球和球之间的碰撞运作不正常？

Question

问题在于碰撞方法，球确实与蝙蝠发生碰撞但发生的事情是......它只会从蝙蝠的下方碰撞到图片框的下边缘。我希望相反的事情发生，所以当球从图片框的顶部掉落时，它会从球棒上反弹回到图片框的顶部。任何帮助将不胜感激，任何问题请询问，谢谢你提前。

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    private Graphics paper;
    private SolidBrush brush, brushBlue, brushGreen, brushRed,
                       brushBlk, brushYellow;
    private int rectX, rectY;

    private int x, y, xDirection, yDirection, xBat;
    private int playCount, brushSize;
    private Random randomNumber;
    private bool btnDisplayBricksClicked;
    private Rectangle ball;
    private Rectangle bat;


    public Form1()
    {
        InitializeComponent();

        paper = picBox.CreateGraphics();
        brush = new SolidBrush(Color.Red);
        randomNumber = new Random();
        btnDisplayBricksClicked = false;

    }

    private void MoveBall()
    {
        timer1.Interval = randomNumber.Next(50, 100);
        timer1.Enabled = true;

        x = x + xDirection;
        y = y + yDirection;

        if (x >= picBox.Width)
            xDirection = -xDirection;

        if (y >= picBox.Height)
            yDirection = -yDirection;

        if (x <= 0)
            xDirection = -xDirection;

        if (y <= 0)
            yDirection = -yDirection;
    }

    private void DrawBall()
    {
        ball = new Rectangle(x, y, brushSize, brushSize);
        brushSize = 15;
        paper.FillEllipse(brush, ball);
    }

    private void DrawBat(Graphics paper)
    {

        brushBlue = new SolidBrush(Color.Blue);

        rectX = xBat - 25;
        rectY = picBox.Height - 30;
        bat = new Rectangle(rectX, rectY, 50, 20);
        paper.FillRectangle(brushBlue, bat);
    }

    private void CheckCollision()
    {
        if (ball.IntersectsWith(bat))
        {


            yDirection = -yDirection;              //problems here

        }          


    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        paper.Clear(Color.LightSteelBlue);

        DrawBall();

        MoveBall();

        DrawBat(paper);

        CheckCollision();

        if (btnDisplayBricksClicked)
        {
            DrawBricks(paper);
        }          

    }