Fix ArgumentNullException in case of IValidatableObject returns null (#25693)

* Fix ArgumentNullException in case of IValidatableObject returns null (#25495)

* Skip validatable null check tests on NETFX (#25495)

* Fix an issue with tests ignore

Author: Zhukov Maxim

src/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/Validator.cs

@@ -433,9 +433,12 @@ private static IEnumerable<ValidationError> GetObjectValidationErrors(object ins
             {
                 var results = validatable.Validate(validationContext);
 
-                foreach (var result in results.Where(r => r != ValidationResult.Success))
+                if (results != null)
                 {
-                    errors.Add(new ValidationError(null, instance, result));
+                    foreach (var result in results.Where(r => r != ValidationResult.Success))
+                    {
+                        errors.Add(new ValidationError(null, instance, result));
+                    }
                 }
             }
 

src/System.ComponentModel.Annotations/tests/ValidatorTests.cs

@@ -190,6 +190,26 @@ public IEnumerable<ValidationResult> Validate(ValidationContext validationContex
             }
         }
 
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Null check not present in .NET Framework. See https://github.com/dotnet/corefx/issues/25495")]
+        public void TryValidateObject_IValidatableObject_Null()
+        {
+            var instance = new ValidatableNull();
+            var context = new ValidationContext(instance);
+
+            var results = new List<ValidationResult>();
+            Assert.True(Validator.TryValidateObject(instance, context, results));
+            Assert.Equal(0, results.Count);
+        }
+
+        public class ValidatableNull : IValidatableObject
+        {
+            public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
+            {
+                return null;
+            }
+        }
+
         [Fact]
         public void TryValidateObject_RequiredNonNull_Success()
         {
@@ -318,6 +338,34 @@ public static void ValidateObject_throws_ValidationException_if_all_properties_a
             Assert.Equal(objectToBeValidated, exception.Value);
         }
 
+        [Fact]
+        public void ValidateObject_IValidatableObject_Success()
+        {
+            var instance = new ValidatableSuccess();
+            var context = new ValidationContext(instance);
+
+            Validator.ValidateObject(instance, context);
+        }
+
+        [Fact]
+        public void ValidateObject_IValidatableObject_Error()
+        {
+            var instance = new ValidatableError();
+            var context = new ValidationContext(instance);
+            var exception = Assert.Throws<ValidationException>(
+                () => Validator.ValidateObject(instance, context));
+            Assert.Equal("error", exception.ValidationResult.ErrorMessage);
+        }
+
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Null check not present in .NET Framework. See https://github.com/dotnet/corefx/issues/25495")]
+        public void ValidateObject_IValidatableObject_Null()
+        {
+            var instance = new ValidatableNull();
+            var context = new ValidationContext(instance);
+
+            Validator.ValidateObject(instance, context);
+        }
         #endregion ValidateObject
 
         #region TryValidateProperty