options = {
                    type: "POST",
                    url: ajaxPage,
                    data:{"Formula":escape(textarea),"FieldNames":escape(fieldNames)},
                    contentType: "application/json;charset=utf-8",
                    dataType: "string",
                    async: false
                };
                returnText = $.ajax(options).responseText;

if (Request["mn"] != null && Request["mn"] == "Detection")
                {
                    Response.ContentType = "application/json";
                    Response.Write(IsExist());
                    Response.End();
                }

options = {
                    type: "POST",
                    url: ajaxPage,
                    data:{"Formula":textarea,"FieldNames":fieldNames},///////////
                   // contentType: "application/json;charset=utf-8",//不要设置contentType为这个，要不服务器端生成不了键值对
                    //dataType: "string",//dataType没有string类型，不过设置为这个也行，会自动变为text
                    dataType: "text",
                    async: false
                };