Re: Unable to initialize COM in VC Dll when invoked via C# application

Hello Ashutosh,

I agree with Igor. If the C++ DLL CoInitialize the thread as MTA:
CoInitializeEx(0, COINIT_MULTITHREADED);
and our current .NET thread has been initialized as STA by the [STAThread]
attribute, for example:

        [DllImport(@"TestDLL.dll", EntryPoint = "Function1")]
        public static extern int Function1();
        [STAThread]
        static void Main()
        {
            Marshal.ThrowExceptionForHR(Function1());
        }
It will throw an exception: "cannot change thread mode after it is set.
(Exception from HRESULT: 0x80010106 (RPC_E_CHANGED_MODE))". The fact is
that, when a thread is first created by the OS as a result of calling
either CreateProcess or CreateThread, the newly created thread has no
associated apartment. Prior to using COM, the new thread must first enter
an apartment by calling CoInitialize(Ex) or OleInitialize. Once a thread
enters an apartment, it is illegal to change apartment types using
CoInitializeEx. Attempts to do so will result in the HRESULT
RPC_E_CHANGED_MODE. However, once a thread completely exits an apartment
using CoUninitialize, it may enter another apartment by calling
CoInitializeEx again. Ashutosh, please let us know the HRESULT value
returned from the C++ DLL CoInit call so that we can have a more accurate
analysis of the symptom.

Thanks
Jialiang Ge (jialge@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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