TextBox.MaxLength属性．
最近留恋于Csdn社区，发现很多网友问一些关于TextBox的MaxLength属性的问题．现将SDK说明列于此，
TextBox.MaxLength 属性
获取或设置文本框中最多允许的字符数
文本框中最多允许的字符数。默认值为 0，表示未设置该属性。
使用 MaxLength 属性限定可以在 TextBox 控件中输入的字符数。
注意 :仅当 TextMode 属性设置为 TextBoxMode.SingleLine 或 TextBoxMode.Password 时，此属性才适用。
<%@ Page Language="C#" AutoEventWireup="True" %>
<html>
<head>
<script runat="server">
protected void AddButton_Click(Object sender, EventArgs e)
{
int Answer;
Answer = Convert.ToInt32(Value1.Text) + Convert.ToInt32(Value2.Text);
AnswerMessage.Text = Answer.ToString();
}
</script>
</head>
<body>
<form runat="server">
<h3> TextBox Example </h3>
<table>
<tr>
<td colspan="5">
Enter integer values into the text boxes. <br/>
Click the Add button to add the two values. <br/>
Click the Reset button to reset the text boxes.
</td>
</tr>
<tr>
<td colspan="5">
</td>
</tr>
<tr align="center">
<td>
<asp:TextBox ID="Value1"
Columns="2"
MaxLength="3"
Text="1"
runat="server"/>
</td>
<td>
+
</td>
<td>
<asp:TextBox ID="Value2"
Columns="2"
MaxLength="3"
Text="1"
runat="server"/>
</td>
<td>
=
</td>
<td>
<asp:Label ID="AnswerMessage"
runat="server"/>
</td>
</tr>
<tr>
<td colspan="2">
<asp:RequiredFieldValidator
ID="Value1RequiredValidator"
ControlToValidate="Value1"
ErrorMessage="Please enter a value.<br/>"
Display="Dynamic"
runat="server"/>
<asp:RangeValidator
ID="Value1RangeValidator"
ControlToValidate="Value1"
Type="Integer"
MinimumValue="1"
MaximumValue="100"
ErrorMessage="Please enter an integer <br/> between than 1 and 100.<br/>"
Display="Dynamic"
runat="server"/>
</td>
<td colspan="2">
<asp:RequiredFieldValidator
ID="Value2RequiredValidator"
ControlToValidate="Value2"
ErrorMessage="Please enter a value.<br/>"
Display="Dynamic"
runat="server"/>
<asp:RangeValidator
ID="Value2RangeValidator"
ControlToValidate="Value2"
Type="Integer"
MinimumValue="1"
MaximumValue="100"
ErrorMessage="Please enter an integer <br/> between than 1 and 100.<br/>"
Display="Dynamic"
runat="server"/>
</td>
<td>
 
</td
</tr>
<tr align="center">
<td colspan="4">
<asp:Button ID="AddButton"
Text="Add"
OnClick="AddButton_Click"
runat="server"/>
</td>
<td>
</td>
</tr>
</table>
</form>
</body>
</html>
示例展示如何使用 MaxLength 属性将 TextBox 控件允许的字符数限定为 3。