C# PicBox 背景透明

private void picBackground_Paint(object sender, PaintEventArgs e)
        {
            foreach (Control C in this.Controls)
            {
                if (C is Label)
                {

                    Label L = (Label)C;

                    L.Visible = false;

                    //设置绘制文字的格式  
                    StringFormat strFmt = new System.Drawing.StringFormat();
                    strFmt.Alignment = StringAlignment.Center; //文本垂直居中  
                    strFmt.LineAlignment = StringAlignment.Center; //文本水平居中  
                    e.Graphics.DrawString(L.Text, this.Font, new SolidBrush(this.ForeColor), new RectangleF(L.Left - picBackground.Left, L.Top - picBackground.Top, L.Width, L.Height), strFmt); 

                }
                else if (C is PictureBox)
                {
                    PictureBox L = (PictureBox)C;
                    if (!L.Name.Equals("picBackground"))
                    {
                        L.Visible = false;
                        ImageAttributes attrib = new ImageAttributes();
                        //Bitmap img = new Bitmap(L.Image);
                        Color color = Color.Transparent;
                        attrib.SetColorKey(color, color);
                        e.Graphics.DrawImage(L.Image, new Rectangle(L.Left - picBackground.Left, L.Top - picBackground.Top, L.Width, L.Height), 0, 0, L.Image.Width, L.Image.Height, GraphicsUnit.Pixel, attrib);
                    }
                }
            }
        }