How to Inspect COM Objects From Visual Basic Dump File?

Question

Background

We have a .NET WinForms application written in C# that interfaces to a handheld store scanner via a console application. The console application is written in good ol' VB6-- no managed code there. The VB6 application consists of several COM objects.

The .NET WinForms application refreshes the data in the scanner by invoking the console application with the right parameters. When the console application starts, it pops up a modal form reminding the user to place the handheld device into its cradle.

Problem

A customer has a bizarre situation in which the call to start the console application appears to hang before it displays the reminder form. If the user presses any key-- even something innocent like Shift or Alt-- the application unfreezes, and the reminder form appears. While it is hung, the CPU usage of the console application is very high.

We have obtained a memory dump from the command line application using ProcDump. I have some experience debugging managed dump files, but this VB 6 dump is strange to me.

We captured several full memory dumps in a row. In some of them, there appears to be COM glue stacks. For example, several dump files show a call stack like this:

msvbm60!BASIC_DISPINTERFACE_GetTICount
msvbm60!_vbaStrToAnsi
msvbm60!IIDIVbaHost
msvbm60!rtcDoEvents
msvbm60!IIDIVbaHost
msvbm60!BASICCLASS_QueryInterface
[our code which I think is trying to create and invoke a COM object]

It doesn't help that the only symbols I have are from our code. The Microsoft symbol server does not have a PDB file for msvbm60.dll (or at least not from their version which is 6.0.98.2).

Questions

I am suspecting there may be some COM threading issue that is happening only on their system.

1) How can I determine the thread state of each thread in a dump file? If this were a managed dump file, I would look at !threads and then !threadstate to figure out the thread states. There is no managed code, so I can't use sos.dll. I didn't see any hints using ~ and !teb.

2) Is there a way to see what COM objects have been created in a dump file? Again, in a managed dump, I can do a !dumpheap to get a list of managed objects. Is there something similar I can find for COM objects?

3) Can I determine the threading model of COM objects in the dump file?

Answer 1

I can only answer question 1. Use !runaway to find the thread or threads consuming the CPU. To get all thread stacks use ~*kb1000.

Answer 2

You can dump thread state by using command:

~*

this will not display 'background' as a state, you will only see running, frozen or suspended.

I'm not sure how you can get information from COM objects, I have never tried but will investigate and get back to you, regards to threading model it will be difficult to infer that without painful monitoring of application state after stepping through and even with that, when you step through all other threads will run unless you use .bpsync 1 which syncs all threads to the current one, but that could cause a hang (e.g. gui thread has now been told to freeze) so I think it will be difficult unless you have access to the source code.