Added tests for binding value types to null

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

@@ -648,16 +648,69 @@ namespace Avalonia.Markup.UnitTests.Data
             };
         }
 
+        [Fact]
+        public void Binding_Producing_Default_Value_Should_Result_In_Correct_Priority()
+        {
+            var defaultValue = StyledPropertyClass.NullableDoubleProperty.GetDefaultValue(typeof(StyledPropertyClass));
+
+            var vm = new NullableValuesViewModel() { NullableDouble = defaultValue };
+            var target = new StyledPropertyClass();
+
+            target.Bind(StyledPropertyClass.NullableDoubleProperty, new Binding(nameof(NullableValuesViewModel.NullableDouble)) { Source = vm });
+
+            Assert.Equal(BindingPriority.LocalValue, target.GetDiagnosticInternal(StyledPropertyClass.NullableDoubleProperty).Priority);
+            Assert.Equal(defaultValue, target.GetValue(StyledPropertyClass.NullableDoubleProperty));
+        }
+
+        [Fact]
+        public void Binding_Non_Nullable_ValueType_To_Null_Reverts_To_Default_Value()
+        {
+            var source = new NullableValuesViewModel { NullableDouble = 42 };
+            var target = new StyledPropertyClass();
+            var binding = new Binding(nameof(source.NullableDouble)) { Source = source };
+
+            target.Bind(StyledPropertyClass.DoubleValueProperty, binding);
+            Assert.Equal(42, target.DoubleValue);
+
+            source.NullableDouble = null;
+
+            Assert.Equal(12.3, target.DoubleValue);
+        }
+
+        [Fact]
+        public void Binding_Nullable_ValueType_To_Null_Sets_Value_To_Null()
+        {
+            var source = new NullableValuesViewModel { NullableDouble = 42 };
+            var target = new StyledPropertyClass();
+            var binding = new Binding(nameof(source.NullableDouble)) { Source = source };
+
+            target.Bind(StyledPropertyClass.NullableDoubleProperty, binding);
+            Assert.Equal(42, target.NullableDouble);
+
+            source.NullableDouble = null;
+
+            Assert.Null(target.NullableDouble);
+        }
+
         private class StyledPropertyClass : AvaloniaObject
         {
             public static readonly StyledProperty<double> DoubleValueProperty =
-                        AvaloniaProperty.Register<StyledPropertyClass, double>(nameof(DoubleValue));
+                        AvaloniaProperty.Register<StyledPropertyClass, double>(nameof(DoubleValue), 12.3);
 
             public double DoubleValue
             {
                 get { return GetValue(DoubleValueProperty); }
                 set { SetValue(DoubleValueProperty, value); }
             }
+            
+            public static StyledProperty<double?> NullableDoubleProperty = 
+                AvaloniaProperty.Register<StyledPropertyClass, double?>(nameof(NullableDoubleProperty), -1);
+
+            public double? NullableDouble
+            {
+                get => GetValue(NullableDoubleProperty);
+                set => SetValue(NullableDoubleProperty, value);
+            }
         }
 
         private class DirectPropertyClass : AvaloniaObject
@@ -676,6 +729,21 @@ namespace Avalonia.Markup.UnitTests.Data
             }
         }
 
+        private class NullableValuesViewModel : INotifyPropertyChanged
+        {
+            public event PropertyChangedEventHandler PropertyChanged;
+
+            private double? _nullableDouble;
+            public double? NullableDouble
+            {
+                get => _nullableDouble; set
+                {
+                    _nullableDouble = value;
+                    PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(NullableDouble)));
+                }
+            }
+        }
+
         private class TestStackOverflowViewModel : INotifyPropertyChanged
         {
             public int SetterInvokedCount { get; private set; }