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































方法三: 自定义控件