While developing and debugging a C# program for a customer I noticed some very strange behaviour: when the program crashed or was terminated from the debugger (without having time to properly close a listening socket), the TCP port would stay open indefinitely. This was very annoying because after the crash I obviously couldn't restart the program without getting errors about the port being already open. Trying to close the socket from a handy program like cports didn't work at all: the process listening on the port was already dead, so it didn't know how to kill it! Not having much time to debug this I logged off and on my Windows account to force the socket to die every time the issue happened.

After implementation and testing of the most important features of the program I decided to look into the problem once more. At first I found some comments about how it's impossible to make Windows close listening sockets of a crashed application or that it closes itself after some minutes, but this sounded like complete nonsense to me. This is not the first network-involving program I write and I never stumbled in this problem before. The fact that it seemed to only happen "sometimes" made me skeptical this was a Windows problem. Didn't really know what to do with this information, so I went on googoling.

I started looking into socket settings or flags, trying to set linger options and timeouts, but that did nothing. I also went so far as to set ReuseAddr to make the errors go away. This option makes the program always connect to the port even if it's occupied, but this didn't solve the problem because now I couldn't detect if there was a REAL other program already listening. Moreover, my users could open the program many times and only one instance would actually receive packets. This was unacceptable at best.

In the end, some post I found and some tries lead me to the right idea: child processes. Since the program had to work with an Oculus Quest (an Android based VR headset), some functions interfaced with it by opening adb and parsing its console output. I quickly figured out the problem appeared only when I used some feature involving adb, and closing its process also freed the TCP port!

When I programmatically launched adb, it became a child process. This would be fine if adb.exe ended after giving me its output but, as any Android developer knows, it stays open in the background waiting for other requests. By default, the child process which outlives its parent inherits almost all of its handles, including sockets. To obtain the desired behaviour, I "just" had to disable the inheritance on the process or on the handle. Too bad there is no way to do it in C# when calling the child process. The introduction of some way to set inheritance flags with pure C# was proposed but as far as I can tell everything stopped there.

I decided to just call kernel32.dll and use Windows APIs to do this. Luckily, the C# Socket class includes a Handle parameter exposing the handle we need.

//Evil windows kernel magic to avoid adb keeping inherited sockets alive when the program crashes

[Flags]
public enum HANDLE_FLAGS : uint
{
     NONE = 0,
     INHERIT = 1,
     PROTECT_FROM_CLOSE = 2
}

[DllImport("kernel32.dll", SetLastError = true)]
private static extern bool SetHandleInformation(IntPtr hObject, HANDLE_FLAGS dwMask, HANDLE_FLAGS dwFlags);

//End of evil windows kernel magic

And when declaring the socket:

Socket socket = new Socket(IPAddress.Any.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
SetHandleInformation(socket.Handle, HANDLE_FLAGS.INHERIT, 0);
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