How to refresh graphics in C#

Question

I have a timer in a panel and when the timer ticks, it changes the coordinates of a rectangle.

I have tried two approaches: 1. Inside onPaint method, 2. Timer calls a function to create graphics and draw the moving rectangle

The first one does not work, but when I switch the windows, it moved once.

The second one works with problem. It is moving but leaving the previous position filled with color, which means the graphics are not refreshed.

I simply use g.FillRectangles() to do that.

Can anyone help me?

P.S. the panel is using a transparent background.

Added: This is a System.Windows.Form.Timer

timer = new Timer();
timer.Enabled = false;
timer.Interval = 100;  /* 100 millisec */
timer.Tick += new EventHandler(TimerCallback);

private void TimerCallback(object sender, EventArgs e)
{
    x+=10;
    y+=10;

    //drawsomething();
    return;
}

1.

protected override void OnPaint(PaintEventArgs e)
{
    Graphics g = e.Graphics;
    g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.None;
    g.FillRectangle(Brushes.Red, x, y, 100, 100);
    base.OnPaint(e);
}

2.

private void drawsomething()
{
    if (graphics == null)
        graphics = CreateGraphics();

    graphics.FillRectangle(Brushes.Red, x, y, 100, 100);
}

Answer 1

Place this.Invalidate() in the TimerCallback event.

private void TimerCallback(object sender, EventArgs e)
{
    x+=10;
    y+=10;

    this.Invalidate();
    return;
}

remove drawsomething function. it is not required here.

Full Code:

public partial class Form1 : Form
{
    Timer timer = new Timer();

    int x;
    int y;
    public Form1()
    {
        InitializeComponent();
        timer.Enabled = true;
        timer.Interval = 100;  /* 100 millisec */
        timer.Tick += new EventHandler(TimerCallback);
    }
    private void TimerCallback(object sender, EventArgs e)
    {
        x += 10;
        y += 10;
        this.Invalidate();
        return;
    }
    protected override void OnPaint(PaintEventArgs e)
    {
        Graphics g = e.Graphics;
        g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.None;
        g.FillRectangle(Brushes.Red, x, y, 100, 100);
        base.OnPaint(e);
    }
}

Answer 2

You need to call the Refresh method of control class after redrawing. It

Forces the control to invalidate its client area and immediately redraw itself and any child controls.

Answer 3

You can use Invalidate method. This method Invalidates a specific region of the control and causes a paint message to be sent to the control.

Details are here: http://msdn.microsoft.com/ru-ru/library/system.windows.forms.panel.invalidate%28v=vs.110%29.aspx