Log errors for failed conversions.

src/Avalonia.Base/Data/Core/BindingExpression.cs

@@ -498,7 +498,7 @@ internal class BindingExpression : IObservable<object?>,
             else if (_targetTypeConverter is not null && value is not null)
             {
                 // Otherwise, if we have a target type converter, convert the value to the target type.
-                value = UpdateAndUnwrap(ConvertFrom(_targetTypeConverter, value, false), ref notification);
+                value = UpdateAndUnwrap(ConvertFrom(_targetTypeConverter, value), ref notification);
             }
         }
 
@@ -574,7 +574,7 @@ internal class BindingExpression : IObservable<object?>,
         return AvaloniaProperty.UnsetValue;
    }
 
-    private object? ConvertFrom(TargetTypeConverter? converter, object value, bool isFallback)
+    private object? ConvertFrom(TargetTypeConverter? converter, object value)
     {
         if (converter is null || _targetProperty is null)
             return value;
@@ -586,10 +586,12 @@ internal class BindingExpression : IObservable<object?>,
 
         var valueString = value?.ToString() ?? "(null)";
         var valueTypeName = value?.GetType().FullName ?? "null";
-        var fallbackMessage = isFallback ? " fallback value " : " ";
-        var ex = new InvalidCastException(
-            $"Cannot convert{fallbackMessage}'{valueString}' ({valueTypeName}) to '{targetType}'.");
-        return new BindingNotification(ex, BindingErrorType.Error);
+        var message = $"Cannot convert '{valueString}' ({valueTypeName}) to '{targetType}'.";
+
+        if (ShouldLogError(out var target))
+            Log(target, message, LogEventLevel.Warning);
+
+        return new BindingNotification(new InvalidCastException(message), BindingErrorType.Error);
     }
 
     private static object? UpdateAndUnwrap(object? value, ref BindingNotification? notification)

tests/Avalonia.Markup.UnitTests/Data/BindingTests_Logging.cs

@@ -121,52 +121,6 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
-        public class NamedElement
-        {
-            [Fact]
-            public void Should_Log_NameScope_Not_Found()
-            {
-                var target = new Decorator { };
-                var root = new TestRoot(target);
-                var binding = new Binding("#source") { TypeResolver = ResolveType };
-
-                using (AssertLog(target, binding.Path, "NameScope not found.", "#source"))
-                {
-                    target.Bind(Control.TagProperty, binding);
-                }
-            }
-
-            [Fact]
-            public void Should_Not_Log_Element_Property_Null_For_Unrooted_Control()
-            {
-                var ns = new NameScope();
-                var source = new Canvas { Name = "source" };
-                var target = new Decorator { };
-                var binding = new Binding("#source.DataContext.Foo") { TypeResolver = ResolveType, NameScope = new(ns) };
-                var container = new StackPanel 
-                { 
-                    [NameScope.NameScopeProperty] = ns,
-                    Children = { source, target }
-                };
-
-                ns.Register(source.Name, source);
-
-                using (AssertNoLog())
-                {
-                    target.Bind(Control.TagProperty, binding);
-                }
-
-                // Sanity check to that the binding works when rooted: make sure that we're not just testing a broken
-                // binding!
-                using (AssertNoLog())
-                {
-                    var root = new TestRoot(container);
-                    root.DataContext = new { Foo = "foo" };
-                    Assert.Equal("foo", target.Tag);
-                }
-            }
-        }
-
         public class VisualAncestor
         {
             [Fact]
@@ -174,8 +128,8 @@ namespace Avalonia.Markup.UnitTests.Data
             {
                 var target = new Decorator { };
                 var root = new TestRoot(target);
-                var binding = new Binding 
-                { 
+                var binding = new Binding
+                {
                     RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor)
                     {
                         AncestorType = typeof(TextBlock),
@@ -230,6 +184,72 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
+        public class NamedElement
+        {
+            [Fact]
+            public void Should_Log_NameScope_Not_Found()
+            {
+                var target = new Decorator { };
+                var root = new TestRoot(target);
+                var binding = new Binding("#source") { TypeResolver = ResolveType };
+
+                using (AssertLog(target, binding.Path, "NameScope not found.", "#source"))
+                {
+                    target.Bind(Control.TagProperty, binding);
+                }
+            }
+
+            [Fact]
+            public void Should_Not_Log_Element_Property_Null_For_Unrooted_Control()
+            {
+                var ns = new NameScope();
+                var source = new Canvas { Name = "source" };
+                var target = new Decorator { };
+                var binding = new Binding("#source.DataContext.Foo") { TypeResolver = ResolveType, NameScope = new(ns) };
+                var container = new StackPanel 
+                { 
+                    [NameScope.NameScopeProperty] = ns,
+                    Children = { source, target }
+                };
+
+                ns.Register(source.Name, source);
+
+                using (AssertNoLog())
+                {
+                    target.Bind(Control.TagProperty, binding);
+                }
+
+                // Sanity check to that the binding works when rooted: make sure that we're not just testing a broken
+                // binding!
+                using (AssertNoLog())
+                {
+                    var root = new TestRoot(container);
+                    root.DataContext = new { Foo = "foo" };
+                    Assert.Equal("foo", target.Tag);
+                }
+            }
+        }
+
+        public class Converter
+        {
+            [Fact]
+            public void Should_Log_Error_For_Unconvertible_Type()
+            {
+                var target = new Decorator { DataContext = new { Foo = new System.Version() } };
+                var root = new TestRoot(target);
+                var binding = new Binding("Foo");
+
+                using (AssertLog(
+                    target,
+                    binding.Path,
+                    "Cannot convert '0.0' (System.Version) to 'Avalonia.Thickness'.",
+                    property: Control.MarginProperty))
+                {
+                    target.Bind(Control.MarginProperty, binding);
+                }
+            }
+        }
+
         public class Fallback
         {
             [Theory]