This example modifies the earlier example Make a bouncing ball animation in C# to display multiple bouncing balls. Much of the basic idea is the same so see that example.

The big difference is that this version stores ball information in arrays instead of single variables. For example, instead of a single BallX variable to store one ball's position, this program uses a BallLocation array of Rectangles to store positions for many balls.

The following code shows how the program initializes its ball data.

// Some drawing parameters.
private static Rectangle[] BallLocation;
private static Point[] BallVelocity;
private static Size FormSize;

// Initialize some random stuff.
private void Form1_Load(object sender, EventArgs e)
{
    // Make random balls.
    Random rand = new Random();
    const int num_balls = 10;
    BallLocation = new Rectangle[num_balls];
    BallVelocity = new Point[num_balls];
    for (int i = 0; i < num_balls; i++)
    {
        int width = rand.Next(10, 40);
        BallLocation[i] = new Rectangle(
            rand.Next(0, this.ClientSize.Width - 2 * width),
            rand.Next(0, this.ClientSize.Height - 2 * width),
            width, width);
        int vx = rand.Next(2, 10);
        int vy = rand.Next(2, 10);
        if (rand.Next(0, 2) == 0) vx = -vx;
        if (rand.Next(0, 2) == 0) vy = -vy;
        BallVelocity[i] = new Point(vx, vy);
    }

    // Save the form's size.
    FormSize = this.ClientSize;

    // Use double buffering to reduce flicker.
    this.SetStyle(
        ControlStyles.AllPaintingInWmPaint |
        ControlStyles.UserPaint |
        ControlStyles.DoubleBuffer,
        true);
    this.UpdateStyles();
}

The following code moves the balls when the program's timer fires its Tick event.

// Move the balls and refresh.
private void tmrMoveBall_Tick(object sender, EventArgs e)
{
    for (int ball_num = 0; ball_num < BallLocation.Length; ball_num++)
    {
        // Move the ball.
        int new_x = BallLocation[ball_num].X + BallVelocity[ball_num].X;
        int new_y = BallLocation[ball_num].Y + BallVelocity[ball_num].Y;
        if (new_x < 0)
        {
            BallVelocity[ball_num].X = -BallVelocity[ball_num].X;
            Boing();
        }
        else if (new_x + BallLocation[ball_num].Width > FormSize.Width)
        {
            BallVelocity[ball_num].X = -BallVelocity[ball_num].X;
            Boing();
        }
        if (new_y < 0)
        {
            BallVelocity[ball_num].Y = -BallVelocity[ball_num].Y;
            Boing();
        }
        else if (new_y + BallLocation[ball_num].Height > FormSize.Height)
        {
            BallVelocity[ball_num].Y = -BallVelocity[ball_num].Y;
            Boing();
        }

        BallLocation[ball_num] = new Rectangle(
            new_x, new_y,
            BallLocation[ball_num].Width,
            BallLocation[ball_num].Height);
    }

    this.Invalidate();
}

Finally the following code shows how the program redraws its surface after the balls move.

// Draw the ball at its current location.
private void Form1_Paint(object sender, PaintEventArgs e)
{
    e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;
    e.Graphics.Clear(this.BackColor);
    for (int i = 0; i < BallLocation.Length; i++)
    {
        e.Graphics.FillEllipse(Brushes.Blue, BallLocation[i]);
        e.Graphics.DrawEllipse(Pens.Black, BallLocation[i]);
    }
}