Console Application: Closing Window vs End of Program

Question

I'm writing a small library to help manage some objects in Excel. I'm testing this DLL using a simple console application that makes calls to the library, then prints the results out. I can then end the program in any of the typical fashions, usually by either hitting return (thus completing the ReadLine call) or hitting the window's close button. However, the reference to the Excel instance behaves differently based on how the program exits.

In my program, if no existing reference to Excel can be found, I use the following line:

_app = new ExcelInterop.Application();

where _app is an instance of Microsoft.Office.Interop.Excel.Application in either a static or singleton class (I've tried both, both have the same results).

Assuming the program creates it's own instance (and doesn't find one already open):

• The instance will remain open if the program is exited by clicking the close window button:
• The instance is released, and no longer appears in the task manager if the program is exited by reaching the end of the code in the Main block

Is there anyway to make all program ends behave like the latter case? Furthermore, this DLL will go on to be used in a WPF application, are there similar concerns in WPF? Or at large, even?

Perhaps most importantly, what are the technical reasons for this behavior?

Answer 1

A console mode program is a pretty hostile place for the COM interop wrapper objects that are needed for an apartment-threaded out-of-process COM server. This program demonstrates the issue:

using System;

class Program {
    static void Main(string[] args) {
        var prg = new Program();
        Console.ReadLine();
    }
    ~Program() {
        Console.WriteLine("Clean-up completed");
        System.Threading.Thread.Sleep(1500);
    }
}

Try it both ways, by pressing Enter and by clicking the Close button. You'll see that the finalizer never gets executed. The operating system terminates the process before it gets a chance to shut down properly when you click the Close button.

Same problem with the finalizers for the COM wrappers. They cannot execute so IUnknown::Release() doesn't get called and the Office program is completely unaware that the client program is no longer there. Windows has its own cleanup for abandoned out-of-process servers but that doesn't work for Office programs for some otherwise mysterious reason.

That explains it, fixing it isn't so easy. You'll have to register a callback that runs when the Close button is clicked. If necessary, set the app reference to null if it is still in scope and force the finalizer to run with GC.Collect() + GC.WaitForPendingFinalizers(). Do keep in mind that this is just a band-aid, not a fix. It won't work when the user aborts your program while you are busy talking to the Office program. Avoiding a console mode project is best.