C# Get Path/Filename by Handle (hWnd) 32 and 64bit

Question

I got the following code to get path/filename by handle:

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern int GetWindowThreadProcessId(IntPtr handle, out uint processId);

    public static string GetProcessPath(IntPtr hwnd)
    {
        uint pid = 0;
        GetWindowThreadProcessId(hwnd, out pid);
        Process proc = Process.GetProcessById((int)pid);
        return proc.MainModule.FileName.ToString();
    }

it works perfect in 32 bit but I get error in 64bit > "Only part of the ReadProcessMemory or WriteProcessMemory request was completed." the project is compiled as x86 (platform target x86).

How can I fix it?

~Thanks Ron

Answer 1

It appears from your question that you've currently compiled your program as a 32-bit application. However, the process you're trying to query (assuming you're running on a 64-bit version of Windows) is undoubtedly a 64-bit one. That kind of thing isn't allowed. Although you can run 32-bit applications on 64-bit versions of Windows, they run under the dedicated Windows on Windows (WOW64) subsystem. That's why you're getting a Win32Exception claiming that "Only part of a ReadProcessMemory or WriteProcessMemory request was completed". I agree it's not the most descriptive of error messages if you don't already know how Windows manages 32-bit and 64-bit processes, but armed with this knowledge it makes at least a little more sense.

The solution is to compile your application as a 64-bit application (x64), or for "Any CPU". Everything should work as expected after that. If possible, I suggest using "Any CPU", which will allow the application to run in 32-bit mode on 32-bit OSes and 64-bit on 64-bit OSes. Which is really the ideal set of circumstances, assuming that:

1. You've written your P/Invoke definitions correctly (i.e., using IntPtr where appropriate, instead of Integer).
2. You're not relying on 3rd-party DLLs (to which you don't have the source code) that were compiled as 32-bit.

Answer 2

I'm not aware of a known problem with this, it smells environmental. The error is very low level, probably the wow64 emulation layer. I can only suggest you punt and use a different way to get the same info. You can use WMI, Win32_Process class. Run a select query on ProcessId, the ExecutablePath property gives you what you are looking for. Use the WMI Code Creator utility to experiment, it auto-generates the C# code you need. The odds are however not zero that this will fail the same way.