需要返回JSon数据集。
ASPX:
$.ajax({
type: "POST",
contentType: "application/json;utf-8",
data: "{strclassid:'" + strclassid + "'}",
url: "/AutoService.asmx/Show",
success: function (msg) {
var myObject = eval('(' + msg.d + ')');
for (var i = 0; i < myObject.length; i++) {
    $("#selectabc").append(("<option value='" + myObject[i].Id + "'>" + myObject[i].Names + "</option>"));
    }

Cs文件中可用

JavaScriptSerializer js = new JavaScriptSerializer();
return js.Serialize(USERS)

序列化
直接生成JSON文件更方便：

       StringBuilder sbGoodsName = new StringBuilder();
       sbGoodsName.Append("[{\"GoodsName\":\"请选择\",\"GoodsID\":\"-1\"}");
       DbDataReader dr = db.ExecuteReader(cmd);
       while (dr.Read())
        {
            sbGoodsName.Append(",{");
            sbGoodsName.Append("\"GoodsName\":" + "\"" + dr["names"].ToString() + "\",");
            sbGoodsName.Append("\"GoodsID\":" + "\"" + dr["id"].ToString() + "\"");
            sbGoodsName.Append("}");
        }
        dr.Close();
        sbGoodsName.Append("]");
        return sbGoodsName.ToString();