Cannot call COM object created from STAThread from oher STA threads

Question

I am new to COM and trying to understand the difference between STA and MTA. I tried to create an example that would show that COM can manage calls to object created in STA that is not thread-safe.

MyCalcServer class here is created using ATL Simple Object. The settings used are the same as in this article:

Threading Model: Apartment
Aggregation: No
Interface: Custom

MyCalcServer COM object is used in another C# project which is:

class Program
{
    [STAThread]
    static void Main(string[] args)
    {
        MyCOMLib.MyCalcServer instance = new MyCOMLib.MyCalcServer();
        string output1;
        instance.ChangeValue("Gant", out output1);
        Console.WriteLine(output1);


        Thread t1 = new Thread(() =>
        {
            while (true)
            {
                string output;
                instance.ChangeValue("Gant", out output);
                Console.WriteLine(output);
            }
        });
        t1.SetApartmentState(ApartmentState.STA);
        t1.Start();

        // :
        // also has t2 and t3 here with similar code
        // :

        t1.Join(); t2.Join(); t3.Join();

    }
}

However, this always results in InvalidCastException (E_NOINTERFACE) raised inside t1's code. I have also tried changing ApartmentState to MTA with no success.

Unable to cast COM object of type 'MyCOMLib.MyCalcServerClass' to interface type 'MyCOMLib.IMyCalcServer'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{B005DB8C-7B21-4898-9DEC-CBEBE175BB21}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE)).

Could anybody please explain what I am doing wrong here?

Maybe the JIT thinks you are not using "instance" and releases it early. try to put Marshal.ReleaseComObject(instance) after the joins. — adrianm
Try to change the first line to MyCOMLib.IMyCalcServer instance = new MyCOMLib.MyCalcServer(); I think only interfaces (not classes) can be marshalled between threads. — adrianm
@adiranm Still got the same exception. Luckily I got this solved. Thanks anyway :) — Gant
+1 for the question, I just hit the same thing. I knew it was due to cross-thread calls on an apartment object, but I didn't understand why the exception complained about casting the object. Now I know - it must look for marshalling support during the cast, and doesn't find it, so gives a generic error. Could be more helpful... — Daniel Earwicker

Dewfy Dewfy · Accepted Answer · 2010-02-01T08:50:23

You explicitly ask COM to create instance for main thread, then you pass this to another thread. Of course in some circumstance it is allowed (for example declare MyCalcServer as multithread).

But in your case it looks you need create proxy for another thread. In regular COM clients it is done by CoMarshalInterThreadInterfaceInStream. There is large article to clarify it http://www.codeproject.com/KB/COM/cominterop.aspx

Cannot call COM object created from STAThread from oher STA threads

2 Answers