Validate DataTemplates

src/Avalonia.Controls/Templates/DataTemplates.cs

@@ -1,3 +1,4 @@
+using System;
 using Avalonia.Collections;
 
 namespace Avalonia.Controls.Templates
@@ -13,6 +14,22 @@ namespace Avalonia.Controls.Templates
         public DataTemplates()
         {
             ResetBehavior = ResetBehavior.Remove;
+            
+            Validate += ValidateDataTemplate;
+        }
+
+        private static void ValidateDataTemplate(IDataTemplate template)
+        {
+            var valid = template switch
+            {
+                ITypedDataTemplate typed => typed.DataType is not null,
+                _ => true
+            };
+            
+            if (!valid)
+            {
+                throw new InvalidOperationException("DataTemplate inside of DataTemplates must have a DataType set. Set DataType property or use ItemTemplate with single template instead.");
+            }
         }
     }
 }

src/Avalonia.Controls/Templates/ITypedDataTemplate.cs

@@ -0,0 +1,10 @@
+using System;
+using Avalonia.Metadata;
+
+namespace Avalonia.Controls.Templates;
+
+public interface ITypedDataTemplate : IDataTemplate
+{
+    [DataType]
+    Type? DataType { get; }
+}

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

@@ -5,7 +5,7 @@ using Avalonia.Metadata;
 
 namespace Avalonia.Markup.Xaml.Templates
 {
-    public class DataTemplate : IRecyclingDataTemplate
+    public class DataTemplate : IRecyclingDataTemplate, ITypedDataTemplate
     {
         [DataType]
         public Type DataType { get; set; }

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

@@ -9,7 +9,7 @@ using Avalonia.Metadata;
 
 namespace Avalonia.Markup.Xaml.Templates
 {
-    public class TreeDataTemplate : ITreeDataTemplate
+    public class TreeDataTemplate : ITreeDataTemplate, ITypedDataTemplate
     {
         [DataType]
         public Type DataType { get; set; }

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

@@ -413,11 +413,11 @@ namespace Avalonia.Markup.Xaml.UnitTests.MarkupExtensions
         xmlns:local='clr-namespace:Avalonia.Markup.Xaml.UnitTests.MarkupExtensions;assembly=Avalonia.Markup.Xaml.UnitTests'
         x:DataType='local:TestDataContext'>
     <ItemsControl Items='{CompiledBinding ListProperty}' Name='target'>
-        <ItemsControl.DataTemplates>
+        <ItemsControl.ItemTemplate>
             <DataTemplate>
                 <TextBlock Text='{CompiledBinding}' Name='textBlock' />
             </DataTemplate>
-        </ItemsControl.DataTemplates>
+        </ItemsControl.ItemTemplate>
     </ItemsControl>
 </Window>";
                 var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);

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

@@ -74,18 +74,18 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
 <Window xmlns='https://github.com/avaloniaui'>
     <DockPanel>
         <TabStrip Name='strip' DockPanel.Dock='Top' Items='{Binding Items}' SelectedIndex='0'>
-          <TabStrip.DataTemplates>
+          <TabStrip.ItemTemplate>
             <DataTemplate>
               <TextBlock Text='{Binding Header}'/>
             </DataTemplate>
-          </TabStrip.DataTemplates>
+          </TabStrip.ItemTemplate>
         </TabStrip>
         <Carousel Name='carousel' Items='{Binding Items}' SelectedIndex='{Binding #strip.SelectedIndex}'>
-          <Carousel.DataTemplates>
+          <Carousel.ItemTemplate>
             <DataTemplate>
               <TextBlock Text='{Binding Detail}'/>
             </DataTemplate>
-          </Carousel.DataTemplates>
+          </Carousel.ItemTemplate>
         </Carousel>
     </DockPanel>
 </Window>";

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

@@ -1,3 +1,4 @@
+using System;
 using Avalonia.Controls;
 using Avalonia.Controls.Presenters;
 using Avalonia.UnitTests;
@@ -132,5 +133,25 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
                 Assert.Same(viewModel.Child.Child, canvas.DataContext);
             }
         }
+        
+        [Fact]
+        public void DataTemplates_Without_Type_Should_Throw()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:sys='clr-namespace:System;assembly=netstandard'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <Window.DataTemplates>
+        <DataTemplate>
+            <Canvas Name='foo'/>
+        </DataTemplate>
+    </Window.DataTemplates>
+    <ContentControl Name='target' Content='Foo'/>
+</Window>";
+                Assert.Throws<InvalidOperationException>(() => (Window)AvaloniaRuntimeXamlLoader.Load(xaml));
+            }
+        }
     }
 }

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

@@ -14,7 +14,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
         {
             using (UnitTestApplication.Start(TestServices.MockPlatformWrapper))
             {
-                var xaml = "<DataTemplates xmlns='https://github.com/avaloniaui'><TreeDataTemplate ItemsSource='{Binding}'/></DataTemplates>";
+                var xaml = "<DataTemplates xmlns='https://github.com/avaloniaui'><TreeDataTemplate DataType='Control' ItemsSource='{Binding}'/></DataTemplates>";
                 var templates = (DataTemplates)AvaloniaRuntimeXamlLoader.Load(xaml);
                 var template = (TreeDataTemplate)(templates.First());