InvalidComObjectException Class

1, Concerning the following COM interop error :

An unhandled exception of type 'System.Runtime.InteropServices.InvalidComObjectException' occurred in <client application>.exe
Additional information: COM object that has been separated from its underlying RCW cannot be used.

The following scenario is one cause of such an error :

1.1 A COM object is created or acquired in one thread (ThreadA).

1.2 This COM object is passed as parameter to a second thread (ThreadB).

1.3 ThreadA then terminates before ThreadB is able to marshal in the COM object.


2. The following C# code demonstrates this :

        [STAThread]
        public static void ThreadA(object data)
        {
            Object newObj = Activator.CreateInstance(Type.GetTypeFromProgID("<some progID>"));
            ISomeInterface objISomeInterface = (ISomeInterface)newObj;
            Thread tB = new Thread(Threads.ThreadB);
            tB.ApartmentState = ApartmentState.STA;
            tB.Start(objISomeInterface);
             // ThreadA terminates right after starting ThreadB.
        }
        [STAThread]
        public static void ThreadB(object data)
        {
             // The InvalidComObjectException will occur on the next line.
            ISomeInterface objISomeInterface = (ISomeInterface)data;
             ...
             ...
             ...
        }

2.1 ThreadA terminates right after creating ThreadB.

2.2 When ThreadB starts, "data" (which contains the original "objISomeInterface" object from ThreadA, will fail when cast to the ISomeInterface COM interface. The error message described in point 1 will be displayed.

2.3 Via anecdotal observation, it is likely that the "objISomeInterface" RCW internally disconnects with its COM interface when it goes out of scope. Hence the error message.


- Bio.