有现成的RequiredFidldValidator，给你写个示例
<%@ Page Language= "c# " Debug= "true "%>
<script language= "C# " runat= "server ">
void Page_Load(Object Source, EventArgs E)
{
}
void submit(object src,EventArgs e){
Response.Write( "提交成功！ ");
}
</script>
<html>
<head>
<title> Webdiyer asp.net示例 </title>
</head>
<body>
<form runat= "server " method= " ">
<asp:radiobuttonlist runat= "server " id= "rbl ">
<asp:ListItem value= "0 "> zero </asp:ListItem>
<asp:ListItem value= "1 "> one </asp:ListItem>
<asp:ListItem value= "2 "> two </asp:ListItem>
<asp:ListItem value= "3 "> three </asp:ListItem>
</asp:RadioButtonList>
<asp:RequiredFieldValidator controlToValidate= "rbl " errorMessage= "RadioButtonList不能不选 " runat= "server "/>
<br>
<asp:DropDownList runat= "server " id= "ddl ">
<asp:ListItem value= "0 "> 请选择 </asp:ListItem>
<asp:ListItem value= "1 "> one </asp:ListItem>
<asp:ListItem value= "2 "> two </asp:ListItem>
<asp:ListItem value= "3 "> three </asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator controlToValidate= "ddl " initialValue= "0 " errorMessage= "DropDownList不能选择第一项 " runat= "server "/>
<p>
<asp:Button runat= "server " id= "btn " Text= "提交 " onclick= "submit "/>
</form>
</body>
</html>
<script language= "C# " runat= "server ">
void Page_Load(Object Source, EventArgs E)
{
}
void submit(object src,EventArgs e){
Response.Write( "提交成功！ ");
}
</script>
<html>
<head>
<title> Webdiyer asp.net示例 </title>
</head>
<body>
<form runat= "server " method= " ">
<asp:radiobuttonlist runat= "server " id= "rbl ">
<asp:ListItem value= "0 "> zero </asp:ListItem>
<asp:ListItem value= "1 "> one </asp:ListItem>
<asp:ListItem value= "2 "> two </asp:ListItem>
<asp:ListItem value= "3 "> three </asp:ListItem>
</asp:RadioButtonList>
<asp:RequiredFieldValidator controlToValidate= "rbl " errorMessage= "RadioButtonList不能不选 " runat= "server "/>
<br>
<asp:DropDownList runat= "server " id= "ddl ">
<asp:ListItem value= "0 "> 请选择 </asp:ListItem>
<asp:ListItem value= "1 "> one </asp:ListItem>
<asp:ListItem value= "2 "> two </asp:ListItem>
<asp:ListItem value= "3 "> three </asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator controlToValidate= "ddl " initialValue= "0 " errorMessage= "DropDownList不能选择第一项 " runat= "server "/>
<p>
<asp:Button runat= "server " id= "btn " Text= "提交 " onclick= "submit "/>
</form>
</body>
</html>