How to create custom validation attribute for MVC

I'd like to create a custom validation attribute for MVC2 for an email address that doesn't inherit from RegularExpressionAttribute but that can be used in client validation. Can anyone point me in the right direction?

I tried something as simple as this:

[AttributeUsage( AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false )]
public class EmailAddressAttribute : RegularExpressionAttribute
{
    public EmailAddressAttribute( )
        : base( Validation.EmailAddressRegex ) { }
}

but it doesn't seem to work for the client. However, if I use RegularExpression(Validation.EmailAddressRegex)] it seems to work fine.

You need to register an adapter for the new attribute in order to enable client side validation.

Since the RegularExpressionAttribute already has an adapter, which is RegularExpressionAttributeAdapter, all you have to do is reuse it.

Use a static constructor to keep all the necessary code within the same class.

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple  = false)]
public class EmailAddressAttribute : RegularExpressionAttribute
{
    private const string pattern = @"^\w+([-+.]*[\w-]+)*@(\w+([-.]?\w+)){1,}\.\w{2,4}$";

    static EmailAddressAttribute()
    {
        // necessary to enable client side validation
        DataAnnotationsModelValidatorProvider.RegisterAdapter(typeof(EmailAddressAttribute), typeof(RegularExpressionAttributeAdapter));
    }

    public EmailAddressAttribute() : base(pattern)
    {
    }
}

For more information checkout this post explaining the complete process. http://haacked.com/archive/2009/11/19/aspnetmvc2-custom-validation.aspx

The CustomValidationAttribute Class MSDN page has a few examples on it now. The Phil Haacked post is out of date.