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A ComboBox with a CheckedListBox as a Dropdown

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22 Nov 2008CPOL4 min read 653.8K   33.5K   110   167
A ComboBox with a CheckedListBox as a dropdown
Image 1

Introduction

This control came out of my need to create a CheckedListBox control which does not occupy as much space on the form while the selected item(s) are visible. Although a few examples already exist using owner-drawn controls (i.e., drawing the ListBox portion of the ComboBox), I wasn't fully satisfied because of some shortcomings, hence I came up with a new version, the CheckedComboBox which has the following features:

  • Read-only text portion, displaying the checked item(s).
  • Each checked item appears in the text portion as a string, each string separated by a custom separator that I may define.
  • List portion stays open until the user finishes the selection.
  • Check/uncheck all items.
  • Display/hide the list portion via the keyboard.
  • Mimic as closely as possible the .NET ComboBox.
  • Use it from the Visual Studio palette (as a custom control).

Implementation

The CheckedComboBox derives from ComboBox. In order to avoid the problem of the list portion disappearing before the user finishes the selection, I decided to catch the DropDown event and display my own list portion which consists of a Form whose client area is completely filled by a CheckListBox. Both the Form and the CheckListBox are my own custom versions since I needed to override some of the default behaviour.

Therefore, my custom list portion appears either on the DropDown event or when the user presses the Down arrow while the CheckedComboBox has the focus. I overrode the OnDropDown(), OnKeyDown(), and the OnKeyPress() methods to:

  1. display the dropdown list, and
  2. prevent any keyboard input, in order to make the text portion read-only.
C#
protected override void OnDropDown(EventArgs e) {
    base.OnDropDown(e);
    DoDropDown();    
}

private void DoDropDown() {
    if (!dropdown.Visible) {
        Rectangle rect = RectangleToScreen(this.ClientRectangle);
        dropdown.Location = new Point(rect.X, rect.Y + this.Size.Height);
        int count = dropdown.List.Items.Count;
        if (count > this.MaxDropDownItems) {
            count = this.MaxDropDownItems;
        } else if (count == 0) {
            count = 1;
        }
        dropdown.Size = new Size(this.Size.Width, 
                       (dropdown.List.ItemHeight + 1) * count);
        dropdown.Show(this);
    }
}

protected override void OnKeyDown(KeyEventArgs e) {
    if (e.KeyCode == Keys.Down) {        
        OnDropDown(null);
    }
    // Make sure that certain keys or combinations are not blocked.
    e.Handled = !e.Alt && !(e.KeyCode == Keys.Tab) &&
        !((e.KeyCode == Keys.Left) || (e.KeyCode == Keys.Right) || 
    (e.KeyCode == Keys.Home) || (e.KeyCode == Keys.End));
    base.OnKeyDown(e);
}

protected override void OnKeyPress(KeyPressEventArgs e) {
    e.Handled = true;
    base.OnKeyPress(e);
}

Now, the list portion, being a Form, will stay open without being closed (hidden actually) until the user either presses Esc (the Escape button) to cancel any changes, Enter to accept all changes, or clicks with the mouse anywhere outside the list, which is the usual behaviour of the ComboBox.

In order to achieve this, I had to catch the Deactivate event of the Form (for the mouse click behaviour) and the keyboard events (for the Esc, Enter behaviour). In addition, if the user presses Del (the Delete button) or Shift + Del while the list portion has the focus, all items will be unchecked/checked, respectively, allowing a useful (to me, anyway :-)) keyboard shortcut. Note how a custom CCBoxEventArgs class has been used in order to distinguish whether the Deactivate message comes from the framework (i.e., the mouse click) or from the keyboard (to which I have control since I'm trapping keystrokes).

C#
protected override void OnDeactivate(EventArgs e) {
    base.OnDeactivate(e);
    CCBoxEventArgs ce = e as CCBoxEventArgs;
    if (ce != null) {
        CloseDropdown(ce.AssignValues);
    } else {

        CloseDropdown(true);
    }
}

Using the Code

The code comes as a demo project which you can run and see as an example of usage. In order to use the CheckedComboBox in your code, you only need one file, CheckedComboBox.cs. Although I have not made it a fully-functional Visual Studio control with all the bows and whistles, once you compile your project, the CheckedComboBox should appear under the Components in your Visual Studio ToolBox palette and you can drag and drop it on to your Form in the Designer.

