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ComboBox with Suggest Ability based on Substring-Search

, 5 Aug 2013 CPOL
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A custom ComboBox that supports live filtering of items by Substring-Search or any other Lamda-Expression

Introduction 

The WinForms ComboBox-Control offers a functionality called AutoComplete if DropDownStyle is set to DropDown. There are different options available, but the focus of this article is on AutoCompleteMode = Suggest with AutoCompleteSource = ListItems. This setting provides a ListBox of suggested items when you type some text in the ComboBox.

The problem is: you cannot define the way the suggested items are filtered/determined. It's always a 'StartsWith'-search.

That's why i decided to write my own class (SuggestComboBox) based on ComboBox which looks more or less the same, but is in fact self-made. The screenshot below shows the difference: on the left hand side the input 'j' matches one more item because it's a Contains-search. 

     

The code and how it works   

At first you need a new class that inherits from ComboBox and a ListBox + BindingList as DataSource that will contain the suggested items. The filtering is done in the OnTextChanged - method via LINQ, the Lamda-Expressions for filtering and ordering can be set by the Properties FilterRule and SuggestListOrderRule (default values are a Contains-filter and ascending alphabetic order).  

To support the expected behavior for keyboard actions (UP, DOWN, ENTER, ESCAPE) you primarily need to override two methods: 

  1. OnPreviewKeyDown, that is called first when you hit a key. Here the actions to be done are listed in a switch-statement for the KeyCode.
  2. ProcessCmdKey, that is always called after the first one. Here only the base functionality is interrupted. 

The remaining methods are mainly to manage the visibility and location/size of the ListBox and need no further explanation. Just read the code! Wink | <img src= (examples are included in the zip-file)  

Update:

To support any DataSource-Type (not just string), i added the PropertySelector: you can pass the selection rule for the databound items. The default setting still assumes that a list of strings is the datasource.

Example:

// assume you bind a list of persons to the ComboBox with 'Name' as DisplayMember:
suggestComboBox.DataSource = new List<person>();
suggestComboBox.DisplayMember = "Name";

// then you have to set the PropertySelector like this:
suggestComboBox.PropertySelector = collection => collection.Cast<person>().Select(p => p.Name);

// the class Person looks something like this:
class Person
{
    public string Name { get; set; }
    public DateTime DateOfBirth { get; set; }
    public int Height { get; set; }
}</person></person>

I also fixed some UI-bugs regarding the location and size of the suggest box (2 Changed-EventHandler added). 

public class SuggestComboBox : ComboBox
{
    #region fields and properties
 
    private readonly ListBox _suggLb = new ListBox { Visible = false, TabStop = false };
    private readonly BindingList<string> _suggBindingList = new BindingList<string>();
    private Expression<Func<ObjectCollection, IEnumerable<string>>> _propertySelector;
    private Func<ObjectCollection, IEnumerable<string>> _propertySelectorCompiled;
    private Expression<Func<string, string, bool>> _filterRule;
    private Func<string, bool> _filterRuleCompiled;
    private Expression<Func<string, string>> _suggestListOrderRule;
    private Func<string, string> _suggestListOrderRuleCompiled;
 
    public int SuggestBoxHeight
    {
        get { return _suggLb.Height; }
        set { if (value > 0) _suggLb.Height = value; }
    }
    /// <summary>
    /// If the item-type of the ComboBox is not string,
    /// you can set here which property should be used
    /// </summary>
    public Expression<Func<ObjectCollection, IEnumerable<string>>> PropertySelector
    {
    	get { return _propertySelector; }
	set
	{
	    if (value == null) return;
	    _propertySelector = value;
	    _propertySelectorCompiled = value.Compile();
	}
    }
 
    ///<summary>
    /// Lambda-Expression to determine the suggested items
    /// (as Expression here because simple lamda (func) is not serializable)
    /// <para>default: case-insensitive contains search</para>
    /// <para>1st string: list item</para>
    /// <para>2nd string: typed text</para>
    ///</summary>
    public Expression<Func<string, string, bool>> FilterRule
    {
        get { return _filterRule; }
        set
        {
            if (value == null) return;
            _filterRule = value;
            _filterRuleCompiled = item => value.Compile()(item, Text);
        }
    }
 
