Validation of Guid

Question

I have a strongly-typed view which has a DropDownListFor attribute on it.

Each item in the dropdown list is represented by a GUID.

What I'm after is a way to validate if a user selects an item from the dropdown list. At present i don't see anyway of doing this using Data Annotations.

Is there anyway of achieving this using Data Annotations so client and server side validation would work.

I'm guessing i need to make a custom method to do this but was wondering if anything already existed.

Answer 1

Actually, you can't use Required attribute with GUIDs (without the method I mention below) because they inherit from struct, and as such their default value is actually an instance of Guid.Empty, which will satisfy the requirements of the Required attribute. Now that being said, it is possible to get what you want you just need to make your property nullable, take this for example...

public class Person
{
    [Required] //Only works because the Guid is nullable
    public Guid? PersonId { get; set;}
    public string FirstName { get; set;}
    public string LastName { get; set;}
}

By marking the GUID nullable (using the ?, or Nullable if you prefer the long way) you let it stay as null when binding against what the browser sent. In your case, just make sure the value of the default option of the dropdown uses an empty string as it's value.

EDIT: The only caveat to this method is you end up having to use something like Person.GetValueOfDefault() everywhere and potentially testing for Guid.Empty. I got tired of doing this and ended up creating my own validation attribute to help simplify validating Guids (and any other types that have default values I want to treat as invalid such as int, DateTime, etc). However I don't have client side validation to go along with this yet, so validation only happens on the server. This can be combined with [Required] (designed to not duplicate functionality of [Required]) if you're ok with using nullable types. This would mean you still have to use GetValueOrDefault(), but at least then you don't have to test for Guid.Empty anymore. The Gist link has some XMLDocs with examples, I left them out here for brevity. I'm currently using it with ASP.NET Core.

EDIT: Updated to fix a bug with Nullable<>, and a bug with treating null as invalid. Added supporting classes to handle client side validation. See Gist for full code.

Gist: RequireNonDefaultAttribute

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
public class RequireNonDefaultAttribute : ValidationAttribute
{
    public RequireNonDefaultAttribute()
        : base("The {0} field requires a non-default value.")
    {
    }

    public override bool IsValid(object value)
    {
        if (value is null)
            return true; //You can flip this if you want. I wanted leave the responsability of null to RequiredAttribute
        var type = value.GetType();
        return !Equals(value, Activator.CreateInstance(Nullable.GetUnderlyingType(type) ?? type));
    }
}