textbox的textmode取为multiline多行时，其maxlength不起作用

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