    ///<summary>
    /// Lambda-Expression to order the suggested items
    /// (as Expression here because simple lamda (func) is not serializable)
    /// <para>default: alphabetic ordering</para>
    ///</summary>
    public Expression<Func<string, string>> SuggestListOrderRule
    {
        get { return _suggestListOrderRule; }
        set
        {
            if (value == null) return;
            _suggestListOrderRule = value;
            _suggestListOrderRuleCompiled = value.Compile();
        }
    }
 
    #endregion
 
    /// <summary>
    /// ctor
    /// </summary>
    public SuggestComboBox()
    {
        // set the standard rules:
        _filterRuleCompiled = s => s.ToLower().Contains(Text.Trim().ToLower());
        _suggestListOrderRuleCompiled = s => s;
        _propertySelectorCompiled = collection => collection.Cast<string>();
 
        _suggLb.DataSource = _suggBindingList;
        _suggLb.Click += SuggLbOnClick;
 
        ParentChanged += OnParentChanged;
    }
 
    /// <summary>
    /// the magic happens here ;-)
    /// </summary>
    /// <param name="e"></param>
    protected override void OnTextChanged(EventArgs e)
    {
        base.OnTextChanged(e);
 
        if (!Focused) return;
 
        _suggBindingList.Clear();
        _suggBindingList.RaiseListChangedEvents = false;
        _propertySelectorCompiled(Items)
             .Where(_filterRuleCompiled)
             .OrderBy(_suggestListOrderRuleCompiled)
             .ToList()
             .ForEach(_suggBindingList.Add);
        _suggBindingList.RaiseListChangedEvents = true;
        _suggBindingList.ResetBindings();
        
        _suggLb.Visible = _suggBindingList.Any(); 
        
        if (_suggBindingList.Count == 1 &&  
                    _suggBindingList.Single().Length == Text.Trim().Length)
        {
            Text = _suggBindingList.Single();
            Select(0, Text.Length);
            _suggLb.Visible = false;
        }
    }
 
    /// <summary>
    /// suggest-ListBox is added to parent control
    /// (in ctor parent isn't already assigned)
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void OnParentChanged(object sender, EventArgs e)
    {
        Parent.Controls.Add(_suggLb);
        Parent.Controls.SetChildIndex(_suggLb, 0);
        _suggLb.Top = Top + Height - 3;
        _suggLb.Left = Left + 3;
        _suggLb.Width = Width - 20;
        _suggLb.Font = new Font("Segoe UI", 9);
    }
 
    protected override void OnLostFocus(EventArgs e)
    {
        // _suggLb can only getting focused by clicking (because TabStop is off)
        // --> click-eventhandler 'SuggLbOnClick' is called
        if (!_suggLb.Focused)
            HideSuggBox();
        base.OnLostFocus(e);
    }
    protected override void OnLocationChanged(EventArgs e)
    {
	base.OnLocationChanged(e);
	_suggLb.Top = Top + Height - 3;
	_suggLb.Left = Left + 3;
    }
    protected override void OnSizeChanged(EventArgs e)
    {
	base.OnSizeChanged(e);
	_suggLb.Width = Width - 20;
    }
 
    private void SuggLbOnClick(object sender, EventArgs eventArgs)
    {
        Text = _suggLb.Text;
        Focus();
    }
 
    private void HideSuggBox()
    {
        _suggLb.Visible = false;
    }
 
    protected override void OnDropDown(EventArgs e)
    {
        HideSuggBox();
        base.OnDropDown(e);
    }
 
