What exactly do socket's Shutdown, Disconnect, Close and Dispose do?

c# .net sockets network-programming winsock
relatively_random picture relatively_random · Feb 5, 2016 · Viewed 11.4k times · Source

It's surprisingly hard to find a simple explanation on what these four methods actually do, aimed at network programming newbies. People usually just state what they believe is the proper way to close a socket in a particular scenario, but not what happens in the background behind each step.

Going by the teach-a-man-to-fish philosophy, can you explain the Shutdown, Disconnect, Close and Dispose methods?

Answer

relatively_random picture relatively_random · Feb 5, 2016

An answer on StackOverflow made me think I have finally reached some glimpse of an understanding. Then I went testing for a bit and here's the summary of a newbie's view. Please correct me if I'm wrong because this is based on inference, not expertise.

Shutdown

Shutdown disables the Send and/or Receive methods, depending on the provided argument. It doesn't disable the underlying protocol handling and it never blocks.

If Send is disabled, it also queues up a zero-byte send packet into the underlying send buffer. When the other side receives this packet, it knows that your socket will no longer send any data.

If Receive is disabled, any data the other side might be trying to send will be lost.

If Receive is disabled without disabling Send, it just prevents the socket from receiving data. Since no zero-byte packet will be sent, the other side won't know anything about it until it tries to send something, and only if the socket's protocol requires acknowledging.

Disconnect

First, Disconnect does the equivalent of Shutdown(SocketShutdown.Both).

Then it blocks, waiting for two things:

  1. For all the queued-up send data to be sent.
  2. For the other side to acknowledge the zero-byte packet (if applicable to the underlying protocol).

If you call Disconnect(false), system resources will be freed.

Close

Close frees system resources. May abruptly stop sending queued-up data. If called with the argument, will wait for the data to be sent, but only up to the specified timeout.

Dispose

Dispose is same as the Close overload without the timeout argument. To be more precise, Close without timeout is the same as Dispose.

If you use the using block on the socket, it will automatically call Dispose.

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