矩形消失而不是移动

Question

我试图使用MyRectangle.Offset(x, y)移动一个矩形，但它没有移动。从我发现它的东西，我正在取消它的副本，而不是矩形本身，所以现在我创建一个具有所需坐标的新矩形，然后用这个替换旧的矩形。它适用于第一次移动，但是当我想第二次移动矩形时，它就会消失。我不知道为什么。以下是我的一些代码：

private void button2_Click(object sender, EventArgs e)
    {
            spawn = "movement";
            pictureBox1.Invalidate();
    }

private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        switch (spawn)
        {
            case "movement":
                foreach(aircraft acft in aircrafts) // aircrafts is an array of of class aircraft I have created. 
                                                    // It consist of many variables (mostly integers), and the rectangle that is supposed to move
                {
                    acft.Move_calculate(e.Graphics);
                }
                spawn = string.Empty; // clearing the string to prevent creating an infinite loop
                break;
                ....

public void Move_calculate(Graphics g)
    {
        speed = nahodnacisla.Next(14, 21);
        if (points[passed].X > x_coordinate && points[passed].Y > y_coordinate) // have four of these conditons (x > or <, y > or <)
        {
            points[passed].X = Bx; // points is an array of Point, those are the points the rectangle will be moving around (move slowly to one, then to another)
            points[passed].Y = By;
            distanceForAlfaX = Bx - x_coordinate; // x_coordinate is currect coordinate of the rectangle
            distanceForAlfaY = By - y_coordinate;
            alpha =(distanceForAlfaY/distanceForAlfaX); // tangens alpha
            x_change = (int)(speed * (Math.Cos(alpha))); // using the tangens counting the rectagle movement relative to x axis
            y_change = (int)Math.Sqrt(((speed * speed) + (x_change * x_change))); // calculate movement relative to y axis using Pythagoras theorem
            x_coordinate += x_change; // update the coordinates
            y_coordinate += y_change;
            Pen p = new Pen(Color.Turquoise, 2);
            r = new Rectangle(x_coordinate, y_coordinate, 5, 5); // replace the original rectangle named r with a new one
            g.DrawRectangle(p, r);
            p.Dispose();

正如我所说，第一次，矩形根据速度设置正确移动。但是当我再次单击“移动”按钮（稍后将定时器设置为两秒）时，它会消失。为什么呢？