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>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