Add the ability to correctly determine if a particular field has been validated.

There are several portions of model validation that attempt to avoid
revalidating if a field has been validated. However the behavior of
ModelStateDictionary makes it difficult to distinguish between an
unvalidated field and a field without validation errors. This change
resolves this issue by letting the caller distinguish between the two
cases.

src/Microsoft.AspNet.Mvc.ModelBinding/Binders/MutableObjectModelBinder.cs

@@ -113,7 +113,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 var validationNode = (ModelValidationNode)sender;
                 var modelState = e.ValidationContext.ModelState;
 
-                if (modelState.IsValidField(validationNode.ModelStateKey))
+                if (modelState.IsValidField(validationNode.ModelStateKey) == null)
                 {
                     // TODO: Revive ModelBinderConfig
                     // string errorMessage =  ModelBinderConfig.ValueRequiredErrorMessageProvider(e.ValidationContext, modelMetadata, incomingValue);
@@ -266,7 +266,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             if (value == null)
             {
                 var modelStateKey = dtoResult.ValidationNode.ModelStateKey;
-                if (bindingContext.ModelState.IsValidField(modelStateKey))
+                if (bindingContext.ModelState.IsValidField(modelStateKey) == null)
                 {
                     if (requiredValidator != null)
                     {
@@ -295,7 +295,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                         ex = targetInvocationException.InnerException;
                     }
                     var modelStateKey = dtoResult.ValidationNode.ModelStateKey;
-                    if (bindingContext.ModelState.IsValidField(modelStateKey))
+                    if (bindingContext.ModelState.IsValidField(modelStateKey) == null)
                     {
                         bindingContext.ModelState.AddModelError(modelStateKey, ex);
                     }
@@ -305,7 +305,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             {
                 // trying to set a non-nullable value type to null, need to make sure there's a message
                 var modelStateKey = dtoResult.ValidationNode.ModelStateKey;
-                if (bindingContext.ModelState.IsValidField(modelStateKey))
+                if (bindingContext.ModelState.IsValidField(modelStateKey) == null)
                 {
                     dtoResult.ValidationNode.Validated += CreateNullCheckFailedHandler(propertyMetadata, value);
                 }
@@ -326,6 +326,12 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 bindingContext.ModelState.AddModelError(modelStateKey, validationResult.Message);
                 addedError = true;
             }
+
+            if (!addedError)
+            {
+                bindingContext.ModelState.MarkFieldValid(modelStateKey);
+            }
+
             return addedError;
         }
 

src/Microsoft.AspNet.Mvc.ModelBinding/ModelState.cs

@@ -11,5 +11,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         {
             get { return _errors; }
         }
+
+        public bool? IsValid { get; set; }
     }
 }

src/Microsoft.AspNet.Mvc.ModelBinding/ModelStateDictionary.cs

@@ -44,9 +44,9 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
         #endregion
 
-        public bool IsValid
+        public bool? IsValid
         {
-            get { return Values.All(modelState => modelState.Errors.Count == 0); }
+            get { return GetValidity(_innerDictionary); }
         }
 
         public ModelState this[[NotNull] string key]
@@ -62,25 +62,39 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
         public void AddModelError([NotNull] string key, [NotNull] Exception exception)
         {
-            GetModelStateForKey(key).Errors.Add(exception);
+            var modelState = GetModelStateForKey(key);
+            modelState.IsValid = false;
+            modelState.Errors.Add(exception);
         }
 
         public void AddModelError([NotNull] string key, [NotNull] string errorMessage)
         {
-            GetModelStateForKey(key).Errors.Add(errorMessage);
+            var modelState = GetModelStateForKey(key);
+            modelState.IsValid = false;
+            modelState.Errors.Add(errorMessage);
         }
 
-        public bool IsValidField([NotNull] string key)
+        public bool? IsValidField([NotNull] string key)
         {
-            // if the key is not found in the dictionary, we just say that it's valid (since there are no errors)
+            var entries = DictionaryHelper.FindKeysWithPrefix(this, key);
-            foreach (var entry in DictionaryHelper.FindKeysWithPrefix(_innerDictionary, key))
+            if (!entries.Any())
             {
-                if (entry.Value.Errors.Count != 0)
+                return null;
-                {
-                    return false;
-                }
             }
-            return true;
+
+            return GetValidity(entries);
+        }
+
+        public void MarkFieldValid([NotNull] string key)
+        {
+            var modelState = GetModelStateForKey(key);
+            if (modelState.IsValid == false)
+            {
+                // TODO We should never end up here from our code
+                throw new InvalidOperationException(Resources.Validation_InvalidFieldCannotBeReset);
+            }
+
+            modelState.IsValid = true;
         }
 
         public void Merge(ModelStateDictionary dictionary)
@@ -113,6 +127,25 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return modelState;
         }
 
