Merge pull request #4150 from MarchingCube/fix-validation-not-found-member

Fix validation of not found members

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

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using Avalonia.Data.Core.Plugins;
 
 namespace Avalonia.Data.Core
@@ -49,6 +50,18 @@ namespace Avalonia.Data.Core
 
             var accessor = plugin?.Start(reference, PropertyName);
 
+            // We need to handle accessor fallback before handling validation. Validators do not support null accessors.
+            if (accessor == null)
+            {
+                reference.TryGetTarget(out object instance);
+
+                var message = $"Could not find a matching property accessor for '{PropertyName}' on '{instance}'";
+
+                var exception = new MissingMemberException(message);
+
+                accessor = new PropertyError(new BindingNotification(exception, BindingErrorType.Error));
+            }
+
             if (_enableValidation && Next == null)
             {
                 foreach (var validator in ExpressionObserver.DataValidators)
@@ -60,15 +73,9 @@ namespace Avalonia.Data.Core
                 }
             }
 
-            if (accessor == null)
+            if (accessor is null)
             {
-                reference.TryGetTarget(out object instance);
+                throw new AvaloniaInternalException("Data validators must return non-null accessor.");
-
-                var message = $"Could not find a matching property accessor for '{PropertyName}' on '{instance}'";
-
-                var exception = new MissingMemberException(message);
-
-                accessor = new PropertyError(new BindingNotification(exception, BindingErrorType.Error));
             }
 
             _accessor = accessor;

tests/Avalonia.Base.UnitTests/Data/Core/ExpressionObserverTests_Property.cs

@@ -578,6 +578,28 @@ namespace Avalonia.Base.UnitTests.Data.Core
             Assert.True(true);
         }
 
+        [Fact]
+        public void Should_Not_Throw_Exception_When_Enabling_Data_Validation_On_Missing_Member()
+        {
+            var source = new Class1();
+            var target = new PropertyAccessorNode("NotFound", true);
+
+            target.Target = new WeakReference<object>(source);
+
+            var result = new List<object>();
+
+            target.Subscribe(x => result.Add(x));
+
+            Assert.Equal(
+                new object[]
+                {
+                    new BindingNotification(
+                        new MissingMemberException("Could not find a matching property accessor for 'NotFound' on 'Avalonia.Base.UnitTests.Data.Core.ExpressionObserverTests_Property+Class1'"),
+                        BindingErrorType.Error),
+                },
+                result);
+        }
+
         private interface INext
         {
             int PropertyChangedSubscriptionCount { get; }