为什么只有在C＃中创建/运行表单时某些代码才起作用？

Question

In the process of turning an application into something more like a background task, I noticed some odd behavior. It works fine when I run something like this (similar to the old code I'm modifying):

using (Foo f = new Foo(stuff)) {
  f.doSomething();

  Application.EnableVisualStyles();
  Application.SetCompatibleTextRenderingDefault(false);
  Application.Run(new Form());
}

The first step towards making this run in the background was of course to remove the references to the form/rendering:

using (Foo f = new Foo(stuff)) {
  f.doSomething();
}

That way doesn't work, however. When I run that, a library that I use (which doesn't have anything to do with Windows forms) no longer works correctly. This is without any of the other changes I planned to do whatsoever. Only removing those three lines. If it is relevant, Foo in the actual code is a class that creates an SSH tunnel (using the SSH.NET library), but it is not referenced (directly or indirectly) through any code in the form/designer, it just needs to be set up/torn down so a connection can be made.

The error occurs before the connection to the tunnel is made, but instead when trying to connect to the remote host and forward the ports. The library reports a connection is made ( IsConnected is true ), but when port forwarding is started, it reports an exception with the message "Session is not connected"

Answer 1

Converting your application to a background service isn't as simple as removing those three lines. In particular, Application.Run(new Form) is what starts the main thread of the application, per MSDN:

Begins running a standard application message loop on the current thread, and makes the specified form visible.

https://msdn.microsoft.com/en-us/library/ms157902(v=vs.110).aspx

You could look into the overload Application.Run() , which would start the main thread without a form - but then you will have issues properly terminating that thread. At the very least, you could verify that your code does in fact work without visibly displaying a form. The only caveat is if your main Form has other calls to your library that now have to be handled elsewhere.

In a Win32-based or Windows Forms application, a message loop is a routine in code that processes user events, such as mouse clicks and keyboard strokes. Every running Windows-based application requires an active message loop, called the main message loop. When the main message loop is closed, the application exits. In Windows Forms, this loop is closed when the Exit method is called, or when the ExitThread method is called on the thread that is running the main message loop.

Most Windows Forms developers will not need to use this version of the method. You should use the Run(Form) overload to start an application with a main form, so that the application terminates when the main form is closed. For all other situations, use the Run(ApplicationContext) overload, which supports supplying an ApplicationContext object for better control over the lifetime of the application.

https://msdn.microsoft.com/en-us/library/ms157900(v=vs.110).aspx

To be honest, you'd probably be best off just starting a new Windows Service or WCF project.