ajax正确返回数据，却进入了error分支

.net 开发：

$.ajax({

        type: "POST",  //post没有数据量限制

        url: "ashx/PostHandle.ashx",

        data: { "datatype": "1", "event_name": event_name, "placename": placename, "starttimestamp": starttimestamp, "endtimestamp": endtimestamp }

        contentType: "text/plain; charset=utf-8",

        dataType: "json",   //必须

        beforeSend: function () {

            //$("#loading_bind")[0].style.visibility = "visible";

        },

        success: function (data) {

            alert('data');

        },

        error: function (XMLHttpRequest, textStatus, errorThrown) {

            alert("服务器响应提交失败，请重试...");

        },

        complete: function () {

            //$("#loading_bind")[0].style.visibility = "hidden";

        }

    });

后台ashx：

public void ProcessRequest (HttpContext context) {

        context.Response.Clear(); //清除所有之前生成的Response内容

        //context.Response.ContentType = "text/plain";

        StreamReader stream = new StreamReader(context.Request.InputStream);

        string xml = stream.ReadToEnd();  //post里面的数据

        XmlDocument doc = new XmlDocument();

        try {

            doc.LoadXml(xml);

        }

        catch(XmlException)

        {

            context.Response.Write("post加载的xml数据不能为空");

            return;

        }

        XmlElement root = doc.DocumentElement;

        string datatype = root.SelectSingleNode("datatype").InnerText;

        switch (datatype)

        {

            case ""://添加线下活动促销

                if (AddEventPromotion(root).Equals("success"))

                { context.Response.Write("success"); }

                break;

            default:

                break;

        }

        context.Response.End();

    }

后台返回数据全部正常，最后执行了：

context.Response.Write("success");

但js中缺执行了ajax的error分支，解决方法：
将js中ajax代码的第6行：   dataType: "json",  注释掉  即可

详见datatype的说明  http://deony2jacob1314.iteye.com/blog/2017093