我尝试了一个全局变量标志bool name draw。但它不工作，它永远不会显示消息离开事件。
我用一个断点进行了检查，它确实到达了if行的leave事件，但没有产生我想要的效果。
我想要的效果，只有当按住鼠标左键，同时移动鼠标，然后如果离开pictureBox2区域，然后显示消息。

private bool draw = false;

public Form1()
{
    InitializeComponent();

    InitializeForm();
}

private void InitializeForm()
{
    textBox2.Text = Properties.Settings.Default.CroppedImagesFolder;
    selectedPath = textBox2.Text;

    if (!string.IsNullOrEmpty(textBox1.Text))
    {
        pictureBox2.Image = Image.FromFile(textBox1.Text);
    }

    checkBoxDrawBorder.Checked = true;
    checkBoxClearRectangles.Checked = true;
    checkBoxSaveRectangles.Checked = true;

    LoadFile(textBox2, "folder.txt");

    if (!string.IsNullOrEmpty(selectedPath) && Directory.Exists(selectedPath))
    {
        LoadFile(textBox1, "image.txt");
        LoadRectangles();
    }
}

public class DrawingRectangle
{
    public Rectangle Rect => new Rectangle(Location, Size);
    public Size Size { get; set; }
    public Point Location { get; set; }
    public Control Owner { get; set; }
    public Point StartPosition { get; set; }
    public Color DrawingcColor { get; set; } = Color.LightGreen;
    public float PenSize { get; set; } = 3f;
}

private void pictureBox2_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left && pictureBox2.Image != null && selectedPath != null)
    {
        currentRect = new DrawingRectangle()
        {
            Location = e.Location,
            Size = Size.Empty,
            StartPosition = e.Location,
            Owner = (Control)sender,
            DrawingcColor = SelectedColor
        };
    }
}

private void pictureBox2_MouseMove(object sender, MouseEventArgs e)
{
    if (currentRect != null && e.Button == MouseButtons.Left)
    {
        draw = true;
        Point mouseLocation = pictureBox2.PointToClient(Cursor.Position);
        int width = mouseLocation.X - currentRect.Location.X;
        int height = mouseLocation.Y - currentRect.Location.Y;
        width = Math.Max(width, 0);
        height = Math.Max(height, 0);
        currentRect.Size = new Size(width, height);
        pictureBox2.Invalidate();
    }
}

private void pictureBox2_MouseUp(object sender, MouseEventArgs e)
{
    if (currentRect != null)
    {
        draw = false;
        DrawingRects.Add(currentRect);
        currentRect = null;
        ProcessRectangles();
    }
}

private void pictureBox2_MouseLeave(object sender, EventArgs e)
{
    if (draw)
    {
        MessageBox.Show("You Leave At: " + MousePosition.X.ToString() + " --- " + MousePosition.Y.ToString());
    }
}

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