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How to have an Auto-Completing ComboBox in .NET

, 2 Jun 2002
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Shows how to derive a class from ComboBox and add an Auto-Complete feature to it. The example uses Managed C++.
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This article is an elementary tutorial that shows you how you can have auto-completing combo-boxes in your Managed C++ programs. Of course it wouldn't be much of a difficulty to convert the code straight to C# or VB .NET. The technique described in this article requires you to have the final release of VS.NET. It will not work on any beta version of VS.NET including beta 2 as I found out after two days of frustration.


Sample Image - image01.gif

The above screenshot shows how it works for a drop down combo box.

Sample Image - image02.gif

This screenshot shows how it works for a simple combo box


Basically what we do here is to derive a class from the ComboBox class. Then we override the OnKeyPress method that is called when a key is pressed. For certain keys we would like to call the base class handler. These keys are the BackSpace key which has some expected behaviour and it would not be polite of our class to mess with expected behaviour. Other keys that have pre-defined roles include the cursor keys. We also need to check the ComboBox style. If it is a drop-list combo box where the edit box is uneditable, we should not do anything funny, and instead should simply call the base class function.

if(e->KeyChar == Windows::Forms::Keys::Back ||
    e->KeyChar == Windows::Forms::Keys::Up ||
    e->KeyChar == Windows::Forms::Keys::Down ||
    e->KeyChar == Windows::Forms::Keys::Right ||
    e->KeyChar == Windows::Forms::Keys::Left ||
    DropDownStyle == ComboBoxStyle::DropDownList )

Okay. So we know when to call the base class. Now for all other cases what we need to do is simple. Take the existing text from the edit box. Search the combo box for an entry that starts with this text. Now we need to set the text in the combo box to this searched and found text, except that all portions of this new text that exceeds the old text should be selected. Thus the user can type any key he wants to and remove all the selected text. That's what auto-completing combo boxes are supposed to do. I have written a simple search function called FirstMatch which takes a string and returns a string.

If our search was successful we can actually drop down our combo box and the advantage is that the selection bar will automatically move to the closest match to what's in the edit box. In our case there will be an exact match of course. I was wondering how to get this effect and it was very pleasing to get it free of effort. But we must remember to check to see if the combo box has the DropDown style set. If it is a simple combo box we don't want to try dropping it down.

String *str = FirstMatch(Text);
    int q1 = Text->Length;
    int q2 = str->Length - Text->Length;
    Text = str;
    if(DropDownStyle == ComboBoxStyle::DropDown)
        DroppedDown = true;
    SelectionStart = q1;
    SelectionLength = q2;
    if(DropDownStyle == ComboBoxStyle::DropDown)
        DroppedDown = false;

Finally we need to inform Windows that the event has been handled. For this we need to set the Handled property of the KeyPressEventArgs object passed to us to true. If we don't set this to true, this message will be send for default processing. Since we've handled it already, there is no need for that.

e->Handled = true;


I've noticed that when you take Run from the Start Menu, there is an auto-completing combo box. But there is a difference there in that the dropped down list is resizable. I haven't figured out how to do that unfortunately and I'd much appreciate it if anyone could give me some pointers. I hope it won't involve any of that owner draw stuff!


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About the Author

Nish Nishant

United States United States
Nish is a real nice guy who has been writing code since 1990 when he first got his hands on an 8088 with 640 KB RAM. Originally from sunny Trivandrum in India, he has been living in various places over the past few years and often thinks it’s time he settled down somewhere.
Nish has been a Microsoft Visual C++ MVP since October, 2002 - awfully nice of Microsoft, he thinks. He maintains an MVP tips and tricks web site - where you can find a consolidated list of his articles, writings and ideas on VC++, MFC, .NET and C++/CLI. Oh, and you might want to check out his blog on C++/CLI, MFC, .NET and a lot of other stuff -
Nish loves reading Science Fiction, P G Wodehouse and Agatha Christie, and also fancies himself to be a decent writer of sorts. He has authored a romantic comedy Summer Love and Some more Cricket as well as a programming book – Extending MFC applications with the .NET Framework.
Nish's latest book C++/CLI in Action published by Manning Publications is now available for purchase. You can read more about the book on his blog.
Despite his wife's attempts to get him into cooking, his best effort so far has been a badly done omelette. Some day, he hopes to be a good cook, and to cook a tasty dinner for his wife.

Comments and Discussions

GeneralMy vote of 2 PinmemberCorey Fournier18-Sep-09 3:48 
GeneralDemo PinmemberJoyBurke23-May-06 8:32 
The demo .exe file doesn't seem to work correctly on my PC (running XP Pro). I just selects the first item in the list. Any ideas?
GeneralRe: Demo PinmemberCorey Fournier18-Sep-09 3:47 
QuestionGreat Works,Can you complete it with C#? Pinmemberkv40008-Jun-05 16:08 
GeneralSlight Change PinmemberSkute3-Feb-04 3:19 
GeneralShell's Auto Complete PinmemberRama Krishna3-Jun-02 8:59 
GeneralRe: Shell's Auto Complete PinmemberNish - Native CPian3-Jun-02 9:11 
GeneralRe: Shell's Auto Complete PinmemberRama Krishna3-Jun-02 9:08 
GeneralRe: Shell's Auto Complete PinmemberNish - Native CPian3-Jun-02 19:20 
GeneralRe: Shell's Auto Complete PinsubeditorJames T. Johnson3-Jun-02 19:49 
GeneralRe: Shell's Auto Complete PinmemberNish - Native CPian3-Jun-02 20:04 

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