+        private static bool? GetValidity(IEnumerable<KeyValuePair<string, ModelState>> entries)
+        {
+            var state = true;
+            foreach (var entry in entries)
+            {
+                var entryState = entry.Value.IsValid;
+                if (entryState == null)
+                {
+                    // If any entries of a field is unvalidated, we'll treat the tree as unvalidated.
+                    return null;
+                }
+                else if (!entryState.Value)
+                {
+                    state = false;
+                }
+            }
+            return state;
+        }
+
         #region IDictionary members
         public void Add(KeyValuePair<string, ModelState> item)
         {
@@ -144,7 +177,6 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             _innerDictionary.CopyTo(array, arrayIndex);
         }
 
-        
         public bool Remove(KeyValuePair<string, ModelState> item)
         {
             return _innerDictionary.Remove(item);

src/Microsoft.AspNet.Mvc.ModelBinding/Properties/Resources.Designer.cs

@@ -59,7 +59,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         /// <summary>
-        /// No encoding found for media type formatter '{0}'. There must be at least one supported encoding registered in order for the media type formatter to read or write content.
+        /// No encoding found for input formatter '{0}'. There must be at least one supported encoding registered in order for the formatter to read content.
         /// </summary>
         internal static string MediaTypeFormatterNoEncoding
         {
@@ -67,12 +67,14 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         /// <summary>
-        /// No encoding found for media type formatter '{0}'. There must be at least one supported encoding registered in order for the media type formatter to read or write content.
+        /// No encoding found for input formatter '{0}'. There must be at least one supported encoding registered in order for the formatter to read content.
         /// </summary>
         internal static string FormatMediaTypeFormatterNoEncoding(object p0)
         {
             return string.Format(CultureInfo.CurrentCulture, GetString("MediaTypeFormatterNoEncoding"), p0);
         }
+
+        /// <summary>
         /// Property '{0}' on type '{1}' is invalid. Value-typed properties marked as [Required] must also be marked with [DataMember(IsRequired=true)] to be recognized as required. Consider attributing the declaring type with [DataContract] and the property with [DataMember(IsRequired=true)].
         /// </summary>
         internal static string MissingDataMemberIsRequired
@@ -264,6 +266,22 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             return string.Format(CultureInfo.CurrentCulture, GetString("ValidationAttributeOnNonPublicProperty"), p0, p1);
         }
 
+        /// <summary>
+        /// A field previously marked invalid should not be marked valid.
+        /// </summary>
+        internal static string Validation_InvalidFieldCannotBeReset
+        {
+            get { return GetString("Validation_InvalidFieldCannotBeReset"); }
+        }
+
+        /// <summary>
+        /// A field previously marked invalid should not be marked valid.
+        /// </summary>
+        internal static string FormatValidation_InvalidFieldCannotBeReset()
+        {
+            return GetString("Validation_InvalidFieldCannotBeReset");
+        }
+
         /// <summary>
         /// A value is required but was not present in the request.
         /// </summary>

src/Microsoft.AspNet.Mvc.ModelBinding/Resources.resx

@@ -165,6 +165,9 @@
   <data name="ValidationAttributeOnNonPublicProperty" xml:space="preserve">
     <value>Non-public property '{0}' on type '{1}' is attributed with one or more validation attributes. Validation attributes on non-public properties are not supported. Consider using a public property for validation instead.</value>
   </data>
+  <data name="Validation_InvalidFieldCannotBeReset" xml:space="preserve">
+    <value>A field previously marked invalid should not be marked valid.</value>
+  </data>
   <data name="Validation_ValueNotFound" xml:space="preserve">
     <value>A value is required but was not present in the request.</value>
   </data>

src/Microsoft.AspNet.Mvc.ModelBinding/Validation/ModelValidationNode.cs

@@ -119,6 +119,13 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // post-validation steps
             var validatedEventArgs = new ModelValidatedEventArgs(validationContext, parentNode);
             OnValidated(validatedEventArgs);
+
+            var modelState = validationContext.ModelState;
+            if (modelState.IsValidField(ModelStateKey) != false)
+            {
+                // If a node or its subtree were not marked invalid, we can consider it valid at this point.
+                modelState.MarkFieldValid(ModelStateKey);
+            }
         }
 
         private void ValidateChildren(ModelValidationContext validationContext)
@@ -149,7 +156,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 // else we could end up with duplicate or irrelevant error messages.
                 var propertyKeyRoot = ModelBindingHelper.CreatePropertyModelName(ModelStateKey, propertyMetadata.PropertyName);
 
