Messing around with ASP.NET MVC 2.0 (in VS2010 RC1), specifically the validation done with DataAnnotations, and created a simple no-op custom validator attribute like this:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public sealed class MyCustomAttribute : ValidationAttribute
{
  public MyCustomAttribute()
    : base("Custom Error Message: {0}")
  {
  }

  public override bool IsValid(object value)
  {
    return true;
  }
}

Similar to the way the sample “PropertiesMustMatchAttribute” is set up - enabled on a class so you attach it to the model, not a property on the model, and set so you can have more than one.

I then created a simple model that used it like this:

[MyCustom]
[MyCustom]
[MyCustom]
public class MyModel
{
  public string SomeField { get; set; }
  public string OtherField { get; set; }
}

Notice I’m using my custom attribute three times there.

I then set up a quick view to edit the model, enabled validation, and put a breakpoint in the “IsValid” method on my attribute.

I ran the editor, hit the “save” button (to post the “edited” content back and get validated) and the call to my custom validation ran only one time.

You know what I missed? I didn’t implement TypeId.

This is documented as being a “unique identifier used to identify two attributes of the same type.” What’s not said there is that if DataAnnotations (or, at least, the MVC validation portion) comes across two attributes of the same type, it’ll only look at one of them. By default, TypeId is just the type of the attribute, so when two attributes of the same type are encountered, they’re considered “the same.”

If you set your AttributeUsage to AllowMultiple, you absolutely must implement TypeId.

You can see this implemented in the MVC “PropertiesMustMatchAttribute” as a new object (so they’re never identical). Updating my simple attribute like this…

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = true)]
public sealed class MyCustomAttribute : ValidationAttribute
{
  public MyCustomAttribute()
    : base("Custom Error Message: {0}")
  {
  }

  public override bool IsValid(object value)
  {
    return true;
  }

  private object _typeId = new object();
  public override object TypeId
  {
    get
    {
      return this._typeId;
    }
  }
}

…causes my custom attribute to run all three times as expected.

What’s the use case for multiple attributes of the same type? In the case of the “PropertiesMustMatchAttribute” sample, maybe you have a model for registering a user - email, email confirmation, new password, new password confirmation. You want to make sure the two email fields match and the two password fields match. Two instances of one attribute, but working on different fields. Without implementing TypeId, only one of the validation checks will happen.

Once you get it working, sure, you can optimize some (for example, the “PropertiesMustMatchAttribute” could store the names of the properties being compared so if someone actually does try to run redundant validation it won’t happen), but that’s left as an implementation detail.