    #region keystroke events
 
    /// <summary>
    /// if the suggest-ListBox is visible some keystrokes
    /// should behave in a custom way
    /// </summary>
    /// <param name="e"></param>
    protected override void OnPreviewKeyDown(PreviewKeyDownEventArgs e)
    {
        if (!_suggLb.Visible)
        {
            base.OnPreviewKeyDown(e);
            return;
        }
 
        switch (e.KeyCode)
        {
            case Keys.Down:
                if (_suggLb.SelectedIndex < _suggBindingList.Count - 1)
                    _suggLb.SelectedIndex++;
                return;
            case Keys.Up:
                if (_suggLb.SelectedIndex > 0)
                    _suggLb.SelectedIndex--;
                return;
            case Keys.Enter:
                Text = _suggLb.Text;
	        Select(0, Text.Length);
	        _suggLb.Visible = false;
                return;
            case Keys.Escape:
                HideSuggBox();
                return;
        }
 
        base.OnPreviewKeyDown(e);
    }
 
    private static readonly Keys[] KeysToHandle  = new[] 
                { Keys.Down, Keys.Up, Keys.Enter, Keys.Escape };
    protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
    {
        // the keysstrokes of our interest should not be processed be base class:
        if (_suggLb.Visible && KeysToHandle.Contains(keyData))
            return true;
        return base.ProcessCmdKey(ref msg, keyData);
    }
 
    #endregion
} 

Remark 

One thing is really annoying i think: every time you hit ENTER or ESCAPE a 'bing'-sound is generated by the framework. I read this is a bug of .NET 4. Anybody has an idea how to fix it properly?

License

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

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About the Author

Physlcu$

Germany Germany
No Biography provided

Comments and Discussions

 
QuestionHow to change width of the dropdown? Pinmemberchiragxyz12316-Dec-14 3:50 
AnswerRe: How to change width of the dropdown? PinmemberHarrySolsem20-Jan-15 23:34 
Questionuse another thread not main thread for this control Pinmember4L4K111-Dec-14 13:24 
AnswerRe: use another thread not main thread for this control PinmemberPhyslcu$12-Dec-14 4:52 
GeneralRe: use another thread not main thread for this control Pinmember4L4K112-Dec-14 8:02 
SuggestionExcellent Work, Pretty Needed! PinmemberChamadness31-May-14 21:02 
QuestionSuggestion drops down after selecting Pinmemberduonglei26-May-14 21:29 
AnswerRe: Suggestion drops down after selecting PinmemberPhyslcu$28-May-14 23:34 
QuestionCombobox somtimes disappears after building Pinmemberduonglei12-May-14 16:27 
AnswerRe: Combobox somtimes disappears after building PinmemberPhyslcu$17-May-14 3:25 
GeneralRe: Combobox somtimes disappears after building Pinmemberduonglei20-May-14 0:35 
AnswerRe: Combobox somtimes disappears after building PinmemberBert Brouns17-Jul-14 5:50 
QuestionFilter on 2 fields of datatable PinmemberMember 107351001-May-14 17:16 
AnswerRe: Filter on 2 fields of datatable PinmemberPhyslcu$3-May-14 1:44 
SuggestionBing Sound Pinmemberbjoernkr27-Mar-14 22:42 
GeneralRe: Bing Sound PinmemberPhyslcu$3-May-14 1:34 
SuggestionAvoid hardcoded font for listbox Pinmemberpaintballkev8123-Feb-14 11:45 
GeneralRe: Avoid hardcoded font for listbox PinmemberPhyslcu$8-Feb-14 8:08 
QuestionSuggest list appears behind other controls PinmemberSirous Weisy3-Feb-14 7:26 
AnswerRe: Suggest list appears behind other controls PinmemberPhyslcu$8-Feb-14 8:05 
GeneralVB.net Version [modified] Pinmemberpaintballkev8123-Feb-14 5:41 
GeneralRe: VB.net Version PinmemberRoneson Senday26-Feb-14 12:35 
GeneralRe: VB.net Version PinmemberRoneson Senday26-Feb-14 14:56 
QuestionSupport for Multiple Words in mixed order? Pinmemberpaintballkev81231-Jan-14 12:07 
AnswerRe: Support for Multiple Words in mixed order? PinmemberPhyslcu$1-Feb-14 3:26 

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