Alternatively, you can very easily use it by manually writing code like shown below to declare and create an instance of a CheckedComboBox:

C#
private CheckedComboBox ccb = new CheckedComboBox();

// If more than 5 items, add a scroll bar to the dropdown.
ccb.MaxDropDownItems = 5;
// Make the "Name" property the one to display, rather than the ToString() 
// representation of the item.
ccb.DisplayMember = "Name";
// Set a separator for how the checked items will appear in the Text portion.
ccb.ValueSeparator = ", ";

In order to add items, use the Items collection...

C#
private string[] coloursArr = { "Red", "Green", "Black", 
                                "White", "Orange", "Yellow", 
                                "Blue", "Maroon", "Pink", "Purple" };

for (int i = 0; i < coloursArr.Length; i++) {
    CCBoxItem item = new CCBoxItem(coloursArr[i], i);
    ccb.Items.Add(item);
}

... where the CCBoxItem is a simple test class I've created as follows (it's in the demo):

C#
public class CCBoxItem {
    private int val;
    public int Value {
        get { return val; }
        set { val = value; }
    }
    private string name;
    public string Name {
        get { return name; }
        set { name = value; }
    }
    public CCBoxItem() {
    }
    public CCBoxItem(string name, int val) {
        this.name = name;
        this.val = val;
    }
    public override string ToString() {
        return string.Format("name: '{0}', value: {1}", name, val);
    }
}

You can also programmatically check an item or set its CheckState to Checked, Unchecked or Indeterminate. Note that the user cannot set an item state to Indeterminate from the User Interface, it can only be programmatically set (unless you want to handle the ItemCheck notification and add your own logic there).

C#
// Check first item (index == 0)
ccb.SetItemChecked(0, true);
// Set the CheckState of the 2nd item in the list to Indeterminate.
ccb.SetItemCheckState(1, CheckState.Indeterminate);

The most interesting events you would probably be catching in your application are when the drop down portion closes (DropDownClosed) and when an item checked state is about to change (ItemCheck). You can register event handlers and handle the events as usual.

Note that the DropDownClosed event is available from the ComboBox directly, but the ItemCheck I had to make available from the CheckListBox.

C#
// Add the handlers to the CheckedComboBox

this.ccb.DropDownClosed += new System.EventHandler(this.ccb_DropDownClosed);
ccb.ItemCheck += new System.Windows.Forms.ItemCheckEventHandler(this.ccb_ItemCheck);

// Handler implementation

private void ccb_DropDownClosed(object sender, EventArgs e) {
    txtOut.AppendText("DropdownClosed\r\n");
    txtOut.AppendText(string.Format("value changed: {0}\r\n", ccb.ValueChanged));
    txtOut.AppendText(string.Format("value: {0}\r\n", ccb.Text));
    // Display all checked items.
    StringBuilder sb = new StringBuilder("Items checked: ");
    // Checked items can be found via the CheckedItems property.
    foreach (CCBoxItem item in ccb.CheckedItems) {
        sb.Append(item.Name).Append(ccb.ValueSeparator);
    }
    sb.Remove(sb.Length-ccb.ValueSeparator.Length, ccb.ValueSeparator.Length);
    txtOut.AppendText(sb.ToString());
    txtOut.AppendText("\r\n");
}

private void ccb_ItemCheck(object sender, ItemCheckEventArgs e) {
    CCBoxItem item = ccb.Items[e.Index] as CCBoxItem;
    txtOut.AppendText(string.Format("Item '{0}' is about to be {1}\r\n", 
                                     item.Name, e.NewValue.ToString()));
}

In the code above, you can see that the checked items are available via the CheckedItems property. Similarly, CheckedIndices will return a collection of checked indices. Both these collections correspond to the .NET CheckedListBox collection, so nothing unfamiliar here.

