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    <p>Death of a socket changes its behavior in several ways, so these methods are both valid :)</p> <p>With both methods you actually check those parts of the socket's behavior that change after disconnection.</p> <blockquote> <p>I don't really understand his statement about calling Receive() to make sure that the remote endpoint has actually received all the data I sent. (Do sockets block receiving until the sending buffer is empty?)</p> </blockquote> <p><code>TCP</code> is reliable protocol, that means that every packet you send must be acknowledged. Acknowledgement implies sending the packets with <code>ACK</code> bit set. These packets may or may not contain additional (payload) data.</p> <p>When the socket is connected, <code>Receive()</code> will block until the socket receives a packet with non-empty payload. But when the socket is disconnected, <code>Receive()</code> will return as soon as the last <code>ACK</code> packet arrives.</p> <p>Calling <code>Receive()</code> ensures that you either <em>receive</em> that last <code>ACK</code> packet from your remote endpoint or a disconnect timeout occurs and you will be able to <em>receive</em> nothing more on this socket.</p> <blockquote> <p>The example on the same page shows how to do it. (I wonder why does it allocate a 1-byte array, even though it calls Send with 0 length?) But a post by Ian Griffiths says that I should read from the socket, not send through it.</p> </blockquote> <p>When <code>send()</code>ing to a socket, you actually try to append some data to the end of the socket queue. Is there is some place left in the buffer, then your <code>Send()</code> returns instantly, if not, the <code>Send()</code> blocks until there is some place.</p> <p>When the socket is in disconnected state, <code>TCP/IP</code> stack prevents all further operations with the buffer, that's why <code>Send()</code> returns an error.</p> <p><code>Send()</code> implements a basic pointer check, this means it fails when a <code>NULL</code> pointer is passed to it. You may probably pass any non-null constant as a pointer, but you better allocate 1 byte instead of making the constant up — just in case.</p> <hr> <p>You may use any method you like, as none of them is resource consuming. As long as they are used for socket connection checking, they are identical.</p> <p>As for me, I would prefer <code>Receive()</code>, as this is what you normally run in a cycle and wait for. You get a non-zero from <code>Receive()</code>, you process data; you get a zero, you process disconnection.</p>
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    1. POHow can I check whether a (TCP) socket is (dis)connected in C#?
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    1. POHow can I check whether a (TCP) socket is (dis)connected in C#?
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    1. COThanks a lot! I still need some clarification though. If I don't send anything through the socket, which method is best to check that the socket is alive before trying to read from it? Should I try to receive, and if I get 0 then the socket has been closed?
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    2. COAlso why does the example allocate a 1-byte array, and then pass it to send() with 0 length? Is this any different from allocating new byte[0]?
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    3. COSee my updated post
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