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));
    }
}

Answer 2

Edited Answer

Upon re-reading your question, it sounds like you just want to know if a value is selected. If that's the case then just apply the RequiredAttribute to the Guid property and make it nullable on the model

public class GuidModel
{
    [Required]
    public Guid? Guid { get; set; }

    public IEnumerable<Guid> Guids { get; set; }
}

then in the strongly typed View (with @model GuidModel)

@Html.ValidationMessageFor(m => m.Guid)
@Html.DropDownListFor(
    m => m.Guid,
    Model.Guids.Select(g => new SelectListItem {Text = g.ToString(), Value = g.ToString()}),
    "-- Select Guid --")

Add the client validation JavaScript script references for client-side validation.

The controller looks like

public class GuidsController : Controller
{
    public GuidRepository GuidRepo { get; private set; }

    public GuidsController(GuidRepository guidRepo)
    {
        GuidRepo = guidRepo;
    }

    [HttpGet]
    public ActionResult Edit(int id)
    {
        var guid = GuidRepo.GetForId(id);
        var guids - GuidRepo.All();

        return View(new GuidModel { Guid = guid, Guids = guids });
    }

    [HttpPost]
    public ActionResult Edit(GuidModel model)
    {
        if (!ModelState.IsValid)
        {
            model.Guids = GuidRepo.All();
            return View(model);
        }

        /* update db */

        return RedirectToAction("Edit");
    }
}

This will ensure that the Guid property is required for a model-bound GuidModel.

Original Answer

I don't believe that there is a ready made Data Annotation Validation attribute that is capable of doing this. I wrote a blog post about one way to achieve this; the post is using an IoC container but you could take the hard coded dependency if you're wanting to get something working.

Something like

public class ValidGuidAttribute : ValidationAttribute
{
    private const string DefaultErrorMessage = "'{0}' does not contain a valid guid";

    public ValidGuidAttribute() : base(DefaultErrorMessage)
    {
    }

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var input = Convert.ToString(value, CultureInfo.CurrentCulture);

        // let the Required attribute take care of this validation
        if (string.IsNullOrWhiteSpace(input))
        {
            return null;
        }

        // get all of your guids (assume a repo is being used)
        var guids = new GuidRepository().AllGuids();

        Guid guid;
        if (!Guid.TryParse(input, out guid))
        {
            // not a validstring representation of a guid
            return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
        }

        // is the passed guid one we know about?
        return guids.Any(g => g == guid) ?
            new ValidationResult(FormatErrorMessage(validationContext.DisplayName)) : null;
    }
}

and then on the model you send into the controller action

public class GuidModel
{
    [ValidGuid]
    public Guid guid { get; set; }
}

This gives you server side validation. You could write client side validation to do this as well, perhaps using RemoteAttribute but I don't see a lot of value in this case as the only people that are going to see this client side validation are people that are messing with values in the DOM; it would be of no benefit to your normal user.