解决jQuery ajax跨域问题，Google、IE、Firefox亲测有效

直接上最后的结果吧

JS：

$.ajax({
            type: "GET",
            async: false,
            crossDomain: true,
            url: "www.test.com/TestHandler.ashx",
            data: { Id: "1"},
            dataType: "jsonp",
            jsonp: "callback",
            jsonpCallback: "callbackHandler",
            success: function (data) {
                //success
            },
            error: function (x, status, error) {
                //error
            }
        });

服务端（这里我用的是ashx一般处理程序）：

public void ProcessRequest(HttpContext context)
        {
            JavaScriptSerializer jsonHelper = new JavaScriptSerializer();

            HttpContext.Current.Response.ContentType = "application/json";
            HttpContext.Current.Response.AddHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS");
            HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Origin, Content-Type,Authorization, Accept, X-Requested-With");
            HttpContext.Current.Response.AddHeader("Access-Control-Allow-Origin", "*");

            var list=数据;
            
            if (string.IsNullOrEmpty(context.Request["callback"]))
            {
                context.Response.Write(jsonHelper.Serialize(list));
            }
            else
            {
                context.Response.Write(string.Format("{0}({1})", context.Request["callback"], jsonHelper.Serialize(list)));
            }
        }