public void AddLineNumbers() { // create & set Point pt to (0,0) Point pt = new Point(0, 0); // get First Index & First Line from richTextBox1 int First_Index = TxtEditorTexto.GetCharIndexFromPosition(pt); int First_Line = TxtEditorTexto.GetLineFromCharIndex(First_Index); // set X & Y coordinates of Point pt to ClientRectangle Width & Height respectively pt.X = ClientRectangle.Width; pt.Y = ClientRectangle.Height; // get Last Index & Last Line from richTextBox1 int Last_Index = TxtEditorTexto.GetCharIndexFromPosition(pt); int Last_Line = TxtEditorTexto.GetLineFromCharIndex(Last_Index); // set Center alignment to LineNumberTextBox LineNumberTextBox.SelectionAlignment = HorizontalAlignment.Center; // set LineNumberTextBox text to null & width to getWidth() function value LineNumberTextBox.Text = ""; LineNumberTextBox.Width = getWidth(); // now add each line number to LineNumberTextBox upto last line for (int i = First_Line; i <= Last_Line; i++) { LineNumberTextBox.Text += i + 1 + "\n"; } }