C# to C++ process with WM_COPYDATA passing struct with strings

From a c# program I want to use WM_COPYDATA with SendMessage to communicate with a legacy c++/cli MFC application.

I want to pass a managed struct containing string objects.

I can find the handle to the c++ application for use with SendMessage fine.

The bit I don't know about is how the struct and it's strings can be marshalled and read at the other end. Especially as it contains non-blittables.

Do people think this is feasible? I'll continue to work on it, but would apprecite someone who's done this sort of thing telling me if it just isn't going to work.

Here is some demo code if it was a c++/cli program and it's not difficult to get it working. However, I'd like this to be in a .Net class library so it can easily be re-used.

//Quick demonstation code only, not correctly styled
int WINAPI WinMain(HINSTANCE hInstance,
                 HINSTANCE hPrevInstance,
                 LPSTR lpCmdLine,
                 int nCmdShow)
{               
    struct MessageInfo
    {
        int     nVersion;
        char   szTest[ 10 ];        
    };

    MessageInfo sMessageInfo;

    sMessageInfo.nVersion = 100;
    strcpy( sMessageInfo.szTest, "TEST");   

    COPYDATASTRUCT CDS;

    CDS.dwData = 1; //just for test
    CDS.cbData = sizeof( sMessageInfo );
    CDS.lpData = &sMessageInfo;

    //find running processes and send them a message
    //can't just search for "MYAPP.exe" as will be called "MYAPP.exe *32" on a 64bit machine
    array<System::Diagnostics::Process^>^allProcesses = System::Diagnostics::Process::GetProcesses();

    for each (System::Diagnostics::Process^ targetProcess in allProcesses)
    {        
        if (targetProcess->ProcessName->StartsWith("MYAPP", System::StringComparison::OrdinalIgnoreCase))
        {
            HWND handle = static_cast<HWND>(targetProcess->MainWindowHandle.ToPointer());

            BOOL bReturnValue = SendMessage( handle, WM_COPYDATA, (WPAR