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