RequiredFieldValidator的属性：InitialValue
InitialValue设置后，
只能保证控件的值和InitialValue值不同；
不能保证控件的值“不为空”。
Code
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>无标题页</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList id="DropDownList1" runat="server">
<asp:ListItem Selected="True">Select a profession</asp:ListItem>
<asp:ListItem>Programmer</asp:ListItem>
<asp:ListItem>Lawyer</asp:ListItem>
<asp:ListItem>Doctor</asp:ListItem>
<asp:ListItem>Artist</asp:ListItem>
</asp:DropDownList>
<asp:RequiredFieldValidator id="RequiredFieldValidator1" runat="server"
ErrorMessage="必须选择"
ControlToValidate="DropDownList1"
InitialValue="Select a profession">
</asp:RequiredFieldValidator><br />
<br />
<asp:TextBox ID="TextBox1" Runat="server">My Initial Value</asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator2" Runat="server"
ErrorMessage="必须改变文本框内容" ControlToValidate="TextBox1"
InitialValue="My Initial Value"></asp:RequiredFieldValidator>
<asp:RequiredFieldValidator ID="RequiredFieldValidator3" Runat="server"
ErrorMessage="不能为空" ControlToValidate="TextBox1">
</asp:RequiredFieldValidator><br />
<br />
<asp:Button ID="Button1" runat="server" Text="Button" /></div>
</form>
</body>
</html>
