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You need to create a message handler (windows hook) See the following microsoft KB articles:<br />
<br />
HOWTO: Build a Windows Message Handler with AddressOf in VB<br />
<a href="http://support.microsoft.com/support/kb/articles/q170/5/70.ASP?LNG=ENG&SA=MSDN&FR=0" rel="ugc nofollow">http://support.microsoft.com/support/kb/articles/q170/5/70.ASP?LNG=ENG&SA=MSDN&FR=0</a>&nbsp;<br />
<br />
HOWTO: Pass String Data Between Applications Using SendMessage<br />
<a href="http://support.microsoft.com/support/kb/articles/Q176/0/58.ASP?LNG=ENG&SA=MSDN&FR=0" rel="ugc nofollow">http://support.microsoft.com/support/kb/articles/Q176/0/58.ASP?LNG=ENG&SA=MSDN&FR=0</a>&nbsp;<br />
<br />
HOWTO: Hook Into a Window's Messages Using AddressOf<br />
<a href="http://support.microsoft.com/support/kb/articles/Q168/7/95.asp?LNG=ENG&SA=MSDN&FR=0" rel="ugc nofollow">http://support.microsoft.com/support/kb/articles/Q168/7/95.asp?LNG=ENG&SA=MSDN&FR=0</a>&nbsp;<br />
<br />
<br />
Cheers!®©<br />
<br />

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You need to create a message handler (windows hook) See the following microsoft KB articles:

HOWTO: Build a Windows Message Handler with AddressOf in VB
http://support.microsoft.com/support/kb/articles/q170/5/70.ASP?LNG=ENG&SA=MSDN&FR=0 [http://support.microsoft.com/support/kb/articles/q170/5/70.ASP?LNG=ENG&SA=MSDN&FR=0] 

HOWTO: Pass String Data Between Applications Using SendMessage
http://support.microsoft.com/support/kb/articles/Q176/0/58.ASP?LNG=ENG&SA=MSDN&FR=0 [http://support.microsoft.com/support/kb/articles/Q176/0/58.ASP?LNG=ENG&SA=MSDN&FR=0] 

HOWTO: Hook Into a Window's Messages Using AddressOf
http://support.microsoft.com/support/kb/articles/Q168/7/95.asp?LNG=ENG&SA=MSDN&FR=0 [http://support.microsoft.com/support/kb/articles/Q168/7/95.asp?LNG=ENG&SA=MSDN&FR=0] 


Cheers!®©



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Really cool sample without any API !!!
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I want the new instance of an app to send a message to the old one. <br />
<br />
For example I start MyProg by clicking &quot;first.txt&quot;. After that I click on &quot;second.txt&quot; and I want the new MyProg to do something like :<br />
<br />
PostMessage(oldMyProgHandl<wbr />e, WM_COMMAND, 0&amp;, &quot;second.txt&quot;)<br />
<br />
<br />
But how do I Get the message ??
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I want the new instance of an app to send a message to the old one. 

For example I start MyProg by clicking "first.txt". After that I click on "second.txt" and I want the new MyProg to do something like :

PostMessage(oldMyProgHandle, WM_COMMAND, 0&, "second.txt")


But how do I Get the message ??

PostMessage... GetMessage???

Visual Basic is Microsoft’s event-driven programming language and integrated development environment (IDE) for its Component Object Model (COM) programming model. It is relatively easy to learn and use because of its graphical development features and BASIC heritage. It has been replaced with VB.NET, and is very similar to VBA (Visual Basic for Applications), the programming language for the Microsoft Office product line.