ValidateModelAttribute.cs

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
{
    /// <summary>
    /// <see cref="https://docs.microsoft.com/en-us/aspnet/web-api/overview/formats-and-model-binding/model-validation-in-aspnet-web-api"/>
    /// </summary>
    public class ValidateModelAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            var nullable = actionContext.ActionArguments.FirstOrDefault(kv => kv.Value == null);
            if (!nullable.Equals(default(KeyValuePair<string, object>)))
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, $"\"{nullable.Key}\" cannot be null");
                    
                return;
            }
            
            if (actionContext.ModelState.IsValid == false)
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(
                    HttpStatusCode.BadRequest, actionContext.ModelState);
            }
        }
    }
}
      
    }

WebApiConfig.cs

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