-                if (modelState.IsValidField(propertyKeyRoot))
+                if (modelState.IsValidField(propertyKeyRoot) == null)
                 {
                     var propertyValidators = GetValidators(validationContext, propertyMetadata);
                     var propertyValidationContext = new ModelValidationContext(validationContext, propertyMetadata);
@@ -168,18 +175,19 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         private void ValidateThis(ModelValidationContext validationContext, ModelValidationNode parentNode)
         {
             var modelState = validationContext.ModelState;
-            if (!modelState.IsValidField(ModelStateKey))
+            if (modelState.IsValidField(ModelStateKey) == false)
             {
-                return; // short-circuit
+                // If any item in the key's subtree has been identified as invalid, short-circuit
+                return;
             }
 
             // If the Model at the current node is null and there is no parent, we cannot validate, and the 
             // DataAnnotationsModelValidator will throw. So we intercept here to provide a catch-all value-required 
             // validation error
+            var modelStateKey = ModelBindingHelper.CreatePropertyModelName(ModelStateKey, ModelMetadata.GetDisplayName());
             if (parentNode == null && ModelMetadata.Model == null)
             {
-                var trueModelStateKey = ModelBindingHelper.CreatePropertyModelName(ModelStateKey, ModelMetadata.GetDisplayName());
+                modelState.AddModelError(modelStateKey, Resources.Validation_ValueNotFound);
-                modelState.AddModelError(trueModelStateKey, Resources.Validation_ValueNotFound);
                 return;
             }
 
@@ -190,8 +198,8 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
                 var validator = validators[i];
                 foreach (var validationResult in validator.Validate(validationContext))
                 {
-                    var trueModelStateKey = ModelBindingHelper.CreatePropertyModelName(ModelStateKey, validationResult.MemberName);
+                    var currentModelStateKey = ModelBindingHelper.CreatePropertyModelName(ModelStateKey, validationResult.MemberName);
-                    modelState.AddModelError(trueModelStateKey, validationResult.Message);
+                    modelState.AddModelError(currentModelStateKey, validationResult.Message);
                 }
             }
         }

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Binders/CompositeModelBinderTest.cs

@@ -54,7 +54,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             Assert.Equal(42, bindingContext.Model);
             
             Assert.True(validationCalled);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
         }
 
         [Fact]
@@ -106,7 +106,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             Assert.True(isBound);
             Assert.Equal(expectedModel, bindingContext.Model);
             Assert.True(validationCalled);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
         }
 
         [Fact]
@@ -132,7 +132,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             // Assert
             Assert.False(isBound);
             Assert.Null(bindingContext.Model);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
             mockListBinder.Verify();
         }
 

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Binders/MutableObjectModelBinderTest.cs

@@ -319,7 +319,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             var modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
             Assert.Equal(1, modelStateDictionary.Count);
 
             // Check Age error.
@@ -377,13 +377,19 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             var modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
-            Assert.Equal(1, modelStateDictionary.Count);
+            Assert.Equal(2, modelStateDictionary.Count);
+
+            // Check Name field
+            ModelState modelState;
+            Assert.True(modelStateDictionary.TryGetValue("theModel.Name", out modelState));
+            Assert.Equal(0, modelState.Errors.Count);
+            Assert.Equal(true, modelState.IsValid);
 
             // Check Age error.
-            ModelState modelState;
             Assert.True(modelStateDictionary.TryGetValue("theModel.Age", out modelState));
             Assert.Equal(1, modelState.Errors.Count);
+            Assert.Equal(false, modelState.IsValid);
 
             var modelError = modelState.Errors[0];
             Assert.Null(modelError.Exception);
@@ -409,7 +415,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             var modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
             Assert.Equal(2, modelStateDictionary.Count);
 
             // Check Age error.
@@ -457,7 +463,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             var modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
             Assert.Equal(1, modelStateDictionary.Count);
 
             // Check City error.
@@ -488,7 +494,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             var modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
             Assert.Equal(1, modelStateDictionary.Count);
 
             // Check ValueTypeRequired error.
@@ -524,7 +530,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             ModelStateDictionary modelStateDictionary = bindingContext.ModelState;
-            Assert.False(modelStateDictionary.IsValid);
+            Assert.Equal(false, modelStateDictionary.IsValid);
             Assert.Equal(1, modelStateDictionary.Count);
 
