如何在richTextBox中添加新行？

我有一个鼠标按下事件，我正在将文本写入richTextBox：

private void pictureBoxSnap_MouseDown(object sender, MouseEventArgs e)
        {
            richTextBox1.Text += "Rectangle Location: " + e.Location + Environment.NewLine;
            Bitmap bmp;
            Point pnt;

            if (e.Button == MouseButtons.Right)
            {
                cm.Show(pictureBoxSnap, e.Location);

                return;
            }

            if (e.Button == MouseButtons.Left)
            {
                if (checkError(listBox1.SelectedIndex, "pictureBoxSnap_MouseDown") == true) { return; } //if listBox1 empty

                if (isCroppedList[listBox1.SelectedIndex] == true) { return; }

                mouseDown = e.Location;

                if (canDrawNormallyList[listBox1.SelectedIndex] == false)
                {
                    bmp = new Bitmap(pictureBoxSnap.Width - 4, pictureBoxSnap.Height - 4);

                    pnt = PointToScreen(pictureBoxSnap.Location);

                    g = Graphics.FromImage(bmp);

                    g.CopyFromScreen(pnt.X + 2, pnt.Y + 2, 0, 0, new Size(bmp.Width, bmp.Height));

                    g.Dispose();

                    windowsBitmaps[listBox1.SelectedIndex] = bmp;
                }
                else
                {
                    if (thumb != IntPtr.Zero)
                    {
                        MessageBoxButtons buttons = MessageBoxButtons.OK;
                        MessageBoxIcon icon = MessageBoxIcon.Error;

                        MessageBox.Show("ERROR in pictureBoxSnap_MouseDown: thumb != IntPtr.Zero", "ERROR", buttons, icon);

                        this.Close();
                    }
                }
            }
        }

在鼠标按下事件中，我正在编写矩形的位置。

然后在DrawRectangle方法中，我想实时在richTextBox1中显示矩形的大小：

private void DrawRectangle(Point pnt)
        {
            g = Graphics.FromImage(img);

            g.Clear(Color.FromKnownColor(KnownColor.Control));

            if (pnt.X == mouseDown.X || pnt.Y == mouseDown.Y)
            {
                g.DrawLine(Pens.Firebrick, mouseDown.X, mouseDown.Y, pnt.X, pnt.Y);
            }
            else
            {
                g.DrawRectangle(Pens.Firebrick, Math.Min(mouseDown.X, pnt.X), Math.Min(mouseDown.Y, pnt.Y),
                            Math.Abs(mouseDown.X - pnt.X), Math.Abs(mouseDown.Y - pnt.Y));
                richTextBox1.Text = "Rectangle Size: " + Math.Abs(mouseDown.X - pnt.X) +
                    Math.Abs(mouseDown.Y - pnt.Y);
            }

            g.Dispose();

            g = frm.CreateGraphics();

            g.DrawImage(img, 0, 0, img.Width, img.Height);

            g.Dispose();
        }

现在的问题是，当我按下鼠标左键单击鼠标时，我看到了位置，但是当我左右移动鼠标时，它在位置线周围被拖动了，而我看到了尺寸线。如果在DrawRectangle方法中我也使+ =，则只是在我多次移动/拖动鼠标aorund时多次添加了大小行，从而用整个填满了richTextBox。