Inherit DataType from template without hardcoding on specific DataTemplate implementation

src/Avalonia.Base/Metadata/TemplateDataTypeAttribute.cs

@@ -0,0 +1,9 @@
+using System;
+
+namespace Avalonia.Metadata;
+
+[AttributeUsage(AttributeTargets.Property)]
+public class TemplateDataTypeAttribute : Attribute
+{
+    
+}

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlDataContextTypeTransformer.cs

@@ -49,6 +49,8 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                     }
                     else if (child is XamlPropertyAssignmentNode pa)
                     {
+                        var templateDataTypeAttribute = context.GetAvaloniaTypes().TemplateDataTypeAttribute;
+                        
                         if (pa.Property.Name == "DataContext"
                             && pa.Property.DeclaringType.Equals(context.GetAvaloniaTypes().StyledElement)
                             && pa.Values[0] is XamlMarkupExtensionNode ext
@@ -56,8 +58,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
                         {
                             inferredDataContextTypeNode = ParseDataContext(context, on, obj);
                         }
-                        else if(context.GetAvaloniaTypes().DataTemplate.IsAssignableFrom(on.Type.GetClrType())
-                            && pa.Property.Name == "DataType"
+                        else if(pa.Property.CustomAttributes.Any(a => a.Type == templateDataTypeAttribute)
                             && pa.Values[0] is XamlTypeExtensionNode dataTypeNode)
                         {
                             inferredDataContextTypeNode = new AvaloniaXamlIlDataContextTypeMetadataNode(on, dataTypeNode.Value.GetClrType());

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs

@@ -26,6 +26,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlType Transitions { get; }
         public IXamlType AssignBindingAttribute { get; }
         public IXamlType DependsOnAttribute { get; }
+        public IXamlType TemplateDataTypeAttribute { get; }
         public IXamlType UnsetValueType { get; }
         public IXamlType StyledElement { get; }
         public IXamlType IStyledElement { get; }
@@ -112,6 +113,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             Transitions = cfg.TypeSystem.GetType("Avalonia.Animation.Transitions");
             AssignBindingAttribute = cfg.TypeSystem.GetType("Avalonia.Data.AssignBindingAttribute");
             DependsOnAttribute = cfg.TypeSystem.GetType("Avalonia.Metadata.DependsOnAttribute");
+            TemplateDataTypeAttribute = cfg.TypeSystem.GetType("Avalonia.Metadata.TemplateDataTypeAttribute");
             AvaloniaObjectBindMethod = AvaloniaObjectExtensions.FindMethod("Bind", IDisposable, false, IAvaloniaObject,
                 AvaloniaProperty,
                 IBinding, cfg.WellKnownTypes.Object);

src/Markup/Avalonia.Markup.Xaml/Templates/DataTemplate.cs

@@ -7,6 +7,7 @@ namespace Avalonia.Markup.Xaml.Templates
 {
     public class DataTemplate : IRecyclingDataTemplate
     {
+        [TemplateDataType]
         public Type DataType { get; set; }
 
         [Content]

src/Markup/Avalonia.Markup.Xaml/Templates/TreeDataTemplate.cs

@@ -11,6 +11,7 @@ namespace Avalonia.Markup.Xaml.Templates
 {
     public class TreeDataTemplate : ITreeDataTemplate
     {
+        [TemplateDataType]
         public Type DataType { get; set; }
 
         [Content]

