WebApi 通过拦截器设置特定的返回格式

public class ActionFilter : ActionFilterAttribute
{ 

       　 /// <summary>

        /// Action执行之后由MVC框架调用
        /// </summary>
        /// <param name="actionExecutedContext"></param>
        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        {
            var requestUrl = actionExecutedContext.Request.RequestUri.AbsolutePath;
            if (requestUrl.Equals("/api/restful/order") && actionExecutedContext.Response != null
              && actionExecutedContext.Response.StatusCode == HttpStatusCode.OK)
            {
  　　　　　　　　//获取原始返回结果
                var datastr = actionExecutedContext.ActionContext.Response.Content.ReadAsAsync<object>().Result;

                //定义需要返回的格式类型
                var responseModel = new ResponseModel()
                {
                    errormsg = "",
                    requestId = ((Titan.Api.Web.Outside.RESTful.Model.ExtApiResultBase)datastr).RequestId 
                };
                HttpResponseMessage result = new HttpResponseMessage
                {
                    Content = new StringContent(JsonConvert.SerializeObject(responseModel),
                    Encoding.GetEncoding("UTF-8"), "application/json")
                };
                actionExecutedContext.Response = result;
            }
            base.OnActionExecuted(actionExecutedContext);
        }　 
}

最后需要在Global中配置     config.Filters.Add(new ActionFilter());