C# :: Trying To Create Custom Validation Attribute

I am trying to create a custom ValidationAttribute in my c# .net code. I have done a lot of searching and have code that should be working but the IsValid method is not firing as far as I can tell.

Model code:

[Required(AllowEmptyStrings = false, ErrorMessage = "required")]
//[Range(2014, 2100, ErrorMessage="Please enter a valid year.")]
//[RegularExpression(@"^d{4}$", ErrorMessage = "Please enter a valid year.")]
[ValidYear(ErrorMessage="Please enter one >= 2014")]
[Display(Name = "Exp. Year")]
public string expYear { get; set; }

And this is the class in that same model:

public class ValidYearAttribute : ValidationAttribute {
protected override ValidationResult IsValid(object value, ValidationContext validationContext){
return new ValidationResult("Something went wrong");
}
}

Why the IsValid is not firing.