"ErrorMessage=" named parameter of custom attributes for validation

Question

I followed a tutorial and made a custom remote validator for my application so that it will work even if the javascript is disabled. It works the same as normal Remote validations by having a specified controller and action and the optional ErrorMessage.

Here is the full code:

public class RemoteEmailCheckAttribute : RemoteAttribute
{
    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        //get the controller
        Type controller = Assembly.GetExecutingAssembly().GetTypes().FirstOrDefault(type => type.Name.ToLower() == string.Format("{0}Controller", this.RouteData["controller"].ToString()).ToLower());
        if (controller != null)
        {
            MethodInfo action = controller.GetMethods().FirstOrDefault(method => method.Name.ToLower() == this.RouteData["controller"].ToString().ToLower());

            if (action != null)
            {
                object instance = Activator.CreateInstance(controller);
                object response = action.Invoke(instance, new object[] { value });
                if (response is JsonResult)
                {
                    object jsonData = ((JsonResult)response).Data;
                    if (jsonData is bool)
                    {
                        return (bool)jsonData ? ValidationResult.Success : new ValidationResult(this.ErrorMessage);
                    }
                }
            }

        }

        return new ValidationResult(this.ErrorMessage);
    }


    public RemoteEmailCheckAttribute(string routeName)
        : base(routeName)
    {

    }


    public RemoteEmailCheckAttribute(string action, string controller)
        : base(action, controller)
    {

    }

    public RemoteEmailCheckAttribute(string action, string controller, string areaName)
        : base(action, controller, areaName)
    {

    }
}

And this is how i set it on my property,

[RemoteEmailCheck("IsEmailTaken","Enrollment",ErrorMessage="Email is already in use. Please pick something else")]
public string email { get; set; }

my question is that, since i will be using this constructor:

 public RemoteEmailCheckAttribute(string action, string controller, string areaName)
 : base(action, controller, areaName)

Since the third parameter is actually string of any name, how was it able to get the specified ErrorMessage=" "?

I have been checking/researching online and read about Named parameters but they are somehow different, please clarify it to me.

Answer 1

When creating Attributes, Any public non-static read-write fields or properties are named parameters.

For more information see Creating Custom Attributes