Exam program launcher

I work with my occupational psychologist friend every Friday morning. Today, whilst we were talking about a program that we are developing, I noticed that her computer's desktop was filled with icons from my programs: normally three icons per program (one manager, one exam and one results program). I suggested writing a 'program launcher' program, which would present the user with a choice of programs to be run, and then run the chosen program. This would cut down the number of desktop icons drastically.

In the end, we decided against such a program (or rather, left it as an optional exercise for the future) as it doesn't improve the working environment. But out of this idea came a better idea: write a program launcher for the various exams that clients sit. What, you might ask? If a program launcher for multiple programs was discarded, why write one for only a subset of those programs?

The difference is this: a client will sit down at one of the computers in the computer room. The secretary will bring up one of the exams, fill in the client's details (surname, forename, id number, etc) then let the client do the exam. When the exam finishes, the secretary will bring up another exam, type in the client's details again and then let the client do this exam. All the exams were written purposely as standalone programs which need no external files and so can be distributed and run at other sites with a minimum of effort. This is why the user's personal details have to be entered into every exam. Use of a launcher program would mean that the personal details need only be entered once, and then the user can get on with all the exams.

The exam program launcher which I designed works on the following basis:

1. The user's personal data are entered into the launcher
2. The launcher saves these data in the computer's registry
3. A list of exams which the user will sit are chosen from the list of all exams (this list comes from a database which holds the exam's name, its location and any command line parameters)
4. The first exam in the chosen list is removed from the list and run
5. When the exam finishes, it notifies the program launcher than it has finished
6. If there are more exams in the chosen list then the launcher loops to step 5
7. The launcher removes the data saved from the registry and exits

Whilst the above isn't exactly rocket science, one will not find many examples of how to do inter-program communication in Windows. In DOS, this probably would have been done with an interrupt; in fact, during the dying days of DOS, I became quite adept at writing TSR programs, initially in assembly language and then in Turbo Pascal, which used the $2F multiplex interrupt for this purpose. But now we are in Windows, and so we must use a message. But which message?

It turns out that Windows has the 'RegisterWindowMessage' function which creates a unique message number for the parameter passed to it. Several programs can call the same function with the same string; the first program that calls the function will create (register) the message number, and every subsequent call will retrieve the same number.

I modified some of the exams to do the following:

• In the 'GetDetails' dialog form, the program checks the registry to see whether it has values. If the exam has been called from the program launcher, then there will be values, whereas if the exam is run directly then there won't be any values.
  
• When the exam finishes, it makes a call to the RegisterWindowMessage to get the special message number and then broadcasts a message to all the running programs with that message number.

If the exam is run directly, nothing untoward happens, but if the exam is run from the launcher, then it will indeed notify the launcher when it completes. The launcher handles this completion message by executing the next exam in the exam list.

It's a bit difficult to explain this asynchronous, inter-program communication in a synchronous manner (ie writing this explanation) and to someone who isn't used to event-based programming, but like every clever idea, it is in fact quite simple (especially in retrospect). One of the neat things about this program is that the exams can also be run directly, with no side effects.