web api该怎么传回json形式的数据

function generateDtb() {
//写入
var txtName = document.getElementById("txtName").value;
//创建数组
var dtb = new Array();
//通过循环把数据写入到数组并返回
for (var i = 0; i < firstGroup.length; i++) {

var row = new Object();
row.Name = txtName;
row.fullMoney = firstGroup[i].value;
row.discount = secondGroup[i].value;
dtb.push(row);
}
return dtb;
}

把数组转换成json串传入到后台：
$(function () {
//点击botton1
$("#lbtnOK").click(function () {
var url = "DiscountManger.aspx?ajax=1";
var dtb = generateDtb();
// var strName = document.getElementById("txtName").value;

if (dtb == null)
{ }
else {
//序列化对象
var postdata = JSON.stringify(dtb);
//异步请求
$.post(url, { json: postdata }, function (json) {
if (json) {
jBox.tip("添加成功！", "提示");
location.reload();
}
else {
jBox.tip("添加失败！", "提示");
location.reload();
}
}, "json")

}