ajax正确返回数据，却进入了error分支

$.ajax({
type: "POST",  //post没有数据量限制
url: "ashx/PostHandle.ashx",
data: { "datatype": "1", "event_name": event_name, "placename": placename, "starttimestamp": starttimestamp, "endtimestamp": endtimestamp }
contentType: "text/plain; charset=utf-8",
dataType: "json",   //必须
beforeSend: function () {
//$("#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";
}
});

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 "1"://添加线下活动促销
if (AddEventPromotion(root).Equals("success"))
{ context.Response.Write("success"); }
break;
default:
break;
}
context.Response.End();
}

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