             // Check ValueTypeRequired error.
@@ -568,7 +574,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.Equal("John", model.FirstName);
             Assert.Equal("Doe", model.LastName);
             Assert.Equal(dob, model.DateOfBirth);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
         }
 
         [Fact]
@@ -593,7 +599,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // Assert
             var person = Assert.IsType<Person>(bindingContext.Model);
             Assert.Equal(123.456m, person.PropertyWithDefaultValue);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
         }
 
         [Fact]
@@ -637,7 +643,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
 
             // Assert
             validationNode.Validate(validationContext);
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
             Assert.Equal(new DateTime(2001, 1, 1), model.DateOfBirth);
         }
 
@@ -684,9 +690,9 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             testableBinder.SetPropertyPublic(bindingContext, propertyMetadata, dtoResult, requiredValidator);
 
             // Assert
-            Assert.True(bindingContext.ModelState.IsValid);
+            Assert.Equal(true, bindingContext.ModelState.IsValid);
             validationNode.Validate(validationContext, bindingContext.ValidationNode);
-            Assert.False(bindingContext.ModelState.IsValid);
+            Assert.Equal(false, bindingContext.ModelState.IsValid);
         }
 
         [Fact]
@@ -706,7 +712,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             testableBinder.SetPropertyPublic(bindingContext, propertyMetadata, dtoResult, requiredValidator);
 
             // Assert
-            Assert.False(bindingContext.ModelState.IsValid);
+            Assert.Equal(false, bindingContext.ModelState.IsValid);
             Assert.Equal("Sample message", bindingContext.ModelState["foo.ValueTypeRequired"].Errors[0].ErrorMessage);
         }
 
@@ -728,7 +734,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             testableBinder.SetPropertyPublic(bindingContext, propertyMetadata, dtoResult, requiredValidator);
 
             // Assert
-            Assert.False(bindingContext.ModelState.IsValid);
+            Assert.Equal(false, bindingContext.ModelState.IsValid);
             Assert.Equal(1, bindingContext.ModelState["foo.NameNoAttribute"].Errors.Count);
             Assert.Equal("This is a different exception." + Environment.NewLine
                        + "Parameter name: value",
@@ -752,7 +758,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             testableBinder.SetPropertyPublic(bindingContext, propertyMetadata, dtoResult, requiredValidator);
 
             // Assert
-            Assert.False(bindingContext.ModelState.IsValid);
+            Assert.Equal(false, bindingContext.ModelState.IsValid);
             Assert.Equal(1, bindingContext.ModelState["foo.Name"].Errors.Count);
             Assert.Equal("This message comes from the [Required] attribute.", bindingContext.ModelState["foo.Name"].Errors[0].ErrorMessage);
         }

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Binders/TypeConverterModelBinderTest.cs

@@ -27,7 +27,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding.Test
             // Assert
             Assert.False(retVal);
             Assert.Null(bindingContext.Model);
-            Assert.False(bindingContext.ModelState.IsValid);
+            Assert.Equal(false, bindingContext.ModelState.IsValid);
             Assert.Equal("Input string was not in a correct format.", bindingContext.ModelState["theModelName"].Errors[0].ErrorMessage);
         }
 

test/Microsoft.AspNet.Mvc.ModelBinding.Test/Validation/ModelStateDictionaryTest.cs

@@ -60,7 +60,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
         }
 
         [Fact]
-        public void IsValidFieldReturnsFalseIfDictionaryDoesNotContainKey()
+        public void IsValidFieldReturnsNullIfDictionaryDoesNotContainKey()
         {
             // Arrange
             var msd = new ModelStateDictionary();
@@ -69,7 +69,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var isValid = msd.IsValidField("foo");
 
             // Assert
-            Assert.True(isValid);
+            Assert.Null(isValid);
         }
 
         [Fact]
@@ -83,7 +83,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var isValid = msd.IsValidField("foo");
 
             // Assert
-            Assert.False(isValid);
+            Assert.Equal(false, isValid);
         }
 
         [Fact]
@@ -97,7 +97,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var isValid = msd.IsValidField("foo");
 
             // Assert
-            Assert.False(isValid);
+            Assert.Equal(false, isValid);
         }
 
         [Fact]
@@ -106,25 +106,25 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // Arrange
             var msd = new ModelStateDictionary()
             {
-                { "foo", new ModelState() { Value = new ValueProviderResult(null, null, null) } }
+                { "foo", new ModelState() { Value = new ValueProviderResult(null, null, null), IsValid = true } }
             };
 
             // Act
             var isValid = msd.IsValidField("foo");
 
