Using DLL that using COM in C#

Question

I have been writing DLL on C++, that will be use in C#. DLL have some function, where I call

hres =  CoInitializeEx(NULL, COINIT_MULTITHREADED); 

and next call

hres =  CoInitializeSecurity(
        NULL, 
        -1,                          // COM authentication
        NULL,                        // Authentication services
        NULL,                        // Reserved
        RPC_C_AUTHN_LEVEL_PKT,   // Default authentication 
        RPC_C_IMP_LEVEL_IMPERSONATE, // Default Impersonation  
        NULL,                        // Authentication info
        EOAC_NONE,                   // Additional capabilities 
        NULL                         // Reserved
        );

There are no error then I trying to use this dll in C++ . But if I call function from DLL via C# application I see Error (80010106) Cannot change thread mode after it is set. I changed

hres =  CoInitializeEx(NULL, COINIT_MULTITHREADED);

to

hres = CoInitialize(NULL);

After this changes error appear after CoInitializeSecurity :

(80010119)    Security must be initialized before any
                          interfaces are marshalled or unmarshalled. It
                          cannot be changed once initialized.

How resolve this trouble?

Answer 1

You could remove the call to CoInitializeEx and CoInitializeSecurity from your DLL. They have already been called on the thread - that's why you get these errors.

However, if your DLL is trying to initialize a COM thread as a multithreaded apartment, and your C# application is calling the DLL on a single-threaded apartment, then you may have a basic incompatibility. It depends whether the line of code that specified COINIT_MULTITHREADED was a deliberate choice with a reason behind it, or just something that seemed to work at the time it was originally written.