WebAPI 2 ValidateModelAttribute not firing

Question

I have a custom ValidateModelAttribute but it's not firing for some reason

using System.Net;
using System.Web.Http.Controllers;
using System.Net.Http;
using System.Web.Http.Filters;

namespace blank.Utilities.CustomAttributes
{
    public class ValidateModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (actionContext.ModelState.IsValid == false)
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, actionContext.ModelState);
            }
        }
    }
}

Here is the Post call

[ValidateModelAttribute]
[HttpPost]
[Route("update")]
public IHttpActionResult Post(ActivityPctCompleteDto actDto) {
    _activityService.UpdatePctComplete(actDto);
    return Ok();
}

I have added fromBody and fromUri in the post but it doesn't work. What am I missing here?

Here is my Dto

public class ActivityPctCompleteDto
{
    [Required(ErrorMessage = "ActivityID is an invalid Guid value.")]
    public Guid? Id { get; set; }

    [Required(ErrorMessage = "PctComplete is an invalid Decimal(5,4) value.")]
    public decimal? PctComplete { get; set; }
}

Answer 1

I've tried your code and action filter works fine for me. To diagnose your problem check the following:

1. Are you using ActionFilterAttribute from System.Web.Http.Filters namespace? There is a common confustion with filters from System.Web.Mvc.
2. What HTTP result do you get for you request? May be it fails on some earlier stage in ASP.NET pipeline, like routing or authentication?
3. Are you sure that exactly this Post() method processes the request? Try setting breakpoint and check whether it hits.