How to ensure only a single instance of my application runs?

Question

Is there support in the Delphi XE VCL for ensuring only a single instance of an application is running?

In the past, I've used library code to control a Mutex which has always seemed complicated. As I'm starting a new project in Delphi XE, I wonder if I need to dig up that old code, or if there is support built into XE already? Or is there another easy to apply code that is nice and modern?

Answer 1

You create a named Mutex when you start the application. Check GetLastError to see if an other instance is already running.

Put this code right after "begin" in your DPR file. Replace the GUID with one of your own. When I need a text constant that's unlikely to be used for anything else, I usually just hit Ctrl+G to get a GUID!

if CreateMutex(nil, True, '6EACD0BF-F3E0-44D9-91E7-47467B5A2B6A') = 0 then   RaiseLastOSError;  if GetLastError = ERROR_ALREADY_EXISTS then   Exit; 

It might look like the code is leaking an handle because it's not saving the return of CreateMutex. It's not. Windows will automatically release the handle when our application is terminated, and that's absolutely fine with us.

Answer 2

I use JCL to do this:

program MyProgram;  uses   JclAppInst;  begin   JclAppInstances.CheckSingleInstance; // Added instance checking   Application.Initialize;   Application.CreateForm(TMainForm, MainForm);   Application.Run; end. 

Documentation for this, and the notification scheme, is at the JCL Wiki.