DrawParentBackground / DrawThemeParentBackground on a top level form

Question

Consider the following example. This was done by setting the TransparencyKey property:

public Form()
{
    this.InitializeComponent();
    this.BackColor = Color.Fuscia;
    this.TransparencyKey = this.BackColor;
}

What I actually want to be able to do is similar to the behavior of the DrawThemeParentBackground function (conveniently wrapped up in .NET as DrawParentBackground), however this does not seem to work for top level forms (only controls).

I have tried to use the TransparencyKey behavior along with overriding the OnPaint method:

protected override void OnPaint(PaintEventArgs e)
{
    base.OnPaint(e);
    e.Graphics.FillRectangle(new SolidBrush(Color.FromArgb(128, 255, 0, 0)), this.ClientRectangle);
}

Result:

Question:

How can I draw the content underneath the ClientRectangle of a top level form?

Answer 1

Is this the effect you want?

If so, you can use two different forms. You do the drawing in the second one.

public partial class Form1 : Form
{
    private Form2 form2;

    public Form1()
    {
        InitializeComponent();
        this.BackColor = Color.White;
        this.TransparencyKey = Color.White;
        this.StartPosition = FormStartPosition.Manual;
        this.Location = new Point(200, 200);
        form2 = new Form2();
        form2.StartPosition = this.StartPosition;
        form2.Location = this.Location;
        form2.Show();

        this.FormClosed += new FormClosedEventHandler(Form1_FormClosed);
        this.LocationChanged += new EventHandler(Form1_LocationChanged);
        this.SizeChanged += new EventHandler(Form1_SizeChanged);
    }

    void Form1_FormClosed(object sender, FormClosedEventArgs e)
    {
        form2.Close();
    }

    void Form1_LocationChanged(object sender, EventArgs e)
    {
        form2.Location = this.Location;
    }

    void Form1_SizeChanged(object sender, EventArgs e)
    {
        form2.Size = this.Size;
    }
}

and

 public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
        ShowInTaskbar = false;
        this.Opacity = 0.8;
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        e.Graphics.FillRectangle(new SolidBrush(Color.FromArgb(255, 0, 0)), this.ClientRectangle);
    }
}