C# – Using a COM dll from C# without a type library

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

.net COM interop doesn't route all COM messages directly back to the caller. If you called COM from an STA, it won't understand how you app can handle re-entrance. This means that failure messages that could just be handled with a retry, end up causing exceptions.

Try implementing IMessageFilter interface. This will allow COM to understand how to pass messages back to your app. In particular, implement the RetryRejectedCall and check if the failure flags and possibly return a timeout value (something like 1000ms) to allow COM to retry after a brief pause.

It's a COM type, so this is the code you'll need to define the interface:

[ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("00000016-0000-0000-C000-000000000046")]
public interface IMessageFilter
{
    [PreserveSig]
    int HandleInComingCall(int dwCallType, IntPtr hTaskCaller, int dwTickCount, IntPtr lpInterfaceInfo);

    [PreserveSig]
    int RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType);

    [PreserveSig]
    int MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType);
}

And this is an example of how you would implement it:

public class MyMessageFilter : IMessageFilter
{
    int IMessageFilter.HandleInComingCall(int dwCallType, IntPtr hTaskCaller,int dwTickCount, IntPtr lpInterfaceInfo)
    {
        // 0 means that it's handled.
        return 0;
    }

    int IMessageFilter.RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType)
    {
        // The return value is the delay (in ms) before retrying.
        return 1000;
    }

    int IMessageFilter.MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType)
    {
        // 1 hear means that the message is still not processed and to just continue waiting.
        return 1;
    }
}

Once you've implemented a message filter you will need to register it using CoRegisterMessageFilter. This is a per-thread registration, so be aware of what thread you are calling it on. The PInvoke signiture is:

[DllImport("ole32.dll")]
static extern int CoRegisterMessageFilter(IMessageFilter lpMessageFilter, out IMessageFilter lplpMessageFilter);

Even if this doesn't work, at the very least, if you log all the messages in the filter you should hopefully get some more information about what is going wrong. Look at the values of the parameters being passed into the message filter. If you look them up they will relate to error/state codes.

_{[Be aware, the IMessageFilter I'm referring to here is different from the System.Windows.Forms.IMessageFilter, so make sure you don't accidentally use the winforms one.]}