Added failing tests for #1701.

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_DataValidation.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Data;
+using Avalonia.UnitTests;
 using Xunit;
 
 namespace Avalonia.Base.UnitTests
@@ -34,10 +35,10 @@ namespace Avalonia.Base.UnitTests
         {
             var target = new Class1();
 
-            target.SetValue(Class1.ValidatedDirectProperty, new BindingNotification(6));
-            target.SetValue(Class1.ValidatedDirectProperty, new BindingNotification(new Exception(), BindingErrorType.Error));
-            target.SetValue(Class1.ValidatedDirectProperty, new BindingNotification(new Exception(), BindingErrorType.DataValidationError));
-            target.SetValue(Class1.ValidatedDirectProperty, new BindingNotification(7));
+            target.SetValue(Class1.ValidatedDirectIntProperty, new BindingNotification(6));
+            target.SetValue(Class1.ValidatedDirectIntProperty, new BindingNotification(new Exception(), BindingErrorType.Error));
+            target.SetValue(Class1.ValidatedDirectIntProperty, new BindingNotification(new Exception(), BindingErrorType.DataValidationError));
+            target.SetValue(Class1.ValidatedDirectIntProperty, new BindingNotification(7));
 
             Assert.Equal(
                 new[]
@@ -73,7 +74,7 @@ namespace Avalonia.Base.UnitTests
             var source = new Subject<object>();
             var target = new Class1
             {
-                [!Class1.ValidatedDirectProperty] = source.ToBinding(),
+                [!Class1.ValidatedDirectIntProperty] = source.ToBinding(),
             };
 
             source.OnNext(new BindingNotification(6));
@@ -92,6 +93,30 @@ namespace Avalonia.Base.UnitTests
                 target.Notifications.AsEnumerable());
         }
 
+        [Fact]
+        public void Bound_Validated_Direct_String_Property_Can_Be_Set_To_Null()
+        {
+            var source = new ViewModel
+            {
+                StringValue = "foo",
+            };
+
+            var target = new Class1
+            {
+                [!Class1.ValidatedDirectStringProperty] = new Binding
+                {
+                    Path = nameof(ViewModel.StringValue),
+                    Source = source,
+                },
+            };
+
+            Assert.Equal("foo", target.ValidatedDirectString);
+
+            source.StringValue = null;
+
+            Assert.Null(target.ValidatedDirectString);
+        }
+
         private class Class1 : AvaloniaObject
         {
             public static readonly StyledProperty<int> NonValidatedProperty =
@@ -104,15 +129,23 @@ namespace Avalonia.Base.UnitTests
                     o => o.NonValidatedDirect,
                     (o, v) => o.NonValidatedDirect = v);
 
-            public static readonly DirectProperty<Class1, int> ValidatedDirectProperty =
+            public static readonly DirectProperty<Class1, int> ValidatedDirectIntProperty =
                 AvaloniaProperty.RegisterDirect<Class1, int>(
-                    nameof(ValidatedDirect),
-                    o => o.ValidatedDirect,
-                    (o, v) => o.ValidatedDirect = v,
+                    nameof(ValidatedDirectInt),
+                    o => o.ValidatedDirectInt,
+                    (o, v) => o.ValidatedDirectInt = v,
+                    enableDataValidation: true);
+
+            public static readonly DirectProperty<Class1, string> ValidatedDirectStringProperty =
+                AvaloniaProperty.RegisterDirect<Class1, string>(
+                    nameof(ValidatedDirectString),
+                    o => o.ValidatedDirectString,
+                    (o, v) => o.ValidatedDirectString = v,
                     enableDataValidation: true);
 
             private int _nonValidatedDirect;
-            private int _direct;
+            private int _directInt;
+            private string _directString;
 
             public int NonValidated
             {
@@ -122,14 +155,20 @@ namespace Avalonia.Base.UnitTests
 
             public int NonValidatedDirect
             {
-                get { return _direct; }
+                get { return _directInt; }
                 set { SetAndRaise(NonValidatedDirectProperty, ref _nonValidatedDirect, value); }
             }
 
-            public int ValidatedDirect
+            public int ValidatedDirectInt
+            {
+                get { return _directInt; }
+                set { SetAndRaise(ValidatedDirectIntProperty, ref _directInt, value); }
+            }
+
+            public string ValidatedDirectString
             {
-                get { return _direct; }
-                set { SetAndRaise(ValidatedDirectProperty, ref _direct, value); }
+                get { return _directString; }
+                set { SetAndRaise(ValidatedDirectStringProperty, ref _directString, value); }
             }
 
             public IList<BindingNotification> Notifications { get; } = new List<BindingNotification>();
@@ -139,5 +178,16 @@ namespace Avalonia.Base.UnitTests
                 Notifications.Add(notification);
             }
         }
+
+        public class ViewModel : NotifyingBase
+        {
+            private string _stringValue;
+
+            public string StringValue
+            {
+                get { return _stringValue; }
+                set { _stringValue = value; RaisePropertyChanged(); }
+            }
+        }
     }
 }

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -444,6 +444,22 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Setting_Bound_Text_To_Null_Works()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var source = new Class1 { Bar = "bar" };
+                var target = new TextBox { DataContext = source };
+
+                target.Bind(TextBox.TextProperty, new Binding("Bar"));
+
+                Assert.Equal("bar", target.Text);
+                source.Bar = null;
+                Assert.Null(target.Text);
+            }
+        }
+
         private static TestServices FocusServices => TestServices.MockThreadingInterface.With(
             focusManager: new FocusManager(),
             keyboardDevice: () => new KeyboardDevice(),
@@ -492,12 +508,19 @@ namespace Avalonia.Controls.UnitTests
         private class Class1 : NotifyingBase
         {
             private int _foo;
+            private string _bar;
 
             public int Foo
             {
                 get { return _foo; }
                 set { _foo = value; RaisePropertyChanged(); }
             }
+
+            public string Bar
+            {
+                get { return _bar; }
+                set { _bar = value; RaisePropertyChanged(); }
+            }
         }
     }
 }