Opening and returning from child WPF application, not enough resources to resume parent WPF application

Question

I have a Visual Studio solution that contains multiple WPF projects. The main project launches a WPF application that acts as the parent/gateway to the other projects. After navigating the menus on the main application window, there is a button that launches another application that displays MS Access DB information. The code to launch the secondary application from the main application is as follows (with some names/paths changed for simplicity):

private void OpenDatabase(object sender, RoutedEventArgs e)
        {
            ShowInTaskbar = false; // prevents parent app from showing in taskbar
            Process DBAPP = new Process();
            DBAPP.StartInfo.FileName = "path\to\DBAPP.exe";
            DBAPP.Start();
            DBAPP.WaitForExit();

            ShowInTaskbar = true; // parent app now shows in taskbar
        } 

The secondary, database application has ShutdownMode="OnExplicitShutdown", then within the mainWindow.cs ( not the App.xaml(.cs) ) I have a handler for the close button as follows:

private void CloseMainMenu(object sender, RoutedEventArgs e)
        {
            Close();
            Application.Current.Shutdown();
        }

When I hit close on the secondary application I return to the first chunk of code and I see an error on the line ShowInTaskbar = true; that says "System.ComponentModel.Win32Exception: 'Not enough quota is available to process this command'"

I only see this error when I have the secondary application running for more than around 3-5 minutes - possibly the database connection or the windows themselves using up resources; if I quickly open the secondary application and close it immediately - returning to the main application - I do not see this error.

Answer 1

From reading other posts on SO (here and here plus countless others) I discovered, that upon returning from the spawned process to the caller process, there is a message queue that needs to be cleared. Directly returning after a blocking call to Process.WaitForExit() may not allow the message queue to be cleared - especially if a significant amount of time has passed - before the caller process (tries to) resumes execution. In order to resolve this, the Process.WaitForExit() call can be put into its own thread; seen here and in my code example below.

Essentially what is being done is that after starting the spawned process, a new thread is started whose only job is to handle the return from Process.WaitForExit() and then die off. If we have a loop that is constantly checking for the thread to be alive every 500ms, there is enough buffer between the thread dying and the caller process attempting to resume execution for the message queue to be cleared; hence no more error.

I was initially seeing the error after about 3-5 minutes but have not seen it with the spawned process running for over 30 minutes and handling many database operations. I would consider this resolved.

private void OpenDatabase(object sender, RoutedEventArgs e)
        {
            Process DBAPP = new Process();
            DBAPP.StartInfo.FileName = "path\to\DBAPP.exe";
            DBAPP.Start();
            Thread DBThread = new Thread(() => waitForProcessExit(DBAPP));
            DBThread.Start();
            while (DBThread.IsAlive) Thread.Sleep(500);
        } 

public static bool waitForProcessExit(Process spawnedProc)
        {
            spawnedProc.WaitForExit();
            return true;
        }