c# - C#如何用背景透明度绘制用户控件

Question

我想知道如何用背景透明度绘制用户控件，即使用户控件在运行时被提升或移动。我的代码是

private void UserControl1_Paint(object sender, PaintEventArgs e)
{
    var g = e.Graphics;
    g.Clear(Color.White);
    g.SmoothingMode = SmoothingMode.HighQuality;//可以反锯齿
    var rectBound = new Rectangle(0, 0, Width-1, Height-1);

    var b = new SolidBrush(Color.FromArgb(0, 122, 204));
    var rect = new Rectangle(2, 2, Width - 4, Height - 4);
    if(!_isSelected)//FillRectangle
        g.FillEllipse(b, rectBound);
    else
        g.FillEllipse(b, rect);

    var pen = new Pen(Color.Yellow);
    pen.DashStyle = DashStyle.DashDot;
    g.DrawLine(pen,10,10,100,10);

    pen.DashStyle = DashStyle.Dash;
    g.DrawLine(pen, 10, 15, 100, 15);

    pen.DashStyle = DashStyle.DashDotDot;
    g.DrawLine(pen, 10, 20, 100, 20);

    pen.DashStyle = DashStyle.Dot;
    g.DrawLine(pen, 10, 25, 100, 25);

    pen.DashStyle = DashStyle.Solid;
    g.DrawLine(pen, 10, 30, 100, 30);

    if (_isSelected)
    {
        pen = new Pen(Color.Black) { DashStyle = DashStyle.Dot, Width = 1 };
        g.DrawRectangle(pen, rectBound);
    }
}

和演示图片在这里

运行时会创建很多用户控件，其中一些可能重叠，那么应该清楚用户控件的背景。如何做到这一点？