1、自定義方式

····/// <summary>
    /// 支援WebAPI伺服器端跨域
    /// 有很多種支援服務端跨域的方式，但是不能夠同時使用
    /// </summary>
    public class ServerCrossDomainAttribute : ActionFilterAttribute
    {
        private const string ORIGIN = "Origin";
        private const string ACCESS_CONTROL_ALLOW_ORIGIN = "
 
Access-Control-Allow-Origin";

        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        {
            #region 跨域
            if (actionExecutedContext.Request.Headers.Contains(ORIGIN))
            {
                string originHeader = actionExecutedContext.Request.Headers.GetValues(ORIGIN).FirstOrDefault();
                
 
if (!string.IsNullOrEmpty(originHeader))
                {
                    #region 第一種支援跨域的方式
                    //actionExecutedContext.Response.Headers.Add(ACCESS_CONTROL_ALLOW_ORIGIN, originHeader);
                    //actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS");
                    
 
//actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, Authorization");
                    #endregion

                    #region 第二種支援跨域方式
                    //actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Origin", "*");
                    //actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS");
                    //actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept, Authorization");
                    #endregion

                    #region 第三種支援跨域方式  在webconfig中配置
                    //< !--< add name = "Access-Control-Allow-Origin" value = "*" />    
                    //< add name = "Access-Control-Allow-Headers" value = "Content-Type, Authorization" />       
                    //< add name = "Access-Control-Allow-Methods" value = "GET, POST, PUT, DELETE, OPTIONS" /> -->
                    #endregion
                }
            }
            #endregion

            
        }
    }

2、使用System.Web.Http.Cors