使图形随滚动轮旋转

namespace WindowsApplication1
{
    public partial class Form1 : Form
    {
        Bitmap memBitmap; //Bitmap封装 GDI+ 位图，此位图由图形图像及其属性的像素数据组成。Bitmap 是用于处理由像素数据定义的图像的对象。
        float currRotation = 10;
        Rectangle rect = new Rectangle(100, 50, 150, 100); //存储一组整数，共四个，表示一个矩形的位置和大小。
        public Form1()
        {
            InitializeComponent();
            memBitmap = new Bitmap(this.Width, this.Height);
            using (Graphics g = Graphics.FromImage(memBitmap))
            {
                g.DrawRectangle(Pens.Red, rect);
            }
        }

        /// <summary>
        /// 滚轮事件
        /// </summary>
        /// <param name="e"></param>

        protected override void OnMouseWheel(MouseEventArgs e)
        {
            currRotation -= 5.0f * e.Delta / SystemInformation.MouseWheelScrollDelta;     // 5 度旋转，无论向上转动滚轮，还是向下转动滚轮，你可以达到预定的45度；
            Invalidate();
        }

        /// <summary>
        /// 画出所需的图形，随滚轮的转动变化 
        /// </summary>
        /// <param name="e"></param>

        protected override void OnPaint(PaintEventArgs e)
        {
            float shiftX = (rect.Left + rect.Right) / 2.0f;
            float shiftY = (rect.Top + rect.Bottom) / 2.0f;
            using (Graphics g = e.Graphics)
            {
                g.TranslateTransform(shiftX, shiftY);
                g.RotateTransform(currRotation);
                g.TranslateTransform(-shiftX, -shiftY);
                g.DrawImageUnscaled(memBitmap, 0, 0);
            }
        }
    }
}