c# - Making a control transparent

Question

I am currently developing a simple image editing tool using Winforms and .NET 3.5 (work environment).

I have a requirement that when the user clicks a select tool button, a square (rectangle in C#) will appear that they can scale between 100x100 and 400x400. I have this bit fixed - the issue comes with making the background of the rectangle transparent.

I'm a little unclear on if transparency is supported in .NET 3.5, I've tried the following:

SetStyle(ControlStyles.SupportsTransparentBackColor, true);
pnlSelectArea.BackColor = Color.Transparent;
pnlSelectArea.ForeColor = Color.Transparent;
selectArea1.BackColor = Color.Transparent;
selectArea1.ForeColor = Color.Transparent;

But this has no effect - any advice would be appreciated.

Answer 1

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);
    }
}