Making a control transparent

You will need to use Opacity property and set it to zero to make form invisible.

If you want to make a control Transparent, as you have tried in your example, See this article

How to: Give Your Control a Transparent Background

It say the code you have written, must be in constructor of the control. Hence, I guess, you will need to create a custom control derived from your pnlSelectArea 's type most probaably a button. In in that custom control's constructor you can write code to set its style and color.

Here is what worked for me with because the other solutions did not work.

This is with transparent UserControl added to ListView/TreeView Control Collection

I know it says ButtonRenderer but it should work for any controls.

In the UserControl:

protected override void OnPaint(PaintEventArgs e)
{
    ButtonRenderer.DrawParentBackground(e.Graphics, this.ClientRectangle, this);
}

in the Parent control:

protected override void WndProc(ref Message m) 
{
    if(m.Msg == 0xF)
        foreach(Control c in this.Controls) { c.Invalidate(); c.Update(); }

    base.WndProc(ref m);
}

This is my special Control which contains an opacity property, it 100% works:

using System;
using System.Collections;
using System.ComponentModel;
using System.ComponentModel.Design;
using System.Drawing;
using System.Windows.Forms;
using System.Windows.Forms.Design;

public class TranspCtrl : Control
{
    public bool drag = false;
    public bool enab = false;
    private int m_opacity = 100;

    private int alpha;
    public TranspCtrl()
    {
        SetStyle(ControlStyles.SupportsTransparentBackColor, true);
        SetStyle(ControlStyles.Opaque, true);
        this.BackColor = Color.Transparent;
    }

    public int Opacity
    {
        get
        {
            if (m_opacity > 100)
            {
                m_opacity = 100;
            }
            else if (m_opacity < 1)
            {
                m_opacity = 1;
            }
            return this.m_opacity;
        }
        set
        {
            this.m_opacity = value;
            if (this.Parent != null)
            {
                Parent.Invalidate(this.Bounds, true);
            }
        }
    }

    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams cp = base.CreateParams;
            cp.ExStyle = cp.ExStyle | 0x20;
            return cp;
        }
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        Graphics g = e.Graphics;
        Rectangle bounds = new Rectangle(0, 0, this.Width - 1, this.Height - 1);

        Color frmColor = this.Parent.BackColor;
        Brush bckColor = default(Brush);

        alpha = (m_opacity * 255) / 100;

        if (drag)
        {
            Color dragBckColor = default(Color);

            if (BackColor != Color.Transparent)
            {
                int Rb = BackColor.R * alpha / 255 + frmColor.R * (255 - alpha) / 255;
                int Gb = BackColor.G * alpha / 255 + frmColor.G * (255 - alpha) / 255;
                int Bb = BackColor.B * alpha / 255 + frmColor.B * (255 - alpha) / 255;
                dragBckColor = Color.FromArgb(Rb, Gb, Bb);
            }
            else
            {
                dragBckColor = frmColor;
            }

            alpha = 255;
            bckColor = new SolidBrush(Color.FromArgb(alpha, dragBckColor));
        }
        else
        {
            bckColor = new SolidBrush(Color.FromArgb(alpha, this.BackColor));
        }

        if (this.BackColor != Color.Transparent | drag)
        {
            g.FillRectangle(bckColor, bounds);
        }

        bckColor.Dispose();
        g.Dispose();
        base.OnPaint(e);
    }

    protected override void OnBackColorChanged(EventArgs e)
    {
        if (this.Parent != null)
        {
            Parent.Invalidate(this.Bounds, true);
        }
        base.OnBackColorChanged(e);
    }

    protected override void OnParentBackColorChanged(EventArgs e)
    {
        this.Invalidate();
        base.OnParentBackColorChanged(e);
    }
}