What is the scope of finalizer thread - per application domain or per process?

Looks like it is indeed just one thread per CLR instance within the process - at the moment, anyway. Here's some code to show that:

Test.cs:

using System;class Test{    static void Main()    {        AppDomain.CreateDomain("First")                 .ExecuteAssembly("ShowFinalizerThread.exe");        AppDomain.CreateDomain("Second")                 .ExecuteAssembly("ShowFinalizerThread.exe");    }}

ShowFinalizerThread.cs:

using System;using System.Threading;class ShowFinalizerThread{    static Random rng = new Random();    ~ShowFinalizerThread()    {        Console.WriteLine("Thread/domain: {0}/{1}",                          Thread.CurrentThread.ManagedThreadId,                          AppDomain.CurrentDomain.FriendlyName);        if (rng.Next(10) == 0)        {            Console.WriteLine("Hanging!");            Thread.Sleep(2000);        }    }    static void Main()    {        new Thread(LoopForever).Start();    }    static void LoopForever()    {        while (true)        {            new ShowFinalizerThread();            GC.Collect();            GC.WaitForPendingFinalizers();            Thread.Sleep(300);        };    }}

Compile each as a console app, then run test.exe (from the command line is easiest, IMO). You'll see that one app domain's finalizer blocks another.

In the future I wouldn't be surprised to see one finalizer thread per core rather than per AppDomain - but it sounds like you'll still have problems :(

You have my deepest sympathy (though not a solution) - once I tracked down a deadlock in an Oracle Blob. We were able to fix that by disposing of it properly, but I know not everything works that nicely - and it was a real pain even finding that one!