System.Drawing.Drawing2D.GraphicsPath shape = new System.Drawing.Drawing2D.GraphicsPath(); 
shape.AddEllipse(0,0,this.Height, this.Width); 
this.Region = new Region(shape); 

class BitmapRegion 
{ 
  public BitmapRegion() 
  { } 
 
 
  /// <summary>  
  /// Create and apply the region on the supplied control 
  /// 創(chuàng)建支持位圖區(qū)域的控件（目前有button和form） 
  /// </summary>  
  /// <param name="control">The Control object to apply the region to控件</param>  
  /// <param name="bitmap">The Bitmap object to create the region from位圖</param>  
  public static void CreateControlRegion(Control control, Bitmap bitmap) 
  { 
    // Return if control and bitmap are null 
    //判斷是否存在控件和位圖 
    if (control == null || bitmap == null) 
      return; 
 
    // Set our control''s size to be the same as the bitmap 
    //設置控件大小為位圖大小 
    control.Width = bitmap.Width; 
    control.Height = bitmap.Height; 
    // Check if we are dealing with Form here  
    //當控件是form時 
    if (control is System.Windows.Forms.Form) 
    { 
      // Cast to a Form object 
      //強制轉換為FORM 
      Form form = (Form)control; 
      // Set our form''s size to be a little larger that the bitmap just  
      // in case the form''s border style is not set to none in the first place  
      //當FORM的邊界FormBorderStyle不為NONE時，應將FORM的大小設置成比位圖大小稍大一點 
      form.Width = control.Width; 
      form.Height = control.Height; 
      // No border  
      //沒有邊界 
      form.FormBorderStyle = FormBorderStyle.None; 
      // Set bitmap as the background image  
      //將位圖設置成窗體背景圖片 
      form.BackgroundImage = bitmap; 
      // Calculate the graphics path based on the bitmap supplied  
      //計算位圖中不透明部分的邊界 
      GraphicsPath graphicsPath = CalculateControlGraphicsPath(bitmap); 
      // Apply new region  
      //應用新的區(qū)域 
      form.Region = new Region(graphicsPath); 
    } 
    // Check if we are dealing with Button here  
    //當控件是button時 
    else if (control is System.Windows.Forms.Button) 
    { 
      // Cast to a button object  
      //強制轉換為 button 
      Button button = (Button)control; 
      // Do not show button text  
      //不顯示button text 
      button.Text = ""; 
 
      // Change cursor to hand when over button  
      //改變 cursor的style 
      button.Cursor = Cursors.Hand; 
      // Set background image of button  
      //設置button的背景圖片 
      button.BackgroundImage = bitmap; 
 
      // Calculate the graphics path based on the bitmap supplied  
      //計算位圖中不透明部分的邊界 
      GraphicsPath graphicsPath = CalculateControlGraphicsPath(bitmap); 
      // Apply new region  
      //應用新的區(qū)域 
      button.Region = new Region(graphicsPath); 
    } 
  } 
  /// <summary>  
  /// Calculate the graphics path that representing the figure in the bitmap  
  /// excluding the transparent color which is the top left pixel.  
  /// //計算位圖中不透明部分的邊界 
  /// </summary>  
/// <param name="bitmap">The Bitmap object to calculate our graphics path from</param>  
  /// <returns>Calculated graphics path</returns>  
  private static GraphicsPath CalculateControlGraphicsPath(Bitmap bitmap) 
  { 
    // Create GraphicsPath for our bitmap calculation  
    //創(chuàng)建 GraphicsPath 
    GraphicsPath graphicsPath = new GraphicsPath(); 
    // Use the top left pixel as our transparent color  
    //使用左上角的一點的顏色作為我們透明色 
    Color colorTransparent = bitmap.GetPixel(0, 0); 
    // This is to store the column value where an opaque pixel is first found.  
  // This value will determine where we start scanning for trailing opaque pixels. 
    //第一個找到點的X 
    int colOpaquePixel = 0; 
    // Go through all rows (Y axis)  
    // 偏歷所有行（Y方向） 
    for (int row = 0; row < bitmap.Height; row++) 
    { 
      // Reset value  
      //重設 
      colOpaquePixel = 0; 
      // Go through all columns (X axis)  
      //偏歷所有列（X方向） 
      for (int col = 0; col < bitmap.Width; col++) 
      { 
    // If this is an opaque pixel, mark it and search for anymore trailing behind  
        //如果是不需要透明處理的點則標記，然后繼續(xù)偏歷 
        if (bitmap.GetPixel(col, row) != colorTransparent) 
        { 
          // Opaque pixel found, mark current position 
          //記錄當前 
          colOpaquePixel = col; 
       // Create another variable to set the current pixel position  
          //建立新變量來記錄當前點 
          int colNext = col; 
        // Starting from current found opaque pixel, search for anymore opaque pixels  
      // trailing behind, until a transparent  pixel is found or minimum width is reached  
          ///從找到的不透明點開始，繼續(xù)尋找不透明點,一直到找到或則達到圖片寬度  
        for (colNext = colOpaquePixel; colNext < bitmap.Width; colNext++) 
            if (bitmap.GetPixel(colNext, row) == colorTransparent) 
              break; 
      // Form a rectangle for line of opaque  pixels found and add it to our graphics path  
          //將不透明點加到graphics path 
      graphicsPath.AddRectangle(new Rectangle(colOpaquePixel, row, colNext - colOpaquePixel, 1)); 
          // No need to scan the line of opaque pixels just found  
          col = colNext; 
        } 
      } 
    } 
    // Return calculated graphics path  
    return graphicsPath; 
  } 
} 

public Form3() 
    { 
      InitializeComponent(); 
      //創(chuàng)建不規(guī)則窗體 
      POINTAPI[] poin; 
      poin = new POINTAPI[5]; 
      poin[0].x = 90; 
      poin[0].y = 90; 
      poin[1].x = this.Width; 
      poin[1].y = 0; 
      poin[2].x = Width; 
      poin[2].y = this.Height / 2; 
      poin[3].x = Width / 2; 
      poin[3].y = Height / 2; 
      poin[4].x = 0; 
      poin[4].y = Width; 
      Boolean flag = true; 
      IntPtr hRgn = CreatePolygonRgn(ref poin[0], 8, 1); 
      SetWindowRgn(this.Handle, hRgn, ref flag); 
      this.BackColor = Color.BurlyWood; 
    } 
    [StructLayout(LayoutKind.Sequential)] 
    private struct POINTAPI 
    { 
      internal int x; 
      internal int y; 
    } 
    [DllImport("gdi32.dll")] 
 private static extern IntPtr CreatePolygonRgn(ref POINTAPI lpPoint,int nCount,int nPolyFillMode); 
    [DllImport("user32.dll")] 
 private static extern IntPtr SetWindowRgn(IntPtr hWnd,IntPtr hRgn, ref Boolean bRedraw); 
    //設置窗體顯示狀態(tài) 
    [DllImport("user32.dll")] 
private static extern int SetWindowPos(IntPtr hwnd,int hWndInsertAfter,int x,int y,int cx,int cy,int wFlags); 
    private void Start_Btn_Click(object sender, EventArgs e) 
    {//始終顯示在前面 
      SetWindowPos(this.Handle, -1, 0, 0, 0, 0, 1); 
    } 
    private void button1_Click(object sender, EventArgs e) 
    { 
      //最小化始終顯示在前面 
      SetWindowPos(this.Handle, -1, 0, 0, 0, 0, 0); 
    }