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<TITLE>Force a ComboBox to drop down</TITLE>
<META NAME="Author" CONTENT="Harold Howe">
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<H3>
Force a ComboBox to drop down
</H3>
<P>
To tell you the truth, I decided that this would be a good FAQ addition because
I see this question pop up frequently in MFC FAQ's, newsgroups, and discussion
boards (MFC groups are a good place to learn about the API, since MFC doesn't
encapsulate a whole heck of a lot). However, while researching how to force a
<TT>ComboBox</TT> to drop down in C++Builder, I realized that this is so simple
that it probably shouldn't be an FAQ. Oh well.
</P>
<P>
To drop a <TT>ComboBox</TT> from code, simply set the <TT>DroppedDown</TT>
property of the <TT>ComboBox</TT> to true. Here is some code:
</P>
<pre>
<b>void</b> <b>__fastcall</b> TForm1<b>:</b><b>:</b>Button1Click<b>(</b>TObject <b>*</b>Sender<b>)</b>
<b>{</b>
ComboBox1<b>-></b>DroppedDown <b>=</b> <b>true</b><b>;</b>
<b>}</b>
</pre>
<P>
<B>Note:</B> The <TT>DroppedDown</TT> property also works in reverse. If you need
to check if a <TT>ComboBox</TT> is dropped down, you can read the <TT>DroppedDown</TT>
property. It will be true if the <TT>ComboBox</TT> is dropped down.
</P>
<P>
<B>Note:</B> The write method of the <TT>DroppedDown</TT> property works by sending a
message to the window handle of the <TT>ComboBox</TT>. Just in case your curious, here is
what the VCL code looks like (pascal converted to C++):
</P>
<pre>
<b>void</b> <b>_fastcall</b> TCustomComboBox<b>:</b><b>:</b>SetDroppedDown<b>(</b><b>bool</b> Value<b>)</b>
<b>{</b>
<font color="navy">// NOTE: must cast bool to pass Value as the WPARAM</font>
<font color="navy">// of the message. The VCL actually casts to LongInt.</font>
SendMessage<b>(</b>Handle<b>,</b> CB_SHOWDROPDOWN<b>,</b> <b>(</b>WPARAM<b>)</b>Value<b>,</b> <font color="blue">0</font><b>)</b><b>;</b>
<b>}</b>
</pre>
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