RadioListBox: A ListBox with Radio Buttons (Winforms version)

• Download source file (C# control) - 1.81 KB
• Download demo project with source - 33.92 KB

Introduction

This is the .NET version of my previous MFC article, CRadioListBox: A ListBox with Radio Buttons. A couple of years ago, I discussed in a Visual C++ forum about a member's request to implement a custom ListBox control similar to MFC's CCheckListBox, but with radio buttons. Initially it appeared to be trivial, since the ListBox control's unique selection version complies with the requirements, but I have concluded that this control has some advantages:

• It is clearer that options are mutually exclusive with radio buttons.
• It is a good alternative to a group of radio buttons because you have to maintain just one control, less memory use.
• It inherits some useful features like scrolling, sorting, data binding and multi-column.
• It will be easier to change options dynamically, as shown in the demo application.
• It will be easier to manage selection events, also shown in the demo application.

Using the Code

To implement RadioListBox into your project, you just need to do a few steps:

• Include RadioListBox.cs into your project.
• Drop a RadioListBox object into your form.
• Change the standard properties of the control, just like a ListBox.
• Countersense to standard ListBox, transparent BackColor property is allowed.

That's all! Now you can use the radio button collection as a regular ListBox. You can add items with the Items.Add() method and query for user selection with the SelectedIndex property.

Fake Transparency

Some .NET controls accept a transparent color as a BackColor property, but ListBox is not one of them. So, transparency requires lots of non-managed tricks. However, transparency is a key feature needed for this control to be useful. It allows the control to acquire a real radio button look and feel, as you can see in the screenshot above. I decided to stay in the managed world by providing fake transparency to the control by overriding the BackColor property to accept it, and saving its own background color brush. When setting the background color to transparent, the control will mimic the parent form or control, even if the form has a non-standard background color.

RadioListBox Internals

The RadioListBox class is derived from Windows Forms' ListBox class with the owner-draw feature. The resumed class definition is the following:

using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms.VisualStyles;

namespace System.Windows.Forms
{
    public class RadioListBox : ListBox
    {
        private StringFormat Align;
        private bool IsTransparent = false;  // Handles the transparent state
        private Brush BackBrush;  // Manages its own background brush

        // Allows the BackColor to be transparent
        public override Color BackColor ...

        // Hides these properties in the designer
        [Browsable(false)]
        public override DrawMode DrawMode ...

        [Browsable(false)]
        public override SelectionMode SelectionMode ...

        // Public constructor
        public RadioListBox() ...

        // Main painting method
        protected override void OnDrawItem(DrawItemEventArgs e) ...

        // Prevent background erasing
        protected override void DefWndProc(ref Message m) ...

        // Other event handlers
        protected override void OnHandleCreated(EventArgs e) ...
        protected override void OnFontChanged(EventArgs e) ...
        protected override void OnParentChanged(EventArgs e) ...
        protected override void OnParentBackColorChanged(EventArgs e) ...
    }
}

The core enhancement is at the OnDrawItem() method. The method does not highlight the selected item as in a standard ListBox control, but draws a radio button instead. It also manages the focus state to draw the focus rectangle properly and the background color according to the transparency attribute. Here is the C# source code:

// Main painting method
protected override void OnDrawItem(DrawItemEventArgs e)
{
    int maxItem = this.Items.Count - 1;

    if (e.Index < 0 || e.Index > maxItem)
    {
        // Erase all background if control has no items
        e.Graphics.FillRectangle(BackBrush, this.ClientRectangle);
        return;
    }

    int size = e.Font.Height; // button size depends on font height, not on item height

    // Calculate bounds for background, if last item paint up to bottom of control
    Rectangle backRect = e.Bounds;
    if (e.Index == maxItem)
        backRect.Height = this.ClientRectangle.Top + 
        this.ClientRectangle.Height - e.Bounds.Top;
    e.Graphics.FillRectangle(BackBrush, backRect);

    // Determines text color/brush
    Brush textBrush;
    bool isChecked = (e.State & DrawItemState.Selected) == DrawItemState.Selected;

    RadioButtonState state = isChecked ? 
        RadioButtonState.CheckedNormal : RadioButtonState.UncheckedNormal;
    if ((e.State & DrawItemState.Disabled) == DrawItemState.Disabled)
    {
        textBrush = SystemBrushes.GrayText;
        state = isChecked ? RadioButtonState.CheckedDisabled : 
                RadioButtonState.UncheckedDisabled;
    }
    else if ((e.State & DrawItemState.Grayed) == DrawItemState.Grayed)
    {
        textBrush = SystemBrushes.GrayText;
        state = isChecked ? RadioButtonState.CheckedDisabled : 
                RadioButtonState.UncheckedDisabled;
    }
    else
    {
        textBrush = SystemBrushes.FromSystemColor(this.ForeColor);
    }

    // Determines bounds for text and radio button
    Size glyphSize = RadioButtonRenderer.GetGlyphSize(e.Graphics, state);
    Point glyphLocation = e.Bounds.Location;
    glyphLocation.Y += (e.Bounds.Height - glyphSize.Height) / 2;

    Rectangle bounds = new Rectangle(e.Bounds.X + glyphSize.Width, e.Bounds.Y, 
        e.Bounds.Width - glyphSize.Width, e.Bounds.Height);

    // Draws the radio button
    RadioButtonRenderer.DrawRadioButton(e.Graphics, glyphLocation, state);

    // Draws the text
    // Bound Datatable? Then show the column written in Displaymember   
    if (!string.IsNullOrEmpty(DisplayMember)) 

        e.Graphics.DrawString(((System.Data.DataRowView)this.Items[e.Index])
            [this.DisplayMember].ToString(),
        e.Font, textBrush, bounds, this.Align);
    else
        e.Graphics.DrawString(this.Items[e.Index].ToString(), 
            e.Font, textBrush, bounds, this.Align);

    // If the ListBox has focus, draw a focus rectangle around the selected item.
    e.DrawFocusRectangle();
}

History

• 23^rd April, 2007: First version
• 14^th September, 2007: Refinements in control rendering (thanks to stephpms); support for bounded data (thanks to PeterDP)
• 1^st September, 2008: Improved background painting; support for large fonts (thanks to rkousha)