EnumDataType() attribute validation error message not displaying

Question

In my .net core 2.0 Web API I am using EnumDataType() validation attribute on my model property. When the validation fails the custom error message is empty. I am not sure why it is happening -

[EnumDataType(typeof(MyEnum), ErrorMessage = "Custom Error Message")]
public MyEnum MyEnumProp {get; set;}

I checked the other properties where I have [Required], [MinLength] and all are generating custom error messages. Am I doing something wrong? Is there any other approach?

Answer 1

It's a frequent confusion between errors detected during deserialization and validation stages.

Say you have the following enum:

public enum MyEnum
{
    None,
    Value1,
    Value2
}

and the following model:

public class TestModel
{
    [Required]
    public int? Id { get; set; }

    [EnumDataType(typeof(MyEnum), ErrorMessage = "Custom Error Message")]
    public MyEnum MyEnumProp { get; set; }
}

When you post the data:

{
  "Id": 123,
  "MyEnumProp": "UnexistingEnumValue"
}

the error will happen during deserialization stage (in Json.NET for this case). Deserializer just has no way to convert string "UnexistingEnumValue" to some of values from MyEnum. In this case deserializer will register following model binding error: Requested value 'UnexistingEnumValue' was not found.

ModelState.IsValid will be set to false, however value of MyEnumProp will be left at its default value of MyEnum.None. Validation performed by EnumDataType attribute will not detect any error, because MyEnum.None is a valid value for MyEnum. That is why you will not see "Custom Error Message" in ModelState errors.

Now if you post the following data:

{
  "Id": 123,
  "MyEnumProp": 5
}

No error will happen during deserialization stage, because following assignment is pretty legal even if it does not make much sense:

MyEnum v = (MyEnum)5;

So deserializer will not detect any errors. However now EnumDataType validation comes into play. And it detects that 5 is not a valid value for MyEnum. ModelState.IsValid is set to false and the error message specified in EnumDataType.ErrorMessage is registered ("Custom Error Message").

If you want to have the same custom message for both deserialization and validation errors, you should go up to the level of deserializer (Json.NET) and use its extensibility points for this purpose.