探讨这个问题的起因是CSDN论坛的一个帖子：
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http://community.csdn.net/Expert/TopicView.asp?id=5460832
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查看客户端生成的HTML代码可以看出，每次点击都执行一次__doPostBack，并且将选择项的参数传递到隐藏表单域之中。 当然，通过脚本和自己的隐藏域可以很轻松得到选择项，但完全在服务器端，也可以得到。
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<form name=”MengXianhui” method=”post” action=”a.aspx?ddfg” id=”MengXianhui”>
<input type=”hidden” name=”__EVENTTARGET” value=”" />
<input type=”hidden” name=”__EVENTARGUMENT” value=”" />
<input type=”hidden” name=”__VIEWSTATE” value=”dDw0NjI1MDk1MzQ7O2w8Q2hlY2tCb3hMaXN0MTowO0NoZWNrQm94TGlzdDE6MTtDaGVja0JveExpc3QxOjI7Q2hlY2tCb3hMaXN0 MTozO0NoZWNrQm94TGlzdDE6NDtDaGVja0JveExpc3QxOjQ7Pj6h9p2gvuU8iLKlA4DU0GtLAoZhCw==” />
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<script language=”javascript” type=”text/javascript”>
<!–
function __doPostBack(eventTarget, eventArgument) {
var theform;
if (window.navigator.appName.toLowerCase().indexOf(“microsoft”) > -1) {
theform = document.MengXianhui;
}
else {
theform = document.forms["MengXianhui"];
}
theform.__EVENTTARGET.value = eventTarget.split(“”).join(“:”);
theform.__EVENTARGUMENT.value = eventArgument;
theform.submit();
}
// –>
</script>
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<div style=”padding:20px”>
<table id=”CheckBoxList1″ border=”0″>
<tr>
<td><input id=”CheckBoxList1_0″ type=”checkbox” name=”CheckBoxList1:0″ onclick=”__doPostBack(‘CheckBoxList10′,”) ” language=”javascript” /><label for=”CheckBoxList1_0″>a</label></td>
</tr><tr>
<td><input id=”CheckBoxList1_1″ type=”checkbox” name=”CheckBoxList1:1″ onclick=”__doPostBack(‘CheckBoxList11′,”)” language=”javascript” /><label for=”CheckBoxList1_1″>b</label></td>
</tr><tr>
<td><input id=”CheckBoxList1_2″ type=”checkbox” name=”CheckBoxList1:2″ checked=”checked” onclick=”__doPostBack(‘CheckBoxList12′,”) ” language=”javascript” /><label for=”CheckBoxList1_2″>c</label></td>
</tr><tr>
<td><input id=”CheckBoxList1_3″ type=”checkbox” name=”CheckBoxList1:3″ checked=”checked” onclick=”__doPostBack(‘CheckBoxList13′,”)” language=”javascript” /><label for=”CheckBoxList1_3″>d</label></td>
</tr><tr>
<td><input id=”CheckBoxList1_4″ type=”checkbox” name=”CheckBoxList1:4″ onclick=”__doPostBack(‘CheckBoxList14′,”) ” language=”javascript” /><label for=”CheckBoxList1_4″>e</label></td>
</tr>
</table>
</div>
</form>
在服务器端得到选择项的做法是分析事件的参数，通过提交过来的事件参数得到。代码如下：
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<%@ Page Language=”C#” AutoEventWireup=”true” EnableViewState=”false” Debug=”true” % >
<!DOCTYPE html PUBLIC ” -//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1- transitional.dtd”>
<script runat=”server”>
void BoxList1_SelectedIndexChanged(object sender, System.EventArgs e)
{
//得到触发事件的选择项
string ClickedItem = Request.Form["__EVENTTARGET"];
ClickedItem = ClickedItem.Split(‘:’)[1];
int index = Int32.Parse(ClickedItem );
Response.Write(CheckBoxList1.Items [index].Text);
}
</script>
<html xmlns=”http://www.w3.org/1999/xhtml”>
<head runat=”server”>
<title>在服务器端得到CheckBoxList 最好所选择项的方法</title>
</head>
<body>
<form id=”MengXianhui” runat=”server”>
<div style=”padding:20px”>
<asp:CheckBoxList AutoPostBack=”true” ID=”CheckBoxList1″ runat=”server”
OnSelectedIndexChanged=”BoxList1_SelectedIndexChanged”>
<asp:ListItem Value=”1″>a</asp:ListItem>
<asp:ListItem>b</asp:ListItem>
<asp:ListItem>c</asp:ListItem>
<asp:ListItem>d</asp:ListItem>
<asp:ListItem>e</asp:ListItem>
</asp:CheckBoxList>
</div>
</form>
</body>
</html>
来源：CSDN