Could you please tell me taht what is the doc_PrintPage in the code At second last lie

private void button1_Click_1(object sender, EventArgs e)
        {
            System.Drawing.Printing.PrintDocument doc = new System.Drawing.Printing.PrintDocument();
            doc.PrintPage += new System.Drawing.Printing.PrintPageEventHandler(doc_PrintPage);
            doc.Print();
        }