Windows Forms GUI hangs when calling OpenFileDialog.ShowDialog()

openFileDialog1->ShowHelp = true;

I put this line in my code then the problem was solved.

I seem to have solved the problem adding the [STAThread] Attribute to the main method. I was told to do so once I ran the program in a debugger - which I hadn't done before because I ran the service from Visual Studio and the client regularly from Windows.

[STAThread]
public static void Main(string[] args)
{
    GUI gui = new GUI();
    gui.ShowDialog();
}

Can anybody explain what exactly is going on though

This tends to be an environmental problem, when you use OpenFileDialog a lot of shell extensions get loaded into your process. A misbehaving one can easily screw up your program. There are a lot of bad ones out there.

Debugging this is difficult, you need an unmanaged debugger since these shell extensions are unmanaged code. You might be able to tell something from the call stack when you break in after the deadlock. Windows debugging symbols required, enable the Microsoft symbol server. But the most effective approach is to use SysInternals' AutoRuns utility. Start by disabling all of the shell extensions that were not produced by Microsoft. Then start re-enabling the ones you cannot live without one by one.

And, as you found out, these shell extension expect to run on an STA thread and fail miserably when they don't get it. The UI thread of a program must always be STA, also to support the clipboard and drag-and-drop and various kinds of controls like WebBrowser. Normally always taken care of automatically by the [STAThread] attribute on the Main() method, put there by the project template. And the Application.Run() call, required to implement the STA contract. Deadlock when you don't.