Add failing unit tests.

tests/Avalonia.Markup.Xaml.UnitTests/MarkupExtensions/ResourceIncludeTests.cs

@@ -44,6 +44,36 @@ namespace Avalonia.Markup.Xaml.UnitTests.MakrupExtensions
                 }
             }
 
+            [Fact]
+            public void Missing_ResourceKey_In_ResourceInclude_Does_Not_Cause_StackOverflow()
+            {
+                var styleXaml = @"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <StaticResource x:Key='brush' ResourceKey='missing' />
+</ResourceDictionary>";
+
+                using (StartWithResources(("test:style.xaml", styleXaml)))
+                {
+                    var xaml = @"
+<Application xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Application.Resources>
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceInclude Source='test:style.xaml'/>
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
+    </Application.Resources>
+</Application>";
+
+                    var loader = new AvaloniaXamlLoader();
+                    var app = Application.Current;
+
+                    loader.Load(xaml, null, app);
+                }
+            }
+
             private IDisposable StartWithResources(params (string, string)[] assets)
             {
                 var assetLoader = new MockAssetLoader(assets);

tests/Avalonia.Markup.Xaml.UnitTests/StyleIncludeTests.cs

@@ -0,0 +1,44 @@
+using System;
+using Avalonia.UnitTests;
+using Xunit;
+
+namespace Avalonia.Markup.Xaml.UnitTests
+{
+    public class StyleIncludeTests : XamlTestBase
+    {
+        [Fact]
+        public void Missing_ResourceKey_In_StyleInclude_Does_Not_Cause_StackOverflow()
+        {
+            var styleXaml = @"
+<Style xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Style.Resources>
+        <StaticResource x:Key='brush' ResourceKey='missing' />
+    </Style.Resources>
+</Style>";
+
+            using (StartWithResources(("test:style.xaml", styleXaml)))
+            {
+                var xaml = @"
+<Application xmlns='https://github.com/avaloniaui'
+             xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Application.Styles>
+        <StyleInclude Source='test:style.xaml'/>
+    </Application.Styles>
+</Application>";
+
+                var loader = new AvaloniaXamlLoader();
+                var app = Application.Current;
+
+                loader.Load(xaml, null, app);
+            }
+        }
+
+        private IDisposable StartWithResources(params (string, string)[] assets)
+        {
+            var assetLoader = new MockAssetLoader(assets);
+            var services = new TestServices(assetLoader: assetLoader);
+            return UnitTestApplication.Start(services);
+        }
+    }
+}