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

@@ -8,11 +8,14 @@ using System.Text;
 using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Controls.Presenters;
+using Avalonia.Controls.Templates;
 using Avalonia.Data.Converters;
 using Avalonia.Data.Core;
 using Avalonia.Input;
 using Avalonia.Markup.Data;
+using Avalonia.Markup.Xaml.Templates;
 using Avalonia.Media;
+using Avalonia.Metadata;
 using Avalonia.UnitTests;
 using XamlX;
 using Xunit;
@@ -455,7 +458,106 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
                 ThrowsXamlTransformException(() => AvaloniaRuntimeXamlLoader.Load(xaml));
             }
         }
+        
+        [Fact]
+        public void IgnoresDataTemplateTypeFromDataTypePropertyIfXDataTypeDefined()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <Window.DataTemplates>
+        <DataTemplate DataType='local:TestDataContextBaseClass' x:DataType='local:TestDataContext'>
+            <TextBlock Text='{CompiledBinding StringProperty}' Name='textBlock' />
+        </DataTemplate>
+    </Window.DataTemplates>
+    <ContentControl x:DataType='local:TestDataContext' Name='target' Content='{CompiledBinding}' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var target = window.FindControl<ContentControl>("target");
+
+                var dataContext = new TestDataContext();
+
+                dataContext.StringProperty = "Initial Value";
+
+                window.DataContext = dataContext;
 
+                window.ApplyTemplate();
+                target.ApplyTemplate();
+                ((ContentPresenter)target.Presenter).UpdateChild();
+
+                Assert.Equal(dataContext.StringProperty, ((TextBlock)target.Presenter.Child).Text);
+            }
+        }
+        
+        [Fact]
+        public void InfersCustomDataTemplateBasedOnAttribute()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <Window.DataTemplates>
+        <local:CustomDataTemplate FancyDataType='local:TestDataContext'>
+            <TextBlock Text='{CompiledBinding StringProperty}' Name='textBlock' />
+        </local:CustomDataTemplate>
+    </Window.DataTemplates>
+    <ContentControl x:DataType='local:TestDataContext' Name='target' Content='{CompiledBinding}' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var target = window.FindControl<ContentControl>("target");
+
+                var dataContext = new TestDataContext();
+
+                dataContext.StringProperty = "Initial Value";
+
+                window.DataContext = dataContext;
+
+                window.ApplyTemplate();
+                target.ApplyTemplate();
+                ((ContentPresenter)target.Presenter).UpdateChild();
+
+                Assert.Equal(dataContext.StringProperty, ((TextBlock)target.Presenter.Child).Text);
+            }
+        }
+
+        [Fact]
+        public void InfersCustomDataTemplateBasedOnAttributeFromBaseClass()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'>
+    <Window.DataTemplates>
+        <local:CustomDataTemplateInherit FancyDataType='local:TestDataContext'>
+            <TextBlock Text='{CompiledBinding StringProperty}' Name='textBlock' />
+        </local:CustomDataTemplateInherit>
+    </Window.DataTemplates>
+    <ContentControl x:DataType='local:TestDataContext' Name='target' Content='{CompiledBinding}' />
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var target = window.FindControl<ContentControl>("target");
+
+                var dataContext = new TestDataContext();
+
+                dataContext.StringProperty = "Initial Value";
+
+                window.DataContext = dataContext;
+
+                window.ApplyTemplate();
+                target.ApplyTemplate();
+                ((ContentPresenter)target.Presenter).UpdateChild();
+
+                Assert.Equal(dataContext.StringProperty, ((TextBlock)target.Presenter.Child).Text);
+            }
+        }
+        
         [Fact]
         public void ResolvesElementNameBinding()
         {
@@ -1324,7 +1426,9 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
         public string StringProperty { get; set; }
     }
 
-    public class TestDataContext : IHasPropertyDerived
+    public class TestDataContextBaseClass {}
+    
+    public class TestDataContext : TestDataContextBaseClass, IHasPropertyDerived
     {
         public string StringProperty { get; set; }
 
@@ -1413,4 +1517,20 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
             return ReferenceEquals(null, parameter) == false;
         }
     }
+
+    public class CustomDataTemplate : IDataTemplate
+    {
+        [TemplateDataType]
+        public Type FancyDataType { get; set; }
+
+        [Content]
+        [TemplateContent]
+        public object Content { get; set; }
+
+        public bool Match(object data) => FancyDataType.IsInstanceOfType(data);
+
+        public IControl Build(object data) => TemplateContent.Load(Content)?.Control;
+    }
+    
+    public class CustomDataTemplateInherit : CustomDataTemplate { }
 }