Question

I have a set of services hosted with WCF Web Api, what I need to do is validate the properties inside the models of the app.

In MVC 3 for example I decorate properties in the model like this:

    [StringLength(30)]
    public string UserName { get; set; }

and then in the controller I proceed like this to verify os the model has met the validation parameters:

    [HttpPost]
    ActionResult Create(Model myModel)
    { 
        if(ModelState.IsValid(){
           Post the model
        }
        else
        {
           Don't post the model
        }
    }

Is there a way to do something similar in WCF Web Api?

Answer 1

Firstly I should say awesome question+answer Daniel

However, I’ve taken it a little further, refined it and added to it.

ValidationHander

I’ve refined this a little. It is now based on a generic HttpOperationHandler so it can take the HttpRequestMessage. The reason for this is so that I can return error messages formatted using the correct media type (from the accept header).

public class ValidationHandler<TResource> : HttpOperationHandler<TResource, HttpRequestMessage, HttpRequestMessage>
{
    public ValidationHandler() : base("response") { }

    protected override HttpRequestMessage OnHandle(TResource model, HttpRequestMessage requestMessage)
    {
        var results = new List<ValidationResult>();
        var context = new ValidationContext(model, null, null);
        Validator.TryValidateObject(model, context, results, true);

        if (results.Count == 0)
        {
            return requestMessage;
        }

        var errorMessages = results.Select(x => x.ErrorMessage).ToArray();

        var mediaType = requestMessage.Headers.Accept.FirstOrDefault();
        var response = new RestValidationFailure(errorMessages);
        if (mediaType != null)
        {
            response.Content = new ObjectContent(typeof (string[]), errorMessages, mediaType);
        }
        throw new HttpResponseException(response);
    }
}

Extension Methods

The 2 you provided stay virtually the same about from the desc paramter no longer being needed when adding the ValidationHandler in the ModelValidationFor method

I’ve added an extra extension method. This is to make sure that all “Resource” classes are validated. This is mainly me being lazy and forgetful. I am forever forgetting to add some class to a list somewhere. (It’s why I write generic windsor installers!)

public static void ValidateAllResourceTypes(this WebApiConfiguration config, string assemblyFilter = "MyCompany*.dll")
{
    var path = Path.GetDirectoryName((new Uri(Assembly.GetExecutingAssembly().CodeBase)).AbsolutePath);
    var dc = new DirectoryCatalog(path, assemblyFilter);
    var assemblies = dc.LoadedFiles.Select(Assembly.LoadFrom).ToList();
    assemblies.ForEach(assembly =>
    {
        var resourceTypes = assembly.GetTypes()
            .Where(t => t.Namespace != null && t.Namespace.EndsWith("Resources"));

        foreach (var resourceType in resourceTypes)
        {
            var configType = typeof(Extensions);
            var mi = configType.GetMethod("ModelValidationFor");
            var mi2 = mi.MakeGenericMethod(resourceType);
            mi2.Invoke(null, new object[] { config });
        }
    });            
}

I made use of the System.ComponentModel.Composition.Hosting namespace (formerly known as MEF) for the DirectoryCatalog class. In this case I’ve just used the namespace ending with “Resources” to find my “Resource” classes. It wouldn’t take much work to change it to use a custom attribute or whatever other way you might prefer to identify which classes are your “Resources”.

RestValidationFailure

This is a little helper class I made to allow consistent behaviour for validation failure responses.

public class RestValidationFailure : HttpResponseMessage
{
    public RestValidationFailure(string[] messages)
    {
        StatusCode = HttpStatusCode.BadRequest;
        foreach (var errorMessage in messages)
        {
            Headers.Add("X-Validation-Error", errorMessage);
        }
    }
}

---

So, now I get a nice list (in my preferred mediatype) of all the validation errors.

Enjoy! 🙂