Restore resource include validation

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/GroupTransformers/XamlMergeResourceGroupTransformer.cs

@@ -24,6 +24,7 @@ internal class XamlMergeResourceGroupTransformer : IXamlAstGroupTransformer
 
         var mergeResourceIncludeType = context.GetAvaloniaTypes().MergeResourceInclude;
         var mergeSourceNodes = new List<XamlPropertyAssignmentNode>();
+        var mergedResourceWasAdded = false;
         foreach (var manipulationNode in resourceDictionaryManipulation.Children.ToArray())
         {
             void ProcessXamlPropertyAssignmentNode(XamlManipulationGroupNode parent, XamlPropertyAssignmentNode assignmentNode)
@@ -37,7 +38,8 @@ internal class XamlMergeResourceGroupTransformer : IXamlAstGroupTransformer
                             && objectInitialization.Manipulation is XamlPropertyAssignmentNode sourceAssignmentNode)
                         {
                             parent.Children.Remove(assignmentNode);
-                            mergeSourceNodes.Add(sourceAssignmentNode);   
+                            mergeSourceNodes.Add(sourceAssignmentNode);
+                            mergedResourceWasAdded = true;
                         }
                         else
                         {
@@ -46,6 +48,12 @@ internal class XamlMergeResourceGroupTransformer : IXamlAstGroupTransformer
                                 valueNode);
                         }
                     }
+                    else if (mergeSourceNodes.Any())
+                    {
+                        throw new XamlDocumentParseException(context.CurrentDocument,
+                            "MergeResourceInclude should always be included last when mixing with other dictionaries inside of the ResourceDictionary.MergedDictionaries.",
+                            valueNode);
+                    }
                 }
             }
             

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

@@ -83,6 +83,34 @@ public class MergeResourceIncludeTests
         Assert.ThrowsAny<XmlException>(() => AvaloniaRuntimeXamlLoader.LoadGroup(documents));
     }
     
+    [Fact]
+    public void MergeResourceInclude_Is_Allowed_After_ResourceInclude()
+    {
+        var documents = new[]
+        {
+            new RuntimeXamlLoaderDocument(new Uri("avares://Tests/Resources1.xaml"), @"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <SolidColorBrush x:Key='brush1'>Red</SolidColorBrush>
+</ResourceDictionary>"),
+            new RuntimeXamlLoaderDocument(new Uri("avares://Tests/Resources2.xaml"), @"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <SolidColorBrush x:Key='brush2'>Blue</SolidColorBrush>
+</ResourceDictionary>"),
+            new RuntimeXamlLoaderDocument(@"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <ResourceDictionary.MergedDictionaries>
+        <ResourceInclude Source='avares://Tests/Resources2.xaml'/>
+        <MergeResourceInclude Source='avares://Tests/Resources1.xaml'/>
+    </ResourceDictionary.MergedDictionaries>
+</ResourceDictionary>")
+        };
+        
+        AvaloniaRuntimeXamlLoader.LoadGroup(documents);
+    }
+    
     [Fact]
     public void MergeResourceInclude_Works_With_Multiple_Resources()
     {