Compare assemblies tab actually renders the diff tree

The "Compare..." right-click flow opened a tab that came up blank because
two stacked bugs masked each other.

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

ILSpy.Tests/Compare/CompareViewRenderTests.cs

@@ -0,0 +1,109 @@
+// 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.Linq;
+using System.Threading.Tasks;
+
+using Avalonia.Controls;
+using Avalonia.Headless.NUnit;
+using Avalonia.VisualTree;
+
+using AwesomeAssertions;
+
+using ICSharpCode.ILSpyX.TreeView;
+
+using ILSpy;
+using ILSpy.AppEnv;
+using ILSpy.Commands;
+using ILSpy.Compare;
+using ILSpy.TreeNodes;
+using ILSpy.ViewModels;
+using ILSpy.Views;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests.Compare;
+
+/// <summary>
+/// End-to-end render checks for the assembly-comparison view. Pure context-menu and engine
+/// tests don't catch the case where the new tab opens but renders empty because the
+/// hierarchical DataGrid was never wired with a model — these tests mount the view, walk
+/// the visual tree, and assert rows materialise.
+/// </summary>
+[TestFixture]
+public class CompareViewRenderTests
+{
+	[AvaloniaTest]
+	public async Task CompareView_Binds_A_HierarchicalModel_When_Opened()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 2);
+
+		var entry = AppComposition.Current.GetExport<global::ILSpy.ContextMenuEntryRegistry>()
+			.Entries.Single(e => e.Metadata.Header == "Compare...").Value;
+		var assemblies = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
+			.Where(a => a.IsLoadedAsValidAssembly).Take(2).ToList();
+		var nodes = assemblies.Select(a =>
+			(SharpTreeNode)vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(a.ShortName)).ToArray();
+
+		entry.Execute(new TextViewContext { SelectedTreeNodes = nodes });
+
+		var view = await window.WaitForComponent<CompareView>();
+		var grid = await view.WaitForComponent<DataGrid>();
+
+		await Waiters.WaitForAsync(() => grid.HierarchicalModel != null,
+			description: "CompareView must populate the DataGrid's HierarchicalModel — without it "
+				+ "the hierarchical column is rendered over an empty source and the tab shows nothing");
+		grid.HierarchicalModel.Should().NotBeNull();
+	}
+
+	[AvaloniaTest]
+	public async Task CompareView_Renders_Rows_For_The_Merged_Tree()
+	{
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 2);
+
+		var entry = AppComposition.Current.GetExport<global::ILSpy.ContextMenuEntryRegistry>()
+			.Entries.Single(e => e.Metadata.Header == "Compare...").Value;
+		var assemblies = vm.AssemblyTreeModel.AssemblyList!.GetAssemblies()
+			.Where(a => a.IsLoadedAsValidAssembly).Take(2).ToList();
+		var nodes = assemblies.Select(a =>
+			(SharpTreeNode)vm.AssemblyTreeModel.FindNode<AssemblyTreeNode>(a.ShortName)).ToArray();
+
+		entry.Execute(new TextViewContext { SelectedTreeNodes = nodes });
+
+		var view = await window.WaitForComponent<CompareView>();
+		var grid = await view.WaitForComponent<DataGrid>();
+
+		// Default state: ShowIdentical=false, so we expect at least one diff row to be present
+		// when comparing two non-empty assemblies against themselves *or* against a sibling.
+		// (Comparing an assembly with itself yields zero diffs; if the two assemblies are the
+		// same instance we toggle ShowIdentical to force the tree to surface rows.)
+		var page = (CompareTabPageModel)view.DataContext!;
+		if (ReferenceEquals(page.LeftAssembly, page.RightAssembly))
+			page.ShowIdentical = true;
+
+		await Waiters.WaitForAsync(
+			() => grid.GetVisualDescendants().OfType<DataGridRow>().Any(),
+			description: "the diff grid must render at least one row once the merged tree is bound");
+	}
+}

ILSpy/Views/CompareView.axaml

@@ -22,8 +22,7 @@
 		          HierarchicalRowsEnabled="True"
 		          GridLinesVisibility="None"
 		          CanUserResizeColumns="False"
-		          SelectionMode="Single"
-		          ItemsSource="{Binding RootEntry.Children}">
+		          SelectionMode="Single">
 			<DataGrid.Columns>
 				<DataGridHierarchicalColumn Header="Diff" Width="*"
 				                            IsReadOnly="True"

