigationText
+			=> Language.TypeToString(GetTypeDefinition(),
+				ConversionFlags.UseFullyQualifiedTypeNames
+				| ConversionFlags.UseFullyQualifiedEntityNames
+				| ConversionFlags.SupportExtensionDeclarations);
+
+		// Avalonia uses the constructor-time marker reference directly. WPF re-resolves
+		// through GetTypeSystemWithCurrentOptionsOrNull so language-version flips refresh
+		// the display string immediately; the Avalonia port doesn't yet expose that helper
+		// (see `extension-methods-tree` follow-ups in the tracker).
+		ITypeDefinition GetTypeDefinition() => MarkerMethod.DeclaringTypeDefinition!;
+
+		protected override void LoadChildren()
+		{
+			var extensionInfo = ContainerTypeDefinition.ExtensionInfo!;
+			var members = extensionInfo.GetMembersOfGroup(MarkerMethod).ToList();
+
+			foreach (var property in members.OfType<IProperty>().OrderBy(p => p.Name, NaturalStringComparer.Instance))
+				this.Children.Add(new PropertyTreeNode(property));
+			foreach (var method in members.OfType<IMethod>().OrderBy(m => m.Name, NaturalStringComparer.Instance))
+			{
+				if (method.MetadataToken.IsNil)
+					continue;
+				this.Children.Add(new MethodTreeNode(method));
+			}
+		}
+
+		public override FilterResult Filter(LanguageSettings settings)
+		{
+			if (Language is not CSharpLanguage)
+				return FilterResult.Hidden;
+
+			var decompilerSettings = TryGetDecompilerSettings();
+			if (decompilerSettings != null && !decompilerSettings.ExtensionMembers)
+				return FilterResult.Hidden;
+
+			return base.Filter(settings);
+		}
+
+		static ICSharpCode.Decompiler.DecompilerSettings? TryGetDecompilerSettings()
+		{
+			try
+			{
+				return AppComposition.Current.GetExport<global::ILSpy.SettingsService>().DecompilerSettings.Clone();
+			}
+			catch
+			{
+				return null;
+			}
+		}
+
+		public override void Decompile(Language language, ITextOutput output, DecompilationOptions options)
+		{
+			Debug.Assert(language is CSharpLanguage);
+			((CSharpLanguage)language).DecompileExtension(GetTypeDefinition(), output, options);
+		}
+	}
+}

ILSpy/TreeNodes/TypeTreeNode.cs

@@ -168,6 +168,20 @@ namespace ILSpy.TreeNodes
 				Children.Add(new TypeTreeNode((TypeDefinitionHandle)nestedType.MetadataToken, module));
 			}
 
+			// C# 14 explicit-extension declaration blocks surface as their own container nodes
+			// — one per (marker, type-params) tuple inside this static class. Filter()-hidden
+			// when the user runs an older C# language version that doesn't recognise them, or
+			// when DecompilerSettings.ExtensionMembers is off.
+			if (typeDef.ExtensionInfo is { } ext)
+			{
+				var parentAssemblyNode = this.Ancestors().OfType<AssemblyTreeNode>().FirstOrDefault();
+				if (parentAssemblyNode != null)
+				{
+					foreach (var group in ext.ExtensionGroups)
+						Children.Add(new ExtensionTreeNode(typeDef, group, parentAssemblyNode));
+				}
+			}
+
 			// Enums look more useful in declaration order than alphabetical.
 			var fields = typeDef.Kind == TypeKind.Enum
 				? typeDef.Fields