C#实现图片格式转换的方法

/// <summary>
    /// 打开图片
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnOpen_Click(object sender, EventArgs e)
    {

      this.fileDialog.Filter = fileFilter;
      this.fileDialog.Multiselect = true;
      this.fileDialog.CheckFileExists = true;
      if (fileDialog.ShowDialog() == DialogResult.OK)
      {
        string[] fileNames = this.fileDialog.FileNames;
        foreach(string fileName in fileNames)
        {
          Bitmap bmp = new Bitmap(fileName);
          //保存图片名称
          bmp.Tag = Path.GetFileNameWithoutExtension(fileName);
          PictureBox box = new PictureBox();
          box.Image = bmp;
          box.Width = 105;
          box.Height = 150;
          box.BorderStyle = BorderStyle.FixedSingle;
          box.Padding = new Padding(2);
          this.flowPnl.Controls.Add(box);
        }
        this.txtFile.Text = Path.GetDirectoryName(fileNames[0]);

      }
    }

/// <summary>
    /// 转换图片
    /// </summary>
    private void convertImage(string dir, string filter,Bitmap bmp)
    {
      string filePath = Path.Combine(dir, string.Format("{0}.{1}", bmp.Tag.ToString(), filter.ToLower()));
      switch (filter)
      {
        case "JPG":
          bmp.Save(filePath, ImageFormat.Jpeg);
          break;
        case "PNG":
          bmp.Save(filePath, ImageFormat.Png);
          break;
        case "GIF":
          bmp.Save(filePath, ImageFormat.Gif);
          break;
        case "BMP":
          bmp.Save(filePath, ImageFormat.Bmp);
          break;
        case "ICO":
          Stream stream = File.Create(filePath);
          Icon icon = Icon.FromHandle(bmp.GetHicon());
          icon.Save(stream);    //  save the icon
          stream.Close();
          break;
      }
    }