Windows Forms

private Panel panel;
private bool paintRectFlag=true;
private Rectangle rect=new Rectangle(20, 20, 300, 200);
private Pen rectPen=Pens.Black;

public Form1() {
    InitializeComponents();
    panel=new Panel();
    panel.Bounds=new Rectangle(…);
    panel.Paint+=panelPaintHandler;
    Controls.Add(panel);
}

protected void panelPaintHandler(object sender, PaintEventArgs e) {
    Graphics g=e.Graphics;
    if (paintRectFlag) g.DrawRectangle(rectPen, rect);
}

protected void buttonClickHandler(object sender, EventArgs e) {
    paintRectFlag=!paintRectFlag;   // toggle visible flag
    panel.Invalidate();
}

farzadmf wrote:

1. Why do you find the PictureBox control a useless one?

farzadmf wrote:

2. Why did you say that we shouldn't call the CreateGraphics method?

farzadmf wrote:

3. What exactly does the Panel.Invalidate() method do and how it can be used here?

farzadmf wrote:

4. Why did you say that either create the pen, font, etc. object as class members or create them inside the Paint event handler and call the Dispose method? Why is the Dispose method necessary here and what's the difference between these two methods you mentioned.

public class ExpandableGroupBox : System.Windows.Forms.GroupBox
{
    public ExpandableGroupBox()
    {
        base.Text = "";
    }

    public override string Text
    {
        get
        {
            return label.Text;
        }
        set
        {
            label.Text = value;
        }
    }
}