// POST api/values

        public HttpResponseMessage Post([FromBody]UserInfo userInfo)

        {

            if (string.IsNullOrWhiteSpace(userInfo.Gender))

            {

                ModelState.AddModelError("Gender", "性别不能为空");

            }

            if (ModelState.IsValid)

            {

                // Do something with the product (not shown).

                return new HttpResponseMessage(HttpStatusCode.OK);

            }

            else

            {

                return Request.CreateErrorResponse(HttpStatusCode.BadRequest, ModelState);

            }

        }

public class UserInfo

    {

        public int Id { get; set; }

        [Required]

        [StringLength(, ErrorMessage = "名字太长了或者太短了", MinimumLength = )]

        public string Name { get; set; }

        [RegularExpression(@"([2-5]\d)", ErrorMessage = "年龄在20-50之间")]

        public int Age { get; set; }

        public string Gender { get; set; }

    }

 public class ValidationAttribute : ActionFilterAttribute

    {

        public override void OnActionExecuting(HttpActionContext actionContext)

        {

            if (!actionContext.ModelState.IsValid)

            {

                actionContext.Response = actionContext.Request.CreateErrorResponse(

                    HttpStatusCode.BadRequest,

                    actionContext.ModelState);

            }

        }

    }

WebApiConfig.cs

   config.Filters.Add(new ValidationAttribute());