Usage
The usage is very simple: just drop the component on the application's main form. Set the MaxInstances integer property: let it have the value of 1 if you want to enable only one instance of the application, set it to 5 (for example) if only 5 instances can be run simultaneously. By default, when TadpInstanceControl detects that maximum running instances have been reached, it brings the last started instance's main form to front and closes the application. You can even customize application's behavior by handing the OnMaxInstancesReached event. The component also exposes the Enabled property - thus enabling you to enable or disable its "task".

Implementation
The TadpInstanceControl component uses file mapping to share data between multiple instances of an application. The idea is presented in the "The file mapping approach" page of the "Controlling the number of application instances" article.

As you will see inside the source (look for the "ThisOnce" TObject type variable), only one copy of this component can be dropped on a form - or an EInstanceControl exception will be raised. This needs to be done since two TadpInstanceControl components would prevent you from running your application even only once.

When the max instances has been reached (MaxInstancesReached procedure inside the source), and the application is going to be terminated, the component hides the application's hidden window to avoid a "flash" on the taskbar.

Installing into a Component palette
First, download the component. The TadpInstanceControl comes as a single unit file (.pas extension). You'll need to add the component into an existing package. Here's "How to Install Custom Component in Delphi (into Existing Package)"

TadpInstanceControl: example application

1. Start a new Delphi project and drop a TadpInstanceControl on a form (from the "delphi.about.com" component palette).
2. By default, the Enabled property is set to True, MaxInstances is set to 1.
3. Set the MaxInstances property to 2.
   
4. Handle the OnMaxInstancesReached event.
   

   Here's an example:

   procedure TForm1.adpInstanceControl1MaxInstancesReached( Sender: TObject; const LastInstanceHandle: Cardinal); begin ShowMessage('Ups I cannot be run! Bye, Bye.'); end;

   Note the LastInstanceHandle parameter in the procedure - it is the handle to the last started instance's main form.

5. Run the application 2 times. Now try to run it one more time ... ups :

   

That's it. Don't forget to download the component; and as always if there are any questions or comments please post them on the Delphi Programming Forum.