jQuery.get、jQuery.getJSON、jQuery.post无法返回JSON问题的解决方法

经过尝试，只需要在$.ajax中加入 contentType: "application/json; charset=utf-8"选项就可以了，这是因为在.net 3.5以后要对contentType进行检查，所以仅仅指定dataType的话.net就不会返回JSON了，那么我们的请求自然也就无法请求到JSON数据了。正确写法就是这样了：

var url = "/Services/AccountService.asmx/UserExists"; 
var userName = $("#txtUserName").val(); 
$.ajax({ 
type: "POST", 
url: url, 
data: '{userName:"'+userName+'"}', 
dataType: "json", 
success: function (json) { 
if (json.d == true) { 
$("#submit").removeAttr("disabled"); 
return; 
} 
$("#submit").attr("disabled", "disabled"); 
} 
});

更正后代码

var url = "/Services/AccountService.asmx/UserExists"; 
var userName = $("#txtUserName").val(); 
$.ajax({ 
type: "POST", 
url: url, 
data: '{userName:"'+userName+'"}', 
dataType: "json", 
contentType: "application/json; charset=utf-8", 
success: function (json) { 
if (json.d == true) { 
$("#submit").removeAttr("disabled"); 
return; 
} 
$("#submit").attr("disabled", "disabled"); 
} 
});

但是在使用$.get、$.getJSON、$.post时确怎么也得不到JSON数据了,写法如下：
$.get代码

var url = "/Services/AccountService.asmx/UserExists"; 
var userName = $("#txtUserName").val(); 
$.get( 
url 
, { userName: userName } 
, function (json) { 
if (json.d == true) { 
$("#submit").removeAttr("disabled"); 
return; 
} 
$("#submit").attr("disabled", "disabled"); 
},"json");

$.getJSON代码

var url = "/Services/AccountService.asmx/UserExists"; 
var userName = $("#txtUserName").val(); 
$.getJSON( 
url 
, { userName: userName } 
, function (json) { 
if (json.d == true) { 
$("#submit").removeAttr("disabled"); 
return; 
} 
$("#submit").attr("disabled", "disabled"); 
});

$.post代码

var url = "/Services/AccountService.asmx/UserExists"; 
var userName = $("#txtUserName").val(); 
$.post( 
url 
, { userName: userName } 
, function (json) { 
if (json.d == true) { 
$("#submit").removeAttr("disabled"); 
return; 
} 
$("#submit").attr("disabled", "disabled"); 
},'json');

用HttpWatch查看请求返回的数据如下：

<?xml version="1.0" encoding="utf-8"?> 
<boolean xmlns="http://tempuri.org/">false</boolean>

看一下jQuery.extend中有关代码：
jQuery.extend

jQuery.extend({ 
get: function( url, data, callback, type ) { 
// shift arguments if data argument was omited 
if ( jQuery.isFunction( data ) ) { 
type = type || callback; 
callback = data; 
data = null; 
} 
return jQuery.ajax({ 
type: "GET", 
url: url, 
data: data, 
success: callback, 
dataType: type 
}); 
}, 
getScript: function( url, callback ) { 
return jQuery.get(url, null, callback, "script"); 
}, 
getJSON: function( url, data, callback ) { 
return jQuery.get(url, data, callback, "json"); 
}, 
post: function( url, data, callback, type ) { 
// shift arguments if data argument was omited 
if ( jQuery.isFunction( data ) ) { 
type = type || callback; 
callback = data; 
data = {}; 
} 
return jQuery.ajax({ 
type: "POST", 
url: url, 
data: data, 
success: callback, 
dataType: type 
}); 
} 
});

原因就是.net 3.5以后要对contentType进行检查，如果不为json的话，就不会返回json,而get、getJSON、post扩展都再次调用了ajax，但是只传了dataType参数，.net 3.5在检查contentType时发现其不是json，便返回了xml.