ASP.NET and STA COM objects

Question

Looking to settle an argument here.

All the stuff I\'ve read on using a COM object created in VB from an ASP.NET page is surrounding with

WARNING - MAKE

Answer 1

Sure you can use a VB COM component in an ASP.NET application. You think that millions of lines of VB code aren't going to get any love in .NET-land? :)

VB COM components are notorious for being compiled using a Single Threaded Apartment (STA) - though not always the case (if I remember correctly).

When using a STA VB component in .NET you might find that you experience:

1. Slower performance
2. Possible memory leak due to blocked finalizer (not trying to be all doom and gloom, just seems to happen more often than not)
3. Hair loss

An easy way to determine MTA v. STA for a thread is to leverage Windbg, connect to the process, load SOS, and execute !threads. You'll see output similar to:

0:015> !threads
ThreadCount: 27
UnstartedThread: 0
BackgroundThread: 17
PendingThread: 0
DeadThread: 10
Hosted Runtime: no
                                      PreEmptive   GC Alloc           Lock
       ID OSID ThreadOBJ    State     GC       Context       Domain   Count APT Exception
  15    1  a28 000d75f0   1808220 Enabled  3823b58c:3823bb08 000d3fe8     0 STA (Threadpool Worker)
  19    2  43c 000dd5f0      b220 Enabled  00000000:00000000 000d3fe8     0 MTA (Finalizer)
  20    3  b94 000f20b0    80a220 Enabled  00000000:00000000 000d3fe8     0 MTA (Threadpool Completion Port)
  21    4  15c 000f5318      1220 Enabled  00000000:00000000 000d3fe8     0 Ukn

Note that one of threads has "STA" listed in the APT column. This thread is a STA thread and your likely to see your VB COM code on its call stack.

References discussing the problem:

• COM Interoperability in the .NET Framework
• Running ASMX services on STA Threads
• Developing High Performance ASP.NET Websites
• ASP.NET Hang and OutOfMemoryException caused by STA Components
• Debugging Tools for Windows

Good Luck!
Z