Application hanging on Serialport .Close method

Question

This one has me stumped.  I've created a class to interface with a device on a comm port.  I've decalred the serial port:

Private WithEvents moPort as System.IO.Ports.SerialPort

In the New event, I create the port and open it, works great.  I communicate just fine, handle everthing going in, coming out no problem.  When I go to close the port, by just a .Close command in a "Disconnect" method I've defined or in the IDisposeable implementation, the entire program hangs up.  I can hit Pause in the IDE and it highlights the "moPort.Close" line, the call stack always shows something like:

[In a sleep, wait, or join]
[External Code]
Executablename!ProgramName.ClassName.Dispose(boolean Disposing = True)
Executablename!ProgramName.ClassName.Dispose()

I've let it set here for a few minutes and it never comes back, I have to hit the "Stop" button in the IDE to shut it down .  Has anyone seen this or have any idea what I'm doing wrong?

Answer 1

If you have the boolean active, can't you go:

If Executablename!ProgramName.ClassName.Dispose() Then
    'Successful close
Else
    MsgBox("Whoopsie!")
End If

Or does it hang before it even registers this?

Answer 2

It hangs in the dispose method itself:

Protected Overridable Sub Dispose(ByVal disposing As Boolean)
     If Not Me.disposedValue Then
          If disposing Then
               ' free unmanaged resources when explicitly called
               Try
                    If moPort IsNot Nothing Then
                         If moPort.IsOpen Then
                              moPort.Close()     '<--- Hangs here
                         End If
                         moPort.Dispose()
                    End If
               Catch ex As System.Exception
                   'don't care
               Finally
                   moPort = Nothing
               End Try
          End If
          ' TODO: free shared unmanaged resources
     End If
     Me.disposedValue = True
End Sub

Answer 3

I don't seem to have the System.IO.Ports namespace. Are you using VS.NET2003 or VS.NET2005?

Answer 4

Sorry, forgot to include that   VS '05 Professional on XP SP2.

Answer 5

Ahh damn. I can't help you sorry. I'm a VS.NET2003 Man.

Answer 6

This problem is a frequent topic on these forums.  This thread is relevant.  It appears the Close() method is prone to deadlock but there has never been a good diagnosis.  Avoid opening and closing the port more than once might help...

Answer 7

Great thread, thank you for suggesting it!  Not sure why it didn't come up when I search here initially for this.. /shrug.  I have one open (called right after the class is initialized) and one close (called just before setting the class object = nothing).  The port remains open and active while the class is in use.

I have a thread in the form the class is created in that polls the comm port every few seconds through this class.   In the form close event I kill the thread (.Abort and = nothing) then I attempt to .Disconnect (.close()) and .Dispose the serial port class I've created.  It still seems to deadlock on either the .Close() in the disconnect or,  if I  comment out the .Disconnect, hangs in the .Close() in the Dispose method.

Any other ideas?

Answer 8

Having the thread that does the reading call Open, Close and Dispose might be a good idea.  I really can't promise it would work. 

Answer 9

The class raises it's own events based on what's received in the DataReceived event for the serial port control, and monitoring isn't begun until after a couple things have been recieved and evaluated from the device it's connected to.  So I'm not sure that I could do that without scrapping what I've got so far and attemtping to code around the deadlock issue. 

Thanks for the ideas, though.

Answer 10

Not sure what difference it makes in the spookyness that is VB.Net,  but I moved the shutdown code to it's own routine (from FormClosing) and explicitly called it prior to form=nothing and that seemes to have resolved the deadlock somehow.

Answer 11

Hmm, try it at least 20 times.  That's classic threading spookiness...

Answer 12

Seems to be working without a lockup.  I just moved the code in the FormClosing to a "Shutdown" method, and rather than just:

formvar = Nothing

I do:

formvar.ShutDown
formvar = Nothing

And that seems to work.  If I do just the = nothing then it locks up again.  It has to be some type of thread-timing issue that is beyond my ability to fathom.

Thanks again 'bugz :)

Answer 13

I would say it's a threading issue: threads are easy when they work, when they break it's extremely hard to figure out what's going on. With the Serial Port, because it raises events on a different thread, it can make it all the more difficult.

I've placed all serial port stuff in its own class in its own thread, and don't have any problems (uh-oh, shouldn't have said that...).