Web.API 2 centralised model state validation

Model state validation is easy with DataAnnotations. However, you will find repeating the next line of code very soon:

…
if(!ModelState.IsValid) return BadRequest(..);
…

Luckily, there is a pretty simple solution – to use ActionFilter:

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

	namespace MyAwesomeWebApi.ActionFilters
	{
	/// <summary>
	/// Returns 400 if the ModelState is invalid.
	/// </summary>
	[AttributeUsage(AttributeTargets.All, AllowMultiple = false, Inherited = true)]
	public class ValidateModelAttribute : ActionFilterAttribute
	{
	public override void OnActionExecuting(HttpActionContext actionContext)
	{
	if (actionContext.ModelState.IsValid)
	return;

	actionContext.Response = actionContext.Request.CreateErrorResponse(HttpStatusCode.BadRequest, actionContext.ModelState);
	}
	}
	}

	using System.Collections.Generic;
	using System.Threading.Tasks;
	using System.Web.Http;
	using System.Web.Http.Description;
	using System.ComponentModel.DataAnnotations;

	namespace MyAwesomeWebApi.Controllers
	{
	public class ValuesController : ApiController
	{
	[ValidateModel]
	public IHttpActionResult Post(MyModel model)
	{
	var valService = new ValService();
	var result = valService.Create(model);
	return Created($"api/values/{result.Id}", result);
	}
	}

	public class MyModel
	{
	[Required]
	public int Id { get; set; }
	}
	}