History

In version 2.0, I have included some minor fixes/enhancements as follows:

  • Exposed the SetItemChecked(int index, bool isChecked) method so that you can programmatically set an item as checked
  • Relevant to the above, also included two more methods to get/set the CheckState:
    C#
    CheckState GetItemCheckState(int index)

    and

    C#
    SetItemCheckState(int index, CheckState state)
  • Added a horizontal scrollbar to the dropdown list, which becomes visible if any of the items' length exceeds the width of the dropdownlist
  • Made the dropdown list "invisible" from the task bar (it used to appear because it is a Form)

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Comments and Discussions

 
Questionchanging items font size Pin
ArielR18-Feb-24 9:35
ArielR18-Feb-24 9:35 
QuestionApp window hides on closing dropdown. Pin
Member 1585783111-Dec-22 22:08
Member 1585783111-Dec-22 22:08 
AnswerRe: App window hides on closing dropdown. Pin
ub_here19-Jan-23 20:47
ub_here19-Jan-23 20:47 
GeneralRe: App window hides on closing dropdown. Pin
Member 167676015-Feb-23 20:55
Member 167676015-Feb-23 20:55 
GeneralRe: App window hides on closing dropdown. Pin
peter nix20-Apr-23 4:54
peter nix20-Apr-23 4:54 
AnswerRe: App window hides on closing dropdown. Pin
Joseph Miloshevsky28-Apr-23 2:47
Joseph Miloshevsky28-Apr-23 2:47 
GeneralRe: App window hides on closing dropdown. Pin
Joseph Miloshevsky28-Apr-23 4:14
Joseph Miloshevsky28-Apr-23 4:14 
QuestionFirst Check doesn't upgrade checked status Pin
FSnowman199128-Jul-22 4:13
FSnowman199128-Jul-22 4:13 
QuestionUsing CheckedComboBox dropdownlist in vb.net Project Pin
Aishwarya Divekar12-Jun-22 22:24
Aishwarya Divekar12-Jun-22 22:24 
QuestionOnMouseMove is not called unless clicked when converted to VB.net Pin
Supriyo Baidya5-Oct-21 4:14
Supriyo Baidya5-Oct-21 4:14 
QuestionUse in a commercial app Pin
rp_8-Jul-21 16:04
rp_8-Jul-21 16:04 
AnswerRe: Use in a commercial app Pin
Stelios Alexandrakis11-Jul-21 6:40
Stelios Alexandrakis11-Jul-21 6:40 
Questionfaster way to clear the list? Pin
TreesOnTheBeach12-May-21 8:30
TreesOnTheBeach12-May-21 8:30 
QuestioncheckedProductGroup.CheckedItems Pin
Member 1510199916-Mar-21 22:28
Member 1510199916-Mar-21 22:28 
AnswerRe: checkedProductGroup.CheckedItems Pin
TreesOnTheBeach12-Apr-21 4:58
TreesOnTheBeach12-Apr-21 4:58 
QuestionDropDown event doesn't fire at all and dropdownclosed event fires when the drop down is first shown Pin
Member 456089129-Apr-20 19:39
Member 456089129-Apr-20 19:39 
QuestionCheckOnClick = false does not work Pin
MVSoftVM3-Feb-20 2:41
MVSoftVM3-Feb-20 2:41 
AnswerRe: CheckOnClick = false does not work Pin
The Magical Magikarp29-Apr-20 16:05
The Magical Magikarp29-Apr-20 16:05 
GeneralRe: CheckOnClick = false does not work Pin
MVSoftVM29-Apr-20 19:49
MVSoftVM29-Apr-20 19:49 
GeneralRe: CheckOnClick = false does not work Pin
The Magical Magikarp29-Apr-20 19:53
The Magical Magikarp29-Apr-20 19:53 
QuestionHow to add items by datasource? Pin
Member 1018411923-Dec-19 7:17
Member 1018411923-Dec-19 7:17 
AnswerRe: How to add items by datasource? Pin
Personal CJ12-Apr-21 4:57
Personal CJ12-Apr-21 4:57 
BugWhen the combo box open, one needs an extra click to get start selecting the check boxes PinPopular
RBoy30-Oct-19 14:51
RBoy30-Oct-19 14:51 
BugFocus is not restored after the checkbox window is hidden PinPopular
RBoy30-Oct-19 12:37
RBoy30-Oct-19 12:37 
GeneralRe: Focus is not restored after the checkbox window is hidden Pin
Member 1466050420-Nov-19 4:08
Member 1466050420-Nov-19 4:08 

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