public class MyActionWebApiAttribute : ActionFilterAttribute{public EnumHeader HeaderCode { set; get; }public override bool AllowMultiple{get{return false;}}public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext){base.OnActionExecuted(actionExecutedContext);HttpResponseMessage response = actionExecutedContext.ActionContext.Response;try{response.Content.Headers.Clear();string HeaderCodeStr = "text/html";if (HeaderCode == EnumHeader.application_xml){HeaderCodeStr = "application/xml";}else if (HeaderCode == EnumHeader.application_json){HeaderCodeStr = "application/json";}response.Content.Headers.Add("Content-Type", HeaderCodeStr + "; charset=utf-8");}catch (Exception ex){}}}//endpublic enum EnumHeader{text_html,application_json,application_xml}//end

public override bool AllowMultiple{get{return false;}}

public static void Register(HttpConfiguration config){config.Routes.MapHttpRoute(name: "DefaultApi",routeTemplate: "api/{controller}/{action}",defaults: new { id = RouteParameter.Optional });config.Filters.Add(new MyActionWebApiAttribute());// 取消注释下面的代码行可对具有 IQueryable 或 IQueryable<T> 返回类型的操作启用查询支持。// 若要避免处理意外查询或恶意查询，请使用 QueryableAttribute 上的验证设置来验证传入查询。// 有关详细信息，请访问 http://go.microsoft.com/fwlink/?LinkId=279712。//config.EnableQuerySupport();// 若要在应用程序中禁用跟踪，请注释掉或删除以下代码行// 有关详细信息，请参阅: http://www.asp.net/web-api
            config.EnableSystemDiagnosticsTracing();}

　　　　[HttpPost][MyActionWebApi(HeaderCode = EnumHeader.application_json)]public Site SiteList(){Site sites = new Site { SiteId = 1, Title = "test", Uri = "http://www.cnblogs.com/hcfan" };return sites;}

public class Site{public int SiteId { get; set; }public string Title { get; set; }public string Uri { get; set; }}