C#代码展示：点击button按钮，在picturebox按住鼠标左键，拖动鼠标画矩形，松开鼠标左键矩形绘制完成，可重复这个操作画多个矩形，并且可以选中任意一个矩形拖动改变位置和改变大小，可以删除，并且根据窗体大小自适应

好的，下面是C#代码：

public partial class Form1 : Form
{
    private List<Rectangle> rectangles = new List<Rectangle>(); // 用于存储矩形
    private Rectangle selectedRectangle; // 选中的矩形
    private bool isDragging; // 是否正在拖动矩形
    private Point mouseOffset; // 鼠标在矩形内的偏移量

    public Form1()
    {
        InitializeComponent();
        pictureBox1.MouseDown += new MouseEventHandler(pictureBox1_MouseDown);
        pictureBox1.MouseMove += new MouseEventHandler(pictureBox1_MouseMove);
        pictureBox1.MouseUp += new MouseEventHandler(pictureBox1_MouseUp);
    }

    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            // 开始绘制矩形
            Rectangle rect = new Rectangle(e.Location, new Size(0, 0));
            rectangles.Add(rect);
            selectedRectangle = rect;
        }
    }

    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            if (selectedRectangle != null)
            {
                // 拖动矩形
                isDragging = true;
                selectedRectangle.Width = e.Location.X - selectedRectangle.X;
                selectedRectangle.Height = e.Location.Y - selectedRectangle.Y;
                pictureBox1.Invalidate(); // 重绘pictureBox
            }
        }
        else
        {
            // 鼠标未按下时，检查是否在矩形内
            foreach (Rectangle rect in rectangles)
            {
                if (rect.Contains(e.Location))
                {
                    selectedRectangle = rect;
                    mouseOffset = new Point(e.Location.X - rect.X, e.Location.Y - rect.Y);
                    pictureBox1.Cursor = Cursors.SizeAll; // 显示拖动光标
                    return;
                }
            }
            selectedRectangle = null;
            pictureBox1.Cursor = Cursors.Default; // 显示默认光标
        }
    }

    private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
    {
        if (e.Button == MouseButtons.Left)
        {
            if (isDragging)
            {
                // 结束拖动矩形
                isDragging = false;
            }
            else
            {
                // 绘制矩形完成
                selectedRectangle.Width = e.Location.X - selectedRectangle.X;
                selectedRectangle.Height = e.Location.Y - selectedRectangle.Y;
                pictureBox1.Invalidate(); // 重绘pictureBox
            }
        }
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {
        // 绘制矩形
        foreach (Rectangle rect in rectangles)
        {
            e.Graphics.DrawRectangle(Pens.Red, rect);
        }
    }

    private void Form1_Resize(object sender, EventArgs e)
    {
        // pictureBox随窗体大小自适应
        pictureBox1.Size = new Size(ClientSize.Width - pictureBox1.Location.X - 20,
                                    ClientSize.Height - pictureBox1.Location.Y - 20);
    }
}

同时，需要在窗体中添加一个PictureBox控件，并将其Dock属性设置为Fill，这样它就会随着窗体的大小自适应。另外，需要在pictureBox的Paint事件中调用Invalidate方法来重绘pictureBox。