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Thank you Sir,
but i found the ease code for validate the text box :
<asp:RegularExpressionValidator ID="RegularExpressionValidator3" runat="server" ErrorMessage="Max 300 char" ValidationExpression="[\s\S]{1,300}" ControlToValidate="TextBox6" ForeColor="Red"></asp:RegularExpressionValidator>
its working well
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http://www.c-sharpcorner.com/uploadfile/bryianTan/limit-number-of-characters-in-textbox-control/default.aspx