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.
The above screenshot shows how it works for a drop down combo box.
This screenshot shows how it works for a simple combo box
Basically what we do here is to derive a class from the
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
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
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
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
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
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
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!
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 - www.voidnish.com
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 - blog.voidnish.com
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.