ILSpy/Views/CompareView.axaml.cs

@@ -16,15 +16,79 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
+using System.ComponentModel;
+
 using Avalonia.Controls;
+using Avalonia.Controls.DataGridHierarchical;
+
+using ICSharpCode.ILSpyX.TreeView;
 
 namespace ILSpy.Compare
 {
 	public partial class CompareView : UserControl
 	{
+		CompareTabPageModel? boundModel;
+
 		public CompareView()
 		{
 			InitializeComponent();
 		}
+
+		protected override void OnDataContextChanged(EventArgs e)
+		{
+			base.OnDataContextChanged(e);
+			DetachFromModel();
+			if (DataContext is CompareTabPageModel model)
+				AttachToModel(model);
+		}
+
+		void AttachToModel(CompareTabPageModel model)
+		{
+			boundModel = model;
+			model.PropertyChanged += OnModelPropertyChanged;
+			Rebind();
+		}
+
+		void DetachFromModel()
+		{
+			if (boundModel == null)
+				return;
+			boundModel.PropertyChanged -= OnModelPropertyChanged;
+			boundModel = null;
+		}
+
+		void OnModelPropertyChanged(object? sender, PropertyChangedEventArgs e)
+		{
+			// ShowIdentical toggles which rows are filtered out — re-running the cascade
+			// through Rebind makes the DataGrid re-evaluate visibility on the next layout
+			// pass without us having to walk the tree by hand.
+			if (e.PropertyName == nameof(CompareTabPageModel.ShowIdentical))
+				Rebind();
+		}
+
+		void Rebind()
+		{
+			var root = boundModel?.RootEntry;
+			if (root == null)
+				return;
+			root.EnsureChildrenFiltered();
+
+			var options = new HierarchicalOptions<SharpTreeNode> {
+				ChildrenSelector = node => {
+					if (node is TreeNodes.ILSpyTreeNode ilspy)
+						ilspy.EnsureChildrenFiltered();
+					else
+						node.EnsureLazyChildren();
+					return node.Children;
+				},
+				IsLeafSelector = node => !node.ShowExpander,
+				VirtualizeChildren = false,
+				IsExpandedPropertyPath = nameof(SharpTreeNode.IsExpanded),
+			};
+			var model = new HierarchicalModel<SharpTreeNode>(options);
+			model.SetRoots(root.Children);
+			DiffGrid.HierarchicalModel = model;
+		}
 	}
 }

ILSpy/Views/ContentTabPageView.axaml

@@ -6,6 +6,7 @@
              xmlns:textView="using:ILSpy.TextView"
              xmlns:metaViews="using:ILSpy.Views"
              xmlns:options="using:ILSpy.Options"
+             xmlns:compare="using:ILSpy.Compare"
              mc:Ignorable="d" d:DesignWidth="600" d:DesignHeight="400"
              x:Class="ILSpy.Views.ContentTabPageView"
              x:DataType="vm:ContentTabPage">
@@ -19,6 +20,7 @@
 		<textView:DecompilerTextView Name="DecompilerView" />
 		<metaViews:MetadataTablePage Name="MetadataView" />
 		<options:OptionsPageView Name="OptionsView" />
+		<compare:CompareView Name="CompareViewSlot" />
 	</Panel>
 
 </UserControl>

ILSpy/Views/ContentTabPageView.axaml.cs

@@ -21,6 +21,7 @@ using System.ComponentModel;
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
 
+using ILSpy.Compare;
 using ILSpy.Options;
 using ILSpy.TextView;
 using ILSpy.ViewModels;
@@ -39,6 +40,7 @@ namespace ILSpy.Views
 		DecompilerTextView decompilerView = null!;
 		MetadataTablePage metadataView = null!;
 		OptionsPageView optionsView = null!;
+		CompareView compareView = null!;
 
 		public ContentTabPageView()
 		{
@@ -46,6 +48,7 @@ namespace ILSpy.Views
 			decompilerView = this.FindControl<DecompilerTextView>("DecompilerView")!;
 			metadataView = this.FindControl<MetadataTablePage>("MetadataView")!;
 			optionsView = this.FindControl<OptionsPageView>("OptionsView")!;
+			compareView = this.FindControl<CompareView>("CompareViewSlot")!;
 			DataContextChanged += (_, _) => RebindPage();
 		}
 
@@ -103,6 +106,17 @@ namespace ILSpy.Views
 				optionsView.IsVisible = false;
 				optionsView.DataContext = null;
 			}
+
+			if (content is CompareTabPageModel compare)
+			{
+				compareView.DataContext = compare;
+				compareView.IsVisible = true;
+			}
+			else
+			{
+				compareView.IsVisible = false;
+				compareView.DataContext = null;
+			}
 		}
 	}
 }