Validating your model with Web API

TodoApiController.cs

using System.Collections.Generic;
using System.Linq;
using System.Web.Http;
using System.Data;

public class TodoApiController : ApiController
{
    private BetterMobileSpaContext db = new BetterMobileSpaContext();

    // GET /api/todoapi
    public IEnumerable<TodoItem> Get()
    {
        return db.TodoItems.ToList();
    }

    // GET /api/todoapi/5
    public TodoItem Get(int id)
    {
        return db.TodoItems.Find(id);
    }

    // POST /api/todoapi
    [ValidateFilter]
    public void Post(TodoItem value)
    {
        if (!ModelState.IsValid) return;

        db.TodoItems.Add(value);
        db.SaveChanges();
    }

    // PUT /api/todoapi/5
    [ValidateFilter]
    public void Put(int id, TodoItem value)
    {
        if (!ModelState.IsValid) return;

        db.Entry(value).State = EntityState.Modified;
        db.SaveChanges();
    }

    // DELETE /api/todoapi/5
    public void Delete(int id)
    {
        TodoItem todoitem = db.TodoItems.Find(id);
        db.TodoItems.Remove(todoitem);
        db.SaveChanges();
    }

    protected override void Dispose(bool disposing)
    {
        db.Dispose();
        base.Dispose(disposing);
    }
}

ValidateFilterAttribute.cs

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

public class ValidateFilterAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        var modelState = actionExecutedContext.ActionContext.ModelState;
        if (!modelState.IsValid)
        {
            var errors = modelState
                .Where(s => s.Value.Errors.Count > 0)
                .Select(s => new KeyValuePair<string, string>(s.Key, s.Value.Errors.First().ErrorMessage))
                .ToArray();

            actionExecutedContext.Result = new HttpResponseMessage<KeyValuePair<string, string>[]>(errors, HttpStatusCode.BadRequest);
        }
        base.OnActionExecuted(actionExecutedContext);
    }
}

I took dkendrick's comment and further refined the attribute, so if anyone else comes here looking for a solution like I did, here's my take on it:

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

namespace VerifyPlatform.ClaimsTransformer.Presentation.Attributes.Filters
{
    public class ModelStateValidationAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (!actionContext.ModelState.IsValid)
            {
                actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            }

            base.OnActionExecuting(actionContext);
        }
    }
}

Hi, thanks for your share!
If model is null (as I use Fiddler) the IsValid = true. Can we handle this case also within a filter scope?

Yes we can handle If model is null

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

namespace TAMS.API.Infrastructure
{
    public class ValidateFilterAttribute : ActionFilterAttribute
    {
        public override void OnActionExecuting(HttpActionContext actionContext)
        {
            if (!actionContext.ModelState.IsValid)
            {
                actionContext.Response = 
                    actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            }else if(actionContext.ActionArguments.FirstOrDefault().Value == null)
            {
                actionContext.ModelState.AddModelError("Model Null", "Form is empty. Please enter some value");
                actionContext.Response =
                   actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
            }
            base.OnActionExecuting(actionContext);
        }
    }
}