[X] do not apply Bindings if DataType doesnt match (#22056)

* [X] do not apply Bindings if DataType doesnt match

Make sure the behavior of bindings is consistent accross implementations
(compiled/not compiled)

fixes a bunch of issues

* fixes

* send binding diagnostic

src/Controls/src/Core/Binding.cs

@@ -4,6 +4,7 @@
 using System.ComponentModel;
 using System.Globalization;
 using System.Reflection;
+using Microsoft.Maui.Controls.Xaml.Diagnostics;
 
 namespace Microsoft.Maui.Controls
 {
@@ -105,6 +106,8 @@ public string UpdateSourceEventName
 			}
 		}
 
+		internal Type DataType { get; set; }
+
 		internal override void Apply(bool fromTarget)
 		{
 			base.Apply(fromTarget);
@@ -120,7 +123,14 @@ internal override void Apply(object context, BindableObject bindObj, BindablePro
 			object src = _source;
 			var isApplied = IsApplied;
 
-			base.Apply(src ?? context, bindObj, targetProperty, fromBindingContextChanged, specificity);
+			var bindingContext = src ?? Context ?? context;
+			if (DataType != null && bindingContext != null && !DataType.IsAssignableFrom(bindingContext.GetType()))
+			{
+				BindingDiagnostics.SendBindingFailure(this, "Binding", "Mismatch between the specified x:DataType and the current binding context");
+				bindingContext = null;
+			}
+
+			base.Apply(bindingContext, bindObj, targetProperty, fromBindingContextChanged, specificity);
 
 			if (src != null && isApplied && fromBindingContextChanged)
 				return;
@@ -131,7 +141,6 @@ internal override void Apply(object context, BindableObject bindObj, BindablePro
 			}
 			else
 			{
-				object bindingContext = src ?? Context ?? context;
 				if (_expression == null)
 					_expression = new BindingExpression(this, SelfPath);
 				_expression.Apply(bindingContext, bindObj, targetProperty, specificity);

src/Controls/src/Core/IXamlDataTypeProvider.cs

@@ -0,0 +1,5 @@
+namespace Microsoft.Maui.Controls.Xaml;
+interface IXamlDataTypeProvider
+{
+	string BindingDataType { get; }
+}

src/Controls/src/Xaml/MarkupExtensions/BindingExtension.cs

@@ -1,11 +1,11 @@
 using System;
 using System.ComponentModel;
+using System.Linq.Expressions;
 using Microsoft.Maui.Controls.Internals;
 
 namespace Microsoft.Maui.Controls.Xaml
 {
 	[ContentProperty(nameof(Path))]
-	[AcceptEmptyServiceProvider]
 	public sealed class BindingExtension : IMarkupExtension<BindingBase>
 	{
 		public string Path { get; set; } = Binding.SelfPath;
@@ -21,13 +21,22 @@ public sealed class BindingExtension : IMarkupExtension<BindingBase>
 
 		BindingBase IMarkupExtension<BindingBase>.ProvideValue(IServiceProvider serviceProvider)
 		{
-			if (TypedBinding == null)
+			if (TypedBinding is null) {
+				Type bindingXDataType = null;
+				if ((serviceProvider.GetService(typeof(IXamlTypeResolver)) is IXamlTypeResolver typeResolver)
+					&& (serviceProvider.GetService(typeof(IXamlDataTypeProvider)) is IXamlDataTypeProvider dataTypeProvider)
+					&& dataTypeProvider.BindingDataType != null)
+				{
+					typeResolver.TryResolve(dataTypeProvider.BindingDataType, out bindingXDataType);
+				}
 				return new Binding(Path, Mode, Converter, ConverterParameter, StringFormat, Source)
 				{
 					UpdateSourceEventName = UpdateSourceEventName,
 					FallbackValue = FallbackValue,
 					TargetNullValue = TargetNullValue,
+					DataType = bindingXDataType,
 				};
+			}
 
 			TypedBinding.Mode = Mode;
 			TypedBinding.Converter = Converter;

src/Controls/src/Xaml/XamlServiceProvider.cs

@@ -27,6 +27,9 @@ internal XamlServiceProvider(INode node, HydrationContext context)
 				IXmlLineInfoProvider = new XmlLineInfoProvider(xmlLineInfo);
 
 			IValueConverterProvider = new ValueConverterProvider();
+
+			if (node is IElementNode elementNode)
+				Add(typeof(IXamlDataTypeProvider), new XamlDataTypeProvider(elementNode));
 		}
 
 		public XamlServiceProvider() => IValueConverterProvider = new ValueConverterProvider();
@@ -262,4 +265,57 @@ public string LookupNamespace(string prefix)
 		public string LookupPrefix(string namespaceName) => throw new NotImplementedException();
 		public void Add(string prefix, string ns) => namespaces.Add(prefix, ns);
 	}
+
+	class XamlDataTypeProvider : IXamlDataTypeProvider
+	{
+		public XamlDataTypeProvider(IElementNode node)
+		{
+			static IElementNode GetParent(IElementNode node)
+			{
+				return node switch
+				{
+					{ Parent: ListNode { Parent: IElementNode parentNode } } => parentNode,
+					{ Parent: IElementNode parentNode } => parentNode,
+					_ => null,
+				};
+			}
+
+			static bool IsBindingContextBinding(IElementNode node)
+			{
+				if (   ApplyPropertiesVisitor.TryGetPropertyName(node, node.Parent, out XmlName name)
+					&& name.NamespaceURI == ""
+					&& name.LocalName == nameof(BindableObject.BindingContext))
+					return true;
+				return false;
+			}
+
+			INode dataTypeNode = null;
+			IElementNode n = node as IElementNode;
+
+			// Special handling for BindingContext={Binding ...}
+			// The order of checks is:
+			// - x:DataType on the binding itself
+			// - SKIP looking for x:DataType on the parent
+			// - continue looking for x:DataType on the parent's parent...
+			IElementNode skipNode = null;
+			if (IsBindingContextBinding(node))
+			{
+				skipNode = GetParent(node);
+			}
+
+			while (n != null)
+			{
+				if (n != skipNode && n.Properties.TryGetValue(XmlName.xDataType, out dataTypeNode))
+				{
+					break;
+				}
+
+				n = GetParent(n);
+			}
+			if (dataTypeNode is ValueNode valueNode)
+				BindingDataType = valueNode.Value as string;
+
+		}
+		public string BindingDataType { get; }
+	}
 }

src/Controls/tests/Xaml.UnitTests/BindingsCompiler.xaml.cs

@@ -28,10 +28,9 @@ public class Tests
 		{
 			[SetUp] public void Setup() => DispatcherProvider.SetCurrent(new DispatcherProviderStub());
 			[TearDown] public void TearDown() => DispatcherProvider.SetCurrent(null);
-
-			[TestCase(false)]
-			[TestCase(true)]
-			public void Test(bool useCompiledXaml)
+
+			[Test] 
+			public void TestCompiledBindings([Values(false, true)]bool useCompiledXaml)
 			{
 				if (useCompiledXaml)
 					MockCompiler.Compile(typeof(BindingsCompiler));
@@ -113,6 +112,13 @@ public void Test(bool useCompiledXaml)
 				layout.BindingContext = new object();
 				Assert.AreEqual(null, layout.label0.Text);
 			}
+
+			[Test] 
+			public void BindingsNotAppliedWithWrongContext([Values(false, true)]bool useCompiledXaml)
+			{
+				var page = new BindingsCompiler(useCompiledXaml) { BindingContext = new {Text="Foo"} };
+				Assert.AreEqual(null, page.label0.Text);
+			}
 		}
 	}