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    <p>You're not waiting for the Winsock control to actually send the "quit" message. The <code>SendData</code> method is asynchronous: it can return before the data has actually been sent across the network. The data is buffered locally on your machine and sent at some later time by the network driver.</p> <p>In your case, you are trying to send the "quit" message and then closing the socket almost immediately afterwards. Because <code>SendData</code> is asynchronous, the call might return before the "quit" message has actually been sent to the server, and therefore the code might close the socket before it has a chance to send the message.</p> <p>It works when you cancel the unloading of the form first and let the timer send the "quit" message because you're giving the socket enough extra time to send the message to the server before the socket is closed. However, I wouldn't count on this always working; it's a coincidence that the extra steps gave the socket enough time to send the message, and it's not guaranteed to always work out that way.</p> <p>You can fix the problem by waiting for the socket to raise a <code>SendCompleted</code> event after you send the "quit" message and before you close the socket. Below is a basic example. Note that the <code>QueryUnload</code> code is much simpler.</p> <pre><code>Private m_bSendCompleted As Boolean Private m_bSocketError As Boolean Private Sub singleSock_Error(ByVal Number As Integer, Description As String, ByVal Scode As Long, ByVal Source As String, ByVal HelpFile As String, ByVal HelpContext As Long, CancelDisplay As Boolean) 'Set error flag so we know if a SendData call failed because of an error' 'A more robust event handler could also store the error information so that' 'it can be properly logged elsewhere' m_bSocketError = True End Sub Private Sub singleSock_SendCompleted() 'Set send completed flag so we know when all our data has been sent to the server' m_bSendCompleted = True End Sub 'Helper routine. Use this to send data to the server' 'when you need to make sure that the client sends all the data.' 'It will wait until all the data is sent, or until an error' 'occurs (timeout, connection reset, etc.).' Private Sub SendMessageAndWait(ByVal sMessage As String) m_bSendCompleted = False singleSock.SendData sMessage singleSock.SendData sMessage Do Until m_bSendCompleted or m_bSocketError DoEvents Loop If m_bSocketError Then Err.Raise vbObjectError+1024,,"Socket error. Message may not have been sent." End If End Sub Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer) 'This is (almost) all the code needed to properly send the quit message' 'and ensure that it is sent before the socket is closed. The only thing' 'missing is some error-handling (because SendMessageAndWait could raise an error).' If UnloadMode = vbFormControlMenu Then Me.WindowState = vbMinimized Cancel = True Else SendMessageAndWait "quit" &amp; vbCrLf singleSock.Close End If End Sub </code></pre> <p>You can make the code cleaner by putting the logic to send a message and wait for it to be sent in a separate class. This keeps the private variables and the event handlers in one place, instead of having them litter your main code. It also makes it easier to re-use the code when you have multiple sockets. I called the class <code>SynchronousMessageSender</code> for lack of a better name. This example also has more complete error handling:</p> <p><strong>SynchronousMessageSender.cls</strong></p> <pre><code>Private WithEvents m_Socket As Winsock Private m_bAttached As Boolean Private m_bSendCompleted As Boolean Private m_bSocketError As Boolean Private Type SocketError Number As Integer Description As String Source As String HelpFile As String HelpContext As Long End Type Private m_LastSocketError As SocketError 'Call this method first to attach the SynchronousMessageSender to a socket' Public Sub AttachSocket(ByVal socket As Winsock) If m_bAttached Then Err.Raise 5,,"A socket is already associated with this SynchronousMessageSender instance." End If If socket Is Nothing Then Err.Raise 5,,"Argument error. 'socket' cannot be Nothing." End If Set m_Socket = socket End Sub Private Sub socket_SendCompleted() m_bSendCompleted = True End Sub Private Sub socket_Error(ByVal Number As Integer, Description As String, ByVal Scode As Long, ByVal Source As String, ByVal HelpFile As String, ByVal HelpContext As Long, CancelDisplay As Boolean) m_bSocketError = True 'Store error information for later use' 'Another option would be to create an Error event for this class' 'and re-raise it here.' With m_lastSocketError .Number = Number .Description = Description .Source = Source .HelpFile = HelpFile .HelpContext = HelpContext End With End Sub 'Sends the text in sMessage and does not return' 'until the data is sent or a socket error occurs.' 'If a socket error occurs, this routine will re-raise' 'the error back to the caller.' Public Sub SendMessage(ByVal sMessage As String) If Not m_bAttached Then Err.Raise 5,,"No socket is associated with this SynchronousMessageSender. Call Attach method first." End If m_bSendCompleted = False m_bSocketError = False m_socket.SendData sMessage &amp; vbCrLf 'Wait until the message is sent or an error occurs' Do Until m_bSendCompleted Or m_bSocketError DoEvents Loop If m_bSocketError Then RaiseLastSocketError End If End Sub Private Sub RaiseLastSocketError() Err.Raise m_lastSocketError.Number, _ m_lastSocketError.Source, _ m_lastSocketError.Description, _ m_lastSocketError.HelpFile, _ m_lastSocketError.HelpContext End Sub </code></pre> <p><strong>Example Use</strong></p> <pre><code>Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer) Dim sender As New SynchronousMessageSender 'Ignore errors since the application is closing...' On Error Resume Next If UnloadMode = vbFormControlMenu Then Me.WindowState = vbMinimized Cancel = True Else Set sender = New SynchronousMessageSender sender.AttachSocket singleSock sender.SendMessage "quit" singleSock.Close End If End Sub </code></pre> <p>By using a separate class, now all the necessary code can be placed in the <code>Form_QueryUnload</code>, which keeps things tidier.</p>
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