How to Show yes/no alert box use c# after buttonclick result

Question

ASPX

<script type = "text/javascript">
function Confirm() {
var confirm_value = document.createElement("INPUT");
confirm_value.type = "hidden";
confirm_value.name = "confirm_value";
if (confirm("Do you want to save data?")) {
confirm_value.value = "Yes";
} else {
confirm_value.value = "No";
}
document.forms[0].appendChild(confirm_value);
}
</script>

Code Behind

protected bool ValidateDuplicate()
{
bool result = true;
Material material = new Material();
MaterialDataAccess materialDA = new MaterialDataAccess();
if (hfdMode.Value.Equals("new") || hfdMode.Value.Equals("draft"))
{
material.Type = "N";
}
else if (hfdMode.Value == "update")
{
material.Type = "U";
}
material.RequestNo = hfRequestNo.Value;
material.Description = tbxShortDescription.Text.ToUpper().Replace("'", "''");
materialDA.Material = material;
materialDA.ValidateCheck();
if (!materialDA.Message.Text.Equals("SUCCESS"))
{
ClientScript.RegisterStartupScript(typeof(Page), "Confirm", "<script type='text/javascript'>callConfirm();</script>");
string confirmValue = Request.Form["confirm_value"];
if (confirmValue == "Yes")
{
// event occures
}
else
{
// do nothing
}
}
return result;
}

I have used above function in inside the button clicl after the success message come continue our further process if not come success to show the popup with yes/no if click no process is stop click no stop the process.but the above code the script is show after the process is completes how to do it?