InternalServerError.cs

 /// <summary>
        /// Url for the POST is http://localhost/api/todo
        /// </summary>
        /// <param name="item">Creates the Todo ITEM</param>
        /// <returns></returns>
        public HttpResponseMessage Post(TodoItem item)
        {
            try
            {
                if (string.IsNullOrEmpty(item.Description))
                {
                    ValidationResult result = new ValidationResult { Name = "Description", ErrorMessage = "Description should not be empty" };
                    return Request.CreateResponse<ValidationResult>(HttpStatusCode.BadRequest, result);
                }
                var response = Request.CreateResponse<TodoItem>(HttpStatusCode.Created, new TodoItem { });
                response.Headers.Location = new Uri(Url.Link("DefaultApi", new { controller = "Todo", id = item.Id }));
                return response;
            }
            catch (Exception ex)
            {
                return Request.CreateResponse(HttpStatusCode.InternalServerError);
            }
        }

SucessfulResponse.cs

 /// <summary>
        /// Url for the POST is http://localhost/api/todo
        /// </summary>
        /// <param name="item">Creates the Todo ITEM</param>
        /// <returns></returns>
        public HttpResponseMessage Post(TodoItem item)
        {
            var response=Request.CreateResponse<TodoItem>(HttpStatusCode.Created, new TodoItem { });
            response.Headers.Location = new Uri(Url.Link("DefaultApi", new { controller = "Todo", id = item.Id }));
            return response;
        } 

TodoController.cs

namespace WebAPIKoans.Controllers
{
    public class TodoController : ApiController
    {
        /// <summary>
        /// Url-- POST http://localhost/api/todo
        /// </summary>
        /// <param name="item">Creates the Todo ITEM</param>
        /// <returns></returns>
        public HttpResponseMessage Post(TodoItem item)
        {
        }
    }
}

validationError.cs

/// <summary>
        /// Url for the POST is http://localhost/api/todo
        /// </summary>
        /// <param name="item">Creates the Todo ITEM</param>
        /// <returns></returns>
        public HttpResponseMessage Post(TodoItem item)
        {
            if (string.IsNullOrEmpty(item.Description))
            {
                ValidationResult result = new ValidationResult { Name = "Description", ErrorMessage = "Description should not be empty" };
                return Request.CreateResponse<ValidationResult>(HttpStatusCode.BadRequest, result);
            }
            var response=Request.CreateResponse<TodoItem>(HttpStatusCode.Created, new TodoItem { });
            response.Headers.Location = new Uri(Url.Link("DefaultApi", new { controller = "Todo", id = item.Id }));
            return response;
        }