Question

Instance A is trying to restore instance B’s window, but I can’t get the B’s window handle. I think the problem is that the window is being minimized by B to the system tray using:

this.Visibility = Visibility.Hidden;

And A is trying to get B’s window handle using:

Process process = Process.GetCurrentProcess();
Process.GetProcessesByName(process.ProcessName).First().MainWindowHandle;

Which is equal to IntPtr.Zero.

I also tried to get the window handle by class name using Spy++ but the class name has a per-instance GUID in the following format:

HwndWrapper[FileName.exe;;ad445199-cf93-48a4-bd24-2f97d54c8af8]

Answer 1

That is because what you want basically doesn’t exists, and the concept of MainWindowHandle is a gross misnomer that sneaked into the .Net Framework for everlasting confusion. From There can be more than one (or zero): Converting a process to a window:

“I have a thread ID. How do I get the
corresponding window?”

You can use the EnumThreadWindows
function to get all the windows on the
thread.

“Yes, I know about EnumThreadWindows,
but how do I get the window that I
want?”

Well, you haven’t said what you wanted
yet.

“I want the window that corresponds to
the thread.”

But which one? How will you decide
among all the windows?

“That’s what I’m asking you!”

But you haven’t yet described what you
want.

“I want the window corresponding to
the thread. Why won’t you answer my
question?”

Note that saying, “I am looking for
the top-level unowned window” is a
step forward, but it still doesn’t
uniquely identify a window. There can
be multiple top-level unowned windows
in a process. For example, Explorer
typically has lots of top-level
unowned windows. There’s the desktop,
the taskbar, your open folder windows,
and property sheets. If you ask for
“the” top-level unowned window of
Explorer, which one do you want?

Perhaps people are getting the idea
that there is a way to uniquely
specify “the” window for a process
because the System.Diagnostics.Process
object has a property called
MainWindowHandle. The documentation
for that property doesn’t do anything
to dispel the notion, either. I have
no idea how that property decides
among multiple top-level unowned
windows.

The topic is also elaborated in MSDN Q&A Get the Main Window:

Q How can I find the main window for a process? I’m writing a Spy-like tool and I need to get the main window (HWND) for a process so I can send it a message like WM_ACTIVATEAPP.

A Which main window do you mean?