suppose initial value is 0 assign this value to ur textbox at design time or load time when user change this value to 10 and again insert 0 then it will work
its better u use javascript
<script type="text/javascript">
function valid() {
var v_type = document.getElementById("<%=Voucher_Type.ClientID%>").value;
//var sales = document.getElementById("editor1").value;
if (v_type == "--Select--" )
{
alert("fill all information");
return false;
}
else {
return true;
}
}
</script>
<asp:button id="btnSave" runat="server" text="Save" onclick="btnSave_Click" onclientclick="return valid();" xmlns:asp="#unknown">