How to make transparent background to ListBox?

Question

I have a c# WinForms application with a picture in the background. I put a ListBox to the window and I would like to change the BackColor to transparent to my ListBox. But the ListBox doesn't support transparent BackColor . What can I do for it?

Answer 1

What you can do should be something like:

Say, we want to create a transparent list box named TansparentListBox . So we need to derive a new class from the ListBox control, set some control styles to make it double buffered, prevent painting the background, and to say that we will do the painting ourselves:

using System;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.ComponentModel;

[ToolboxBitmap(typeof(ListBox)),
    DesignerCategory("")]
public class TransparentListBox : ListBox
{

    public TransparentListBox() : base()
    {
        SetStyle(
            ControlStyles.Opaque |
            ControlStyles.AllPaintingInWmPaint |
            ControlStyles.ResizeRedraw |
            ControlStyles.UserPaint |
            ControlStyles.OptimizedDoubleBuffer, true);
        DrawMode = DrawMode.OwnerDrawFixed;
    }
    //...

Now we need to override both the OnPaint and OnDrawItem events to do the drawing. The DrawThemeParentBackground function is required to copy the parent's background, just the region of our control. Also we have a new member, the SelectionBackColor property, the background color of the selected items:

    //...
    public Color SelectionBackColor { get; set; } = Color.DarkOrange;

    [DllImport("uxtheme", ExactSpelling = true)]
    private extern static int DrawThemeParentBackground(
        IntPtr hWnd, 
        IntPtr hdc, 
        ref Rectangle pRect
        );

    protected override void OnPaint(PaintEventArgs e)
    {
        var g = e.Graphics;
        var rec = ClientRectangle;

        IntPtr hdc = g.GetHdc();
        DrawThemeParentBackground(this.Handle, hdc, ref rec);
        g.ReleaseHdc(hdc);

        using (Region reg = new Region(e.ClipRectangle))
        {
            if (Items.Count > 0)
            {
                for (int i = 0; i < Items.Count; i++)
                {
                    rec = GetItemRectangle(i);

                    if (e.ClipRectangle.IntersectsWith(rec))
                    {
                        if ((SelectionMode == SelectionMode.One && SelectedIndex == i) ||
                            (SelectionMode == SelectionMode.MultiSimple && SelectedIndices.Contains(i)) ||
                            (SelectionMode == SelectionMode.MultiExtended && SelectedIndices.Contains(i)))
                            OnDrawItem(new DrawItemEventArgs(g, Font, rec, i, DrawItemState.Selected, ForeColor, BackColor));
                        else
                            OnDrawItem(new DrawItemEventArgs(g, Font, rec, i, DrawItemState.Default, ForeColor, BackColor));

                        reg.Complement(rec);
                    }
                }
            }
        }            
    }

    protected override void OnDrawItem(DrawItemEventArgs e)
    {
        if (e.Index < 0) return;

        var rec = e.Bounds;
        var g = e.Graphics;

        g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;

        if ((e.State & DrawItemState.Selected) == DrawItemState.Selected)
            using (SolidBrush sb = new SolidBrush(SelectionBackColor))
                g.FillRectangle(sb, rec);

        using (SolidBrush sb = new SolidBrush(ForeColor))
        using (StringFormat sf = new StringFormat { Alignment = StringAlignment.Near, LineAlignment = StringAlignment.Center })
            g.DrawString(GetItemText(Items[e.Index]), Font, sb, rec, sf);
    }
    //...

The hard part is done. To finish up, the drawing should be refreshed on occurrence of certain events, otherwise it will be a visual chaos:

    //...
    protected override void OnSelectedIndexChanged(EventArgs e)
    {
        base.OnSelectedIndexChanged(e);
        Invalidate();
    }

    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);
        Invalidate();
    }

    protected override void OnMouseWheel(MouseEventArgs e)
    {
        base.OnMouseWheel(e);
        Invalidate();
    }
    //...

Also, a refresh is required when using both, the vertical and horizontal scroll bars. Doing that is possible by overriding the WndProc since the ListBox control does not have any scroll events.

    //...
    private const int WM_KILLFOCUS  = 0x8;
    private const int WM_VSCROLL    = 0x115;
    private const int WM_HSCROLL    = 0x114;

    protected override void WndProc(ref Message m)
    {
        if (m.Msg != WM_KILLFOCUS && 
            (m.Msg == WM_HSCROLL || m.Msg == WM_VSCROLL))
            Invalidate();
        base.WndProc(ref m);
    }
}

That's it. Puzzle up, rebuild and try.