HttpContext.Current.Response.AppendHeader("Access-Control-Allow-Origin", "*");

<system.webServer>
<httpProtocol>
<customHeaders>
<add name="Access-Control-Allow-Origin" value="*" />
<add name="Access-Control-Allow-Headers" value="Content-Type" />
<add name="Access-Control-Allow-Methods" value="*" />
</customHeaders>
</httpProtocol>
</system.webServer>

 /// <summary>
 /// 跨域
 /// </summary>
 public class Cores:ActionFilterAttribute
 {
 public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
 {
 base.OnActionExecuted(actionExecutedContext);
 actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Origin","*");
 }
 }

 public static class WebApiConfig
 {
 public static void Register(HttpConfiguration config)
 {
 // Web API 配置和服務
 // 將 Web API 配置為僅使用不記名令牌身份驗證。
 config.SuppressDefaultHostAuthentication();
 config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));
 // Web API 路由
 config.MapHttpAttributeRoutes();
 config.Filters.Add(new Cores());
 config.Routes.MapHttpRoute(
 name: "DefaultApi",
 routeTemplate: "api/{controller}/{id}",
 defaults: new { id = RouteParameter.Optional }
 );
 }
 }