             // Assert
-            Assert.True(isValid);
+            Assert.Equal(true, isValid);
         }
 
         [Fact]
         public void IsValidPropertyReturnsFalseIfErrors()
         {
             // Arrange
-            var errorState = new ModelState() { Value = GetValueProviderResult("quux", "quux") };
+            var errorState = new ModelState() { Value = GetValueProviderResult("quux", "quux"), IsValid = false };
             errorState.Errors.Add("some error");
             var dictionary = new ModelStateDictionary()
             {
-                { "foo", new ModelState() { Value = GetValueProviderResult("bar", "bar") } },
+                { "foo", new ModelState() { Value = GetValueProviderResult("bar", "bar"), IsValid = true } },
                 { "baz", errorState }
             };
 
@@ -132,7 +132,7 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             var isValid = dictionary.IsValid;
 
             // Assert
-            Assert.False(isValid);
+            Assert.Equal(false, isValid);
         }
 
         [Fact]
@@ -141,15 +141,15 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             // Arrange
             var dictionary = new ModelStateDictionary()
             {
-                { "foo", new ModelState() { Value = GetValueProviderResult("bar", "bar") } },
+                { "foo", new ModelState() { IsValid = true, Value = GetValueProviderResult("bar", "bar") } },
-                { "baz", new ModelState() { Value = GetValueProviderResult("quux", "bar") } }
+                { "baz", new ModelState() { IsValid = true, Value = GetValueProviderResult("quux", "bar") } }
             };
 
             // Act
             var isValid = dictionary.IsValid;
 
             // Assert
-            Assert.True(isValid);
+            Assert.Equal(true, isValid);
         }
 
         [Fact]
@@ -218,9 +218,74 @@ namespace Microsoft.AspNet.Mvc.ModelBinding
             Assert.Equal("some value", modelState.Value.ConvertTo(typeof(string)));
         }
 
-        private static ValueProviderResult GetValueProviderResult(object rawValue, string attemptedValue)
+        [Fact]
+        public void GetFieldValidity_ReturnsUnvalidated_IfNoEntryExistsForKey()
         {
-            return new ValueProviderResult(rawValue, attemptedValue, CultureInfo.InvariantCulture);
+            // Arrange
+            var dictionary = new ModelStateDictionary();
+            dictionary.SetModelValue("user.Name", GetValueProviderResult());
+
+            // Act
+            var isValidField = dictionary.IsValidField("not-user");
+
+            // Assert
+            Assert.Equal(null, isValidField);
+        }
+
+        [Fact]
+        public void GetFieldValidity_ReturnsUnvalidated_IfAnyItemInSubtreeIsInvalid()
+        {
+            // Arrange
+            var dictionary = new ModelStateDictionary();
+            dictionary["user.Address"] = new ModelState { IsValid = true };
+            dictionary.SetModelValue("user.Name", GetValueProviderResult());
+            dictionary.AddModelError("user.Age", "Age is not a valid int");
+
+            // Act
+            var isValidField = dictionary.IsValidField("user");
+
+            // Assert
+            Assert.Equal(null, isValidField);
+        }
+
+        [Theory]
+        [InlineData("user")]
+        [InlineData("user.Age")]
+        public void GetFieldValidity_ReturnsInvalid_IfAllKeysAreValidatedAndAnyEntryIsInvalid(string key)
+        {
+            // Arrange
+            var dictionary = new ModelStateDictionary();
+            dictionary["user.Address"] = new ModelState { IsValid = true };
+            dictionary["user.Name"] = new ModelState { IsValid = true };
+            dictionary.AddModelError("user.Age", "Age is not a valid int");
+
+            // Act
+            var isValidField = dictionary.IsValidField(key);
+
+            // Assert
+            Assert.Equal(false, isValidField);
+        }
+
+        [Fact]
+        public void GetFieldValidity_ReturnsValid_IfAllKeysAreValid()
+        {
+            // Arrange
+            var dictionary = new ModelStateDictionary();
+            dictionary["user.Address"] = new ModelState { IsValid = true };
+            dictionary["user.Name"] = new ModelState { IsValid = true };
+
+            // Act
+            var isValidField = dictionary.IsValidField("user");
+
+            // Assert
+            Assert.Equal(true, isValidField);
+        }
+
+        private static ValueProviderResult GetValueProviderResult(object rawValue = null, string attemptedValue = null)
+        {
+            return new ValueProviderResult(rawValue ?? "some value", 
+                                           attemptedValue ?? "some value", 
+                                           CultureInfo.InvariantCulture);
         }
     }
 }