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" Content="Microsoft Visual Studio .NET 7.1">

<Meta name="CODE_LANGUAGE" Content="C#">

<Meta name="vs_defaultClientScript" content="JavaScript">

<Meta name="vs_targetSchema" content="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
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