WinFrame App C＃游戏设计：左右移动图片框

Question

I would like my pictureboxes to move left and right, so far they're only moving right and disappearing when they reach the edge. In the code there are 3 enemies, they can all move to the right, how can I make these move left when they hit "the wall"? The Player can move both directions.

{
Random _random;


public MainWindow()
{
    InitializeComponent();
    _random = new Random();
}

private void MainWindow_Load(object sender, EventArgs e)
{

    Size s = new System.Drawing.Size(800, 600);
    this.ClientSize = s; 
    this.FormBorderStyle = FormBorderStyle.FixedSingle;
}

private void MainWindow_KeyDown(object sender, KeyEventArgs e)
{
    {
        if (e.KeyCode == Keys.Left)
        {
            Player.Left -= 20;
        }

        if (e.KeyCode == Keys.Right)
        {
            Player.Left += 20;
        }

    }
}

private void timer1_Tick(object sender, EventArgs e)
{
    int z = _random.Next(0, 10);
    int x = _random.Next(0, 20);
    int y = _random.Next(0, 30);
    LargeEnemy.Left += z;
    MediumEnemy.Left += x;
    SmallEnemy.Left += y;

}

Answer 1

Using your code I made this, first make 3 global bools: bool LargeGoLeft = true, MediumGoLeft = true, SmallGoLeft = true;
Then put the code in your timer like so:

    bool moveLeft = false;
    public Form1()
    {
        InitializeComponent();
    }

    private void timer1_Tick(object sender, EventArgs e)
    {
        if((pictureBox1.Left + pictureBox1.Width) >= this.Width)
        {
            moveLeft = true;
        }
        if(pictureBox1.Left < 0)
        {
            moveLeft = false;
        }
        if(moveLeft)
        {
            pictureBox1.Left -= 15;
        }
        if (!moveLeft)
        {
            pictureBox1.Left += 15;
        }
    }

This is the version I was able test and it works perfectly with a picturebox