asp:textboxonkeypress="isnum()"id="TextBox1"runat="server">/asp:textbox>

scriptlanguage="javascript">
functionisnum()
{
if(event.keyCode45||event.keyCode>57)
{
event.keyCode=0;
}
}
/script>

try
{
Convert.ToDouble(TextBox1.Text.Trim());
}
catch
{
Response.Write(" ");
TextBox1.Text="";
Page.RegisterStartupScript("focus"," ");
}