jquery ajax js得不到后台返回的值

JScript code:

proname = $("#txt_proName").val();
$.ajax({
type: "post",
url: "../FormHandler/ItemAllHandler.ashx",
data: { proname: proname, meth: "GetZhaobggJd" },
datatype: "text",
beforeSend: function() {
$('div#lbl_rpt_nozbgg').show('normal');
},
success: function(msg) {
alert(msg);//得不到值
},
complete: function() {
$('div#lbl_rpt_nozbgg').hide('normal');
}
});



//程序已经请求到后台这里了，设置断点得到异常
C# code:

ProjectAndEntBll projectentbll = new ProjectAndEntBll();
DataSet ds = projectentbll.GetZhaobggJd();
if (ds != null)
{
context.Response.Write("true");
try
{
context.Response.End();//这里出异常
}
catch (Exception e) {
e.ToString();// e = {由于代码已经过优化或者本机框架位于调用堆栈之上，无法计算表达式的值。}
}
}
else
{
context.Response.Write("false");