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public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            config.Filters.Add(new ValidateModelAttribute());

            // ...
        }
}

using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Filters;
using System.Web.Http.ModelBinding;

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

public class ProductsController : ApiController
{
    [ValidateModel]
    public HttpResponseMessage Post(Product product)
    {
        // ...
    }
}