If you are using C# then try this.
This will allow the user to enter the numeric values.
<asp:TextBox ID="txt_ID" runat="server" ></asp:TextBox>
//Add Extender
<asp:FilteredTextBoxExtender ID="txt_ID_FilteredTextBoxExtender" FilterType="Numbers" runat="server" Enabled="True" TargetControlID="txt_ID"> </asp:FilteredTextBoxExtender>
To know about the FilteredTextBoxExtender [
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