What is causing ObjectDisposedException from SerialPort while debugging .NET winform?

Question

Vista SP1 Visual Studio 2008 SP1 .NET 3.5 SP1 C#

I have a winforms app I'm playing with that uses a SerialPort object as a private variable. When the application is compiled and executed, it works great. It also works running in debug mode wihtout any breakpoints. 90% of the time when I stop at a breakpoint and try to step through code I get an 'unhandled exception ocurred' dialog with these details:

System.ObjectDisposedException was unhandled Message="Safe handle has been closed" Source="mscorlib" ObjectName="" StackTrace: at Microsoft.Win32.Win32Native.SetEvent(SafeWaitHandle handle) at System.Threading.EventWaitHandle.Set() at System.IO.Ports.SerialStream.AsyncFSCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOverlapped) at System.Threading.IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped pOVERLAP) InnerException:

The frustrating thing is, I do not have to be stepping over serial-related code! I just have to have done something with the port. So I might read a string, manipulate the string, add two numbers together, whatever, and then BANG.

Again, this works just fine when NOT debugging, or when debugging wihtout any breakpoints. There seems to be something about stopping at a breakpoint that makes the CLR dispose the SerialStream on a different thread.

There is a lot of chatter online about problems with renoving USB devices causing this. But I'm using the build-in motherboard port on COM1.

I don't think I had this issue in .NET 2.0 so I may have to go back to that...

I need to simplify the application quite a bit before I can post code - but has anyone seen behavior like this in the debugger before?

Thanks so much!

Answer 1

Perhaps your port is getting closed by the OS as it does not get a response from your application (it is stopped at a breakpoint).

Answer 2

Well I'm not so sure this is an answer, but there was definately something about that project. It was originally written in 2.0 and converted to 3.5 by VS2008. I created a new project in C#-Express 2008 adding the original classes one-by-one and it works like a charm now! No idea what is different.

Answer 3

I had the same problem just this morning. Surprisingly, it simply has gone away when I DISABLED the following options in VS2008 Tools->Options->Debugging->General:

• "Enable the exception assistant"
• "Enable .NET Framework source stepping"
• "Step over properties and operators"
• "Enable property evaluation and other implicit function calls"

I have no idea why, but it worked for me.

Answer 4

I have this too. This must be some kind of bug with the debugger. The above advice worked: Disable the "Enable property evaluation and other implicit function calls."

I have a class with properties that do serial I/O. I thought that perhaps the debugger was helpfully trying to display the property value when I hovered the mouse over it, thus doing the IO from the debugger thread. But that doesn't seem to be the case. I'm really unsure what the cause is.