Asp.net Confirm Box

Question

How I can I handle this logic in asp.net webforms

 

protected void btnSelectInvioces_Click(object source, EventArgs
{
    try
    {
        // some code here
        if(a == b)
        {
            // open confirmation box
            if(OK)  // if click ok button of confirm box
            {
                // proceed further...
            }
        }
    }
    catch()
    { }
}

 

Thanks