asp.net validation to make sure textbox has integer values

simpally add this code:

<asp:FilteredTextBoxExtender ID="txtAltitudeMin_FilteredTextBoxExtender" runat="server" Enabled="True" TargetControlID="txtAltitudeMin" FilterType="Numbers"></asp:FilteredTextBoxExtender>

In C# asp.net WebForms

txtBoxNumber.Attributes.Add("onkeypress", "javascript:if(event.which!=8 && event.which!=0 && (event.which<43 || event.which>57)){return false;};");