方法一： 验证控件验证(经实践可行)
Setting the Maxength of a TextBox when it is in Multiline, You can use RegularExpressionValidator control as shown below
<asp:TextBox ID="txtConclusion" MaxLength="200" TextMode="MultiLine" Height="100px" Width="400px" runat="server" />
<asp:RegularExpressionValidator
ID="txtConclusionValidator1" ControlToValidate="txtConclusion"
Text="超过200字" ValidationExpression="^[\s\S]{0,200}$"
runat="server" />方法二：弹出对话框提示
1.html代码
<HTML>
<HEAD>
<title>WebForm6</title>
<meta name="GENERATOR" Cnotallow="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" Cnotallow="C#">
<meta name="vs_defaultClientScript" cnotallow="JavaScript">
<meta name="vs_targetSchema" cnotallow="http://schemas.microsoft.com/intellisense/ie5">
<script language="javascript">
function isOver(sText,len)
{
var intlen=sText.value.length;
if (intlen>len)
{
alert("The content length must Less than or Equal "+len);
sText.focus();
sText.select();
}
}
</script>
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<asp:TextBox id="txtName" style="Z-INDEX: 102; LEFT: 200px; POSITION: absolute; TOP: 104px" runat="server"
TextMode="MultiLine" Height="112px" Width="271px"></asp:TextBox>
</form>
</body>
</HTML>2.cs代码
private void Page_Load(object sender, System.EventArgs e)
{
this.txtName.Attributes.Add("onblur","isOver(this,1000);");
}方法三： 自定义控件
Multiline TextBox with MaxLength Validation
http://www.codeproject.com/KB/aspnet/Textarea_Length_Validator.aspx