Handling .Net's model state with ValidationFilter attribute

This is a simple ValidationFilter class which you can attach to any .net controller action method:

// ValidationFilter.cs

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

namespace MyApp.Web.Infrastructure.Filters
    /// <summary>
    /// This action attribute is used to validate the view model. It will throw 422 - Unprocessable
    /// entity in case model (modelState) was not validated.
    /// <see href="http://stackoverflow.com/questions/11686690/handle-modelstate-validation-in-asp-net-web-api"/>
    /// <seealso href="http://stackoverflow.com/a/3291292/413785"/>
    /// </summary>
    public class ValidationFilter : ActionFilterAttribute
        public override void OnActionExecuting(HttpActionContext actionContext)
            var modelState = actionContext.ModelState;

            var model = actionContext.ActionArguments["model"];
            if (model != null)
                var modelType = model.GetType();
                dynamic modelObject = Convert.ChangeType(model, modelType);
                if (modelObject != null && modelType.GetMethod("Validate") != null)

            if (!modelState.IsValid)
                actionContext.Response = actionContext.Request.CreateErrorResponse((HttpStatusCode)422, modelState);


What it provides is a “one-line” way to handle invalid model states. For example - instead of doing ModelState.IsValid check on each POST/PUT action, and then handling it manually again and again (by basically copying and pasting the same chunk of code all around your controllers) you can simply reuse the [ValidationFilter] attribute in front of your action and it will check whether the ModelState is valid. If it is, the action method will execute whatever code it has inside of it, and if it’s not the WebAPI will respond with a 422 (Unprocessable Entity) together with the associated ModelState error messages.

For example if you had this model/action combination:

// CustomerModel.cs

public class CustomerModel
    public int FirstName { get; set; }

    public int LastName { get; set; }
// CustomerController.cs

public IHttpActionResult PostCustomer(CustomerModel model)
    return Ok();

…and you didn’t send any data to the WebAPI in your POST request, this is the 422 response that you would get back:

// response.json

  "message": "The request is invalid.",
  "modelState": {
    "model.FirstName": [
      "The FirstName field is required."
    "model.LastName": [
      "The LastName field is required."

This response can then be further parsed/handled by the client-side without problems.