Preventing multiple instances when launching from an associated file...

I have a Delphi program (say MyProgram.Exe) that is associated with a particular file extension - lets say '.XXX' files for arguments sake.

When I double click on the file Fred.XXX in explorer, it launches MyProgram.Exe.  This is good.  If I double click another file in Explorer (say Fred2.XXX) it launches *another* copy of MyProgram.Exe.  This is Bad.

I can prevent this simply by putting some code in the start of MyProgram.Exe that looks for another instance running and, if it finds it, sends a message to the other instance to tell it what file to load before terminating itself.

This seems very messy though, and it prevents the user from EVER running two copies of MyProgram.Exe together.

Other programs (such as Microsoft Word) seems to handle this properly, so how do I do it my own software ??

Thanks in advance,

Mac.
mac_roweAsked:
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ahalyaConnect With a Mentor Commented:
I am making my comment as an answer, so that you can evaluate it.


Others have suggested solutions to avoid a second instance, and AFAIK your question was NOT to always avoid a second instance, but to avoid it at certian times.

I think my solution would do the trick.
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ckanetaCommented:
how about having the new instance of the program send the old instance of the program a WM_CLOSE type message and having the old one shut down.
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XANCommented:
I think you should describe a global variable (App) that will be created in initialization section - something like the next

  initialization
    if not Assigned(App) then
      App := TApp.Create();

  or

you can do the check in TApp.Create()

  finalization
    FreeAndNil(App);

I do so and it works :)
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reddarinCommented:
Wouldn't you be able to check for the command line and if it is a file that is already open by your app just set focus to the open one and exit. An absence of the command line would allow the app to just open normally.

reddarin
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ahalyaCommented:
I *think* that explorer will not launch a second instance, if you use DDE to communicate with your app.  You need to process the DDE messages from your app.

The following set of registry key should get you going, i'd think.

[HKEY_CLASSES_ROOT\.XXX]
(Default)  = YourDoc

[HKEY_CLASSES_ROOT\YourDoc\shell\open\command]
(default) = "yourapp /ddeex"

[HKEY_CLASSES_ROOT\YourDoc\shell\open\ddeexec]
(defualt) = "Your DDE Commands if your app is already running"

[HKEY_CLASSES_ROOT\YourDoc\shell\open\ddeexec\ifexec]
(defualt) = "Your DDE Commands if your app is NOT already running"

I think this last registry key is optional. if it's not specified Explorer will use the same DDE commnds from the "ddeexec" key.
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XANCommented:
Actually, programs like MS Word use COM-interfaces and OLE objects. To prevent load second instance it uses ActiveObject
At first you need register the OLE-object as ActiveObject, then you can use it so many time as you want
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EpsylonCommented:
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SteveWaiteCommented:
Sorry but I may misunderstand your question, however..
Sounds like you are nearly there but you need to sand back a message too.
As you say the second instance sends the file name to the first instance. If the first instance does not have the same file loaded as that in the message then it sends a message back for the second instance to load the new file and stay as a second instance. Otherwise it sends back a message that it has the same file loaded and the second instance will then close.


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ptmcompCommented:
I think the easiest way is to create a mutex (in the .dpr unit). If your second instance tries to create a second mutex, it fails. Then you can look for your App's window (FindWindow) and send a message to it.
ptm.
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EpsylonCommented:
This mutex stuff is descibed in solution 2 of the link I metioned in my previous comment.
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yk030299Commented:
listen
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ptmcompCommented:
Just, use this mutex stuff...
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mac_roweAuthor Commented:
Adjusted points from 200 to 300
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mac_roweAuthor Commented:
Sorry for the amount of time it took me to implement your answer.

I've added an extra 100 points as compensation.

Yes, you were the only one who answered the basic question.

Thanks,

Mac.
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ahalyaCommented:
Oh.. you didn't have to.  

Thx anyway.
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