Extending the MVC RequiredAttribute

问题

I have an extended class of RequiredAttribute that doesn't send error messages back. If I check it in the debugger the text is there alright.

public class VierRequired : RequiredAttribute
{
    public VierRequired(string controlName)
    {
        //...
    }

    public string VierErrorMessage
    {
        get { return ErrorMessage; }
        set { ErrorMessage = value; }
    }

    // validate true if there is any data at all in the object
    public override bool IsValid(object value)
    {
        if (value != null && !string.IsNullOrEmpty(value.ToString()))
            return true;

        return false; // base.IsValid(value);
    }
}

I call it like this

[VierRequired("FirstName", VierErrorMessage = "Please enter your first name")]
public string FirstName { get; set; }

And the mvc-view

<%: Html.TextBoxFor(model => model.FirstName, new { @class = "formField textBox" })%>
<%: Html.ValidationMessageFor(model => model.FirstName)%>

It works if I use the normal Required annotation

[Required(ErrorMessage = "Please enter your name")]
public string FirstName { get; set; }

But the custom does not send any error message back

回答1:

I also had a problem with client side validation when I created my own derivative of the RequiredAttribute. To fix it you need to register your data annotation like so:

DataAnnotationsModelValidatorProvider.RegisterAdapter(
            typeof(VierRequired),
            typeof(RequiredAttributeAdapter));

Simply call this in your Application_Start() method and client side validation should work as normal.

If your attribute is not working when you are POST-ing your form then this would indicate to me that there is something wrong with the logic in your attribute (check you IsValid method). I am also not sure what you are trying to achieve with your derived data annotation; your logic looks like it is trying to do pretty much what the default attribute does anyway:

Taken from the MSDN documentation:

A validation exception is raised if the property is null, contains an empty string (""), or contains only white-space characters.

来源：https://stackoverflow.com/questions/12573362/extending-the-mvc-requiredattribute