您无需等待 Winsock 控件实际发送“退出”消息。该SendData方法是异步的：它可以在数据实际通过网络发送之前返回。数据在您的机器上本地缓冲，稍后由网络驱动程序发送。
在您的情况下，您尝试发送“退出”消息，然后几乎立即关闭套接字。因为SendData是异步的，调用可能会在“退出”消息实际发送到服务器之前返回，因此代码可能会在有机会发送消息之前关闭套接字。
当您首先取消表单的卸载并让计时器发送“退出”消息时，它会起作用，因为您为套接字提供了足够的额外时间，以便在套接字关闭之前将消息发送到服务器。但是，我不会指望这总是有效的。巧合的是，额外的步骤给了套接字足够的时间来发送消息，并且不能保证总是这样。
SendCompleted您可以通过在发送“退出”消息后和关闭套接字之前等待套接字引发事件来解决此问题。下面是一个基本的例子。请注意，QueryUnload代码要简单得多。
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" & vbCrLf
singleSock.Close
End If
End Sub
您可以通过将发送消息的逻辑放在单独的类中等待它发送来使代码更简洁。这将私有变量和事件处理程序保存在一个地方，而不是让它们乱扔您的主代码。当您有多个套接字时，它还可以更轻松地重用代码。SynchronousMessageSender由于没有更好的名字，我给班级打电话。这个例子还有更完整的错误处理：
SynchronousMessageSender.cls
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 & 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
示例使用
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
通过使用一个单独的类，现在所有必要的代码都可以放在 中Form_QueryUnload，这让事情变得更整洁。