how to create message box in public static void method

I have a public static method and I would like a message to be displayed if certain values are selected. This is in asp.net so adding using System.Windows.Forms; causes problems because I am using using System.Web.UI.WebControls;. So how do I create a message?

public static void UpdateSerialQtyRcvd(int SerNoID, int QtyRcvd)
     {
            if (SerNo.QtyRcvd != 1)
            {
                if (SerNo.Reason == "")
                {
                    //message
                }
            }
        }

via Chebli Mohamed