Allow nested BindingNotifications.

src/Avalonia.Base/Data/BindingNotification.cs

@@ -57,7 +57,6 @@ public class BindingNotification
         /// <param name="value">The binding value.</param>
         public BindingNotification(object? value)
         {
-            Debug.Assert(value is not BindingNotification);
             _value = value;
         }
 

src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs

@@ -187,13 +187,20 @@ protected void SetValue(object? valueOrNotification)
             else if (notification.ErrorType == BindingErrorType.DataValidationError)
             {
                 if (notification.HasValue)
-                    SetValue(notification.Value, notification.Error);
+                {
+                    if (notification.Value is BindingNotification n)
+                        SetValue(n);
+                    else
+                        SetValue(notification.Value, notification.Error);
+                }
                 else
+                {
                     SetDataValidationError(notification.Error!);
+                }
             }
             else
             {
-                SetValue(notification.Value, null);
+                SetValue(notification.Value);
             }
         }
         else

tests/Avalonia.Base.UnitTests/Data/Core/BindingExpressionTests.DataValidation.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.ComponentModel.DataAnnotations;
 using Avalonia.Data;
 using Avalonia.UnitTests;
 using Xunit;
@@ -271,6 +272,39 @@ public void Updates_Data_Validation_For_Null_Value_In_Property_Chain()
         GC.KeepAlive(data);
     }
 
+    [Fact]
+    public void Updates_Data_Validation_For_Required_DataAnnotation()
+    {
+        var data = new DataAnnotationsViewModel();
+        var target = CreateTargetWithSource(
+            data,
+            o => o.RequiredString,
+            enableDataValidation: true);
+
+        AssertBindingError(
+            target,
+            TargetClass.StringProperty,
+            new DataValidationException("String is required!"),
+            BindingErrorType.DataValidationError);
+    }
+
+    [Fact]
+    public void Handles_Indei_And_DataAnnotations_On_Same_Class()
+    {
+        // Issue #15201
+        var data = new IndeiDataAnnotationsViewModel();
+        var target = CreateTargetWithSource(
+            data,
+            o => o.RequiredString,
+            enableDataValidation: true);
+
+        AssertBindingError(
+            target,
+            TargetClass.StringProperty,
+            new DataValidationException("String is required!"),
+            BindingErrorType.DataValidationError);
+    }
+
     public class ExceptionViewModel : NotifyingBase
     {
         private int _mustBePositive;
@@ -341,6 +375,42 @@ public IndeiViewModel? Inner
         public override IEnumerable? GetErrors(string propertyName) => null;
     }
 
+    private class DataAnnotationsViewModel : NotifyingBase
+    {
+        private string? _requiredString;
+
+        [Required(ErrorMessage = "String is required!")]
+        public string? RequiredString
+        {
+            get { return _requiredString; }
+            set { _requiredString = value; RaisePropertyChanged(); }
+        }
+    }
+
+    private class IndeiDataAnnotationsViewModel : IndeiBase
+    {
+        private string? _requiredString;
+
+        [Required(ErrorMessage = "String is required!")]
+        public string? RequiredString
+        {
+            get { return _requiredString; }
+            set { _requiredString = value; RaisePropertyChanged(); }
+        }
+
+        public override bool HasErrors => RequiredString is null;
+
+        public override IEnumerable? GetErrors(string propertyName)
+        {
+            if (propertyName == nameof(RequiredString) && RequiredString is null)
+            {
+                return new[] { "String is required!" };
+            }
+
+            return null;
+        }
+    }   
+
     private static void AssertNoError(TargetClass target, AvaloniaProperty property)
     {
         Assert.False(target.BindingNotifications.TryGetValue(property, out var notification));