Can not Return Custom Error Message in Custom Validator

Question

I've wrote a Custom Validation for Guid Type Like this:

 public class ValidGuidAttribute : ValidationAttribute
{

    public override bool IsValid(object value)
    {
        string msg = "please select a device";
        ErrorMessage = msg;
        var input = Convert.ToString(value);
        if (string.IsNullOrEmpty(input) )
        {

            return false;
        }

        Guid guid;
        if(!Guid.TryParse(input,out guid))
        {
            return false;
        }
        if (guid == Guid.Empty)
        {
            return false;
        }
        return true;
    }
}

and when I inspect element on the browser I have this :

<span class="field-validation-valid" data-valmsg-for="DeviceGroupId" data-valmsg-replace="true"></span>

and when I don't fill that filed and post form , I've got this error Message :

The value 'please select a device' is not valid for device group.

I want just get this :

please select a device

and In ViewModel I have :

        [DisplayName("device group"),ValidGuid(ErrorMessage = "please enter")]
    public Guid DeviceGroupId { get; set; }

Hamid Mosalla · Accepted Answer · 2017-03-07 10:45:51Z

1

It seems that custom validation has some problem with the type Guid, when I change the type to string, your current implementation works:

It's weird, in that validation for the validity of Guid type is performed automatically, because when I remove the custom validation attibute and check for validity, I get the same error you get:

answered Mar 7, 2017 at 10:45

Hamid Mosalla

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Comments

Usman · Accepted Answer · 2017-03-09 04:19:45Z

this is default validation message which i think mvc is validating before checking ValidGuid and to override this message you can try

 public class ValidGuidAttribute : ValidationAttribute
    {
        public override bool IsValid(object value)
        {           
            ErrorMessage = "please select a device";
            var input = Convert.ToString(value);
            if (string.IsNullOrEmpty(input))
            {

                return false;
            }

            Guid guid;
            if (!Guid.TryParse(input, out guid))
            {
                return false;
            }
            if (guid == Guid.Empty)
            {
                return false;
            }
            return true;
        }

        public class ValidGuid : DataAnnotationsModelValidator<ValidGuidAttribute>
        {
            public ValidGuid(ModelMetadata metadata, ControllerContext context, ValidGuidAttribute attribute)
                : base(metadata, context, attribute)
            {
                if (!attribute.IsValid(context.HttpContext.Request.Form[metadata.PropertyName]))
                {

                        var propertyName = metadata.PropertyName;


                 if (context.Controller.ViewData.ModelState[propertyName] != null)
                    {
                           context.Controller.ViewData.ModelState[propertyName].Errors.Clear();
                            context.Controller.ViewData.ModelState[propertyName].Errors.Add(attribute.ErrorMessage);
                        }

                }
            }
        }
    }

and add this line to your Global.asax.cs file

DataAnnotationsModelValidatorProvider.RegisterAdapter(typeof(ValidGuidAttribute), typeof(ValidGuidAttribute.ValidGuid));

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