Handle XamlWriter.Save failing on non-public internal types (#780)

When WPF's XamlWriter.Save encounters non-public types like System.Windows.Baml2006.KeyRecord (common in .NET 11 with  merged ResourceDictionaries and DataTemplates), it throws InvalidOperationException. Catch that in ToXamlString and fall back to producing only the PNG snapshot instead of crashing the test.

Author: SimonCropp

src/Directory.Build.props

@@ -3,7 +3,7 @@
   <PropertyGroup>
     <LangVersion>preview</LangVersion>
     <NoWarn>CS1591;CS0649;CS8632;CA1416;NU1608;NU1109</NoWarn>
-    <Version>4.2.0</Version>
+    <Version>4.2.1</Version>
     <AssemblyVersion>1.0.0</AssemblyVersion>
     <LangVersion>preview</LangVersion>
     <PackageTags>Xaml, Wpf, Verify</PackageTags>

src/Directory.Packages.props

@@ -4,21 +4,21 @@
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
   </PropertyGroup>
   <ItemGroup>
-    <PackageVersion Include="MarkdownSnippets.MsBuild" Version="28.1.0" />
+    <PackageVersion Include="MarkdownSnippets.MsBuild" Version="28.2.0" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
     <PackageVersion Include="NUnit" Version="4.5.1" />
     <PackageVersion Include="NUnit3TestAdapter" Version="6.2.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.172" />
     <PackageVersion Include="Shipwreck.Phash.Bitmaps" Version="0.5.0" />
     <PackageVersion Include="System.Drawing.Common" Version="10.0.6" />
-    <PackageVersion Include="Verify" Version="31.15.0" />
+    <PackageVersion Include="Verify" Version="31.16.1" />
     <PackageVersion Include="Verify.DiffPlex" Version="3.1.2" />
-    <PackageVersion Include="Verify.NUnit" Version="31.15.0" />
+    <PackageVersion Include="Verify.NUnit" Version="31.16.1" />
     <PackageVersion Include="Verify.Phash" Version="3.1.0" />
     <PackageVersion Include="Microsoft.Sbom.Targets" Version="4.1.5" />
     <PackageVersion Include="Argon" Version="0.33.5" />
-    <PackageVersion Include="DiffEngine" Version="19.1.2" />
-    <PackageVersion Include="EmptyFiles" Version="8.17.2" />
+    <PackageVersion Include="DiffEngine" Version="19.1.3" />
+    <PackageVersion Include="EmptyFiles" Version="8.18.1" />
     <PackageVersion Include="SimpleInfoName" Version="3.2.0" />
   </ItemGroup>
 </Project>

src/Tests/DataTemplateWindow.xaml

@@ -0,0 +1,23 @@
+<Window x:Class="Tests.DataTemplateWindow"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Height="200"
+        Width="300"
+        Title="DataTemplateWindow">
+  <Window.Resources>
+    <ResourceDictionary>
+      <ResourceDictionary.MergedDictionaries>
+        <ResourceDictionary>
+          <DataTemplate DataType="{x:Type TabItem}">
+            <TextBlock Text="{Binding Header}" />
+          </DataTemplate>
+        </ResourceDictionary>
+      </ResourceDictionary.MergedDictionaries>
+    </ResourceDictionary>
+  </Window.Resources>
+  <TabControl>
+    <TabItem Header="Tab1">
+      <TextBlock Text="Content" />
+    </TabItem>
+  </TabControl>
+</Window>

src/Tests/DataTemplateWindow.xaml.cs

@@ -0,0 +1,9 @@
+using System.Windows;
+
+namespace Tests;
+
+public partial class DataTemplateWindow : Window
+{
+    public DataTemplateWindow() =>
+        InitializeComponent();
+}

src/Tests/TheTests.DataTemplateWindow.verified.png

@@ -0,0 +1,15 @@
+<DataTemplateWindow
+  Title="DataTemplateWindow"
+  Width="300"
+  Height="200"
+  xmlns="clr-namespace:Tests;assembly=Tests"
+  xmlns:av="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+  <av:TabControl>
+    <av:TabItem
+      IsSelected="True"
+      Header="Tab1">
+      <av:TextBlock
+        Text="Content" />
+    </av:TabItem>
+  </av:TabControl>
+</DataTemplateWindow>

src/Tests/TheTests.DataTemplateWindow.verified.xml

@@ -37,4 +37,10 @@ public async Task UserControl()
 
     #endregion
 
+    [Test]
+    public async Task DataTemplateWindow()
+    {
+        var window = new DataTemplateWindow();
+        await Verify(window);
+    }
 }

src/Tests/TheTests.cs

@@ -1,4 +1,4 @@
-using System.Windows;
+using System.Windows;
 
 using VerifyTests.Xaml;
 
@@ -25,21 +25,26 @@ public static void Initialize()
     static ConversionResult ElementToImage(FrameworkElement element, IReadOnlyDictionary<string, object> context)
     {
         var pngStream = WpfUtils.ScreenCapture(element);
-        return new(
-            null,
-            [
-                new("xml", element.ToXamlString()),
-                new("png", pngStream)
-            ]);
+        var xaml = element.ToXamlString();
+        var targets = new List<Target> { new("png", pngStream) };
+        if (xaml != null)
+        {
+            targets.Insert(0, new("xml", xaml));
+        }
+
+        return new(null, targets);
     }
 
     static ConversionResult WindowToImage(Window window, IReadOnlyDictionary<string, object> context)
     {
         var pngStream = WpfUtils.ScreenCapture(window);
-        return new(null,
-            [
-                new("xml", window.ToXamlString()),
-                new("png", pngStream)
-            ]);
+        var xaml = window.ToXamlString();
+        var targets = new List<Target> { new("png", pngStream) };
+        if (xaml != null)
+        {
+            targets.Insert(0, new("xml", xaml));
+        }
+
+        return new(null, targets);
     }
-}
+}

src/Verify.Xaml/VerifyXaml.cs

@@ -19,23 +19,33 @@ static class WpfUtils
             NewLineOnAttributes = true,
         };
 
-    public static string ToXamlString(this FrameworkElement element)
+    public static string? ToXamlString(this FrameworkElement element)
     {
-        var document = new XDocument();
-        using (var writerToDoc = document.CreateWriter())
+        try
         {
-            XamlWriter.Save(element, writerToDoc);
-        }
+            var document = new XDocument();
+            using (var writerToDoc = document.CreateWriter())
+            {
+                XamlWriter.Save(element, writerToDoc);
+            }
 
-        Purge(document);
+            Purge(document);
 
-        var builder = new StringBuilder();
-        using (var writerToString = XmlWriter.Create(builder, writerSettings))
+            var builder = new StringBuilder();
+            using (var writerToString = XmlWriter.Create(builder, writerSettings))
+            {
+                document.WriteTo(writerToString);
+            }
+
+            return builder.ToString();
+        }
+        catch (InvalidOperationException)
         {
-            document.WriteTo(writerToString);
+            // XamlWriter.Save cannot serialize non-public types that may appear
+            // in the visual tree (e.g. System.Windows.Baml2006.KeyRecord).
+            // Return null so callers can still produce the PNG snapshot.
+            return null;
         }
-
-        return builder.ToString();
     }
 
     public static Stream ScreenCapture(FrameworkElement element)
@@ -99,4 +109,4 @@ static void Purge(XDocument document)
             }
         }
     }
-}
+}