让Web API支持$format参数的方法

 public static class WebApiConfig
     {
         public static void Register(HttpConfiguration config)
         {
             // Web API 配置和服务

             // Web API 路由
             config.MapHttpAttributeRoutes();

             config.Routes.MapHttpRoute(
                 name: "DefaultApi",
                 routeTemplate: "api/{controller}/{id}",
                 defaults: new { id = RouteParameter.Optional }
             );

             config.Routes.MapHttpRoute(
                  name: "ActionApi",
                  routeTemplate: "{controller}/{action}/{id}",
                  defaults: new { id = RouteParameter.Optional }
              );

             config.Formatters.JsonFormatter.AddQueryStringMapping("$format", "json", "application/json");
             config.Formatters.XmlFormatter.AddQueryStringMapping("$format", "xml", "application/xml");

         }
     }

WebAPI 只使用JSON格式响应

  public static class WebApiConfig
     {
         public static void Register(HttpConfiguration config)
         {
             // Web API 配置和服务

             // Web API 路由
             config.MapHttpAttributeRoutes();

             config.Routes.MapHttpRoute(
                 name: "DefaultApi",
                 routeTemplate: "api/{controller}/{id}",
                 defaults: new { id = RouteParameter.Optional }
             );

             config.Routes.MapHttpRoute(
                  name: "ActionApi",
                  routeTemplate: "{controller}/{action}/{id}",
                  defaults: new { id = RouteParameter.Optional }
              );

             config.Formatters.XmlFormatter.SupportedMediaTypes.Clear();
         }
     }

使用方式
http://localhost:8081/api/home?$format=json
http://localhost:8081/api/home?$format=xml

参考资料：https://code.msdn.microsoft.com/Support-format-in-ASPNET-e3785b2a