C# – C++ calling managed COM object can’t find dependent assemblies

ccom

I've created and registered a managed COM library in C# on my development machine. I've successfully registered it and created a .tlb file with regasm, and successfully imported the tlb into a c++ console app used for testing.

My COM assembly is called "efcAPI.dll" and it references another assembly that has not been set up for COM or registered in anyway called "efcServerDiscovery.dll". This second dll contains some code used by my COM dll and exists in the same folder as efcAPI.dll.

Everything concerning loading the COM assembly works fine. I can create instances of my classes defined in the COM and call methods from them. However when I call certain methods that use the code defined in efcServerDiscovery.dll I get a _com_error which reports that it could not load file or assembly 'efcServerDiscovery'.

I've verified that everywhere on my hard drive where efcAPI.dll exists there's a copy of efcServerDiscovery.dll (which is just the location I built and registered efcAPI.dll from). I've also attempted to place efcAPI.dll and efcServerDiscovery.dll in the same directory as the c++ app with no success.

Any suggestions as to where the c++ app is looking for the assembly or how to discover where it's looking would be great!

Best Answer

Yes, this is a problem with COM components having non-COM dependencies. Windows doesn't consider the location of the COM DLL when it searches for dependent DLLs. The normal search rules are in effect, the folder that contains the EXE first, Windows directories, current working directory, PATH environment. The location of the COM server does not play a role.

Assuming you don't want to deploy to the EXE folder, none of these are good places to store your DLL, although plenty of installers made the desperation move of storing it in c:\windows\system32 or modify the system PATH environment variable.

One thing you could do is P/Invoke SetDllDirectory() in your C# code before running any code in the DLL. Using Assembly.GetExecutingAssembly().Location will do it. That is however not a safe thing to do though, it might alter the search rules for the app that uses your component.

The only real fix is to install the DLL in the Windows side-by-side cache (WinSxS) and to include a manifest in your C# executable. Given the state of the documentation, I can only wish you the best of luck.

Related Solutions

C# – Calling C# COM object

First of all, why do you want to call that C# Assembly (that you've made comvisible) in your other C# project via COM ? That is not necessary ...

Ontopic: If you've created a tlb file, then you shouldn't do anything special. You can just reference the 'runtime callable wrapper' of the c#assembly you've created.

C# – How to run an async Task method synchronously

Here's a workaround I found that works for all cases (including suspended dispatchers). It's not my code and I'm still working to fully understand it, but it does work.

It can be called using:

customerList = AsyncHelpers.RunSync<List<Customer>>(() => GetCustomers());

Code is from here

public static class AsyncHelpers
{
    /// <summary>
    /// Execute's an async Task<T> method which has a void return value synchronously
    /// </summary>
    /// <param name="task">Task<T> method to execute</param>
    public static void RunSync(Func<Task> task)
    {
        var oldContext = SynchronizationContext.Current;
        var synch = new ExclusiveSynchronizationContext();
        SynchronizationContext.SetSynchronizationContext(synch);
        synch.Post(async _ =>
        {
            try
            {
                await task();
            }
            catch (Exception e)
            {
                synch.InnerException = e;
                throw;
            }
            finally
            {
                synch.EndMessageLoop();
            }
        }, null);
        synch.BeginMessageLoop();

        SynchronizationContext.SetSynchronizationContext(oldContext);
    }

    /// <summary>
    /// Execute's an async Task<T> method which has a T return type synchronously
    /// </summary>
    /// <typeparam name="T">Return Type</typeparam>
    /// <param name="task">Task<T> method to execute</param>
    /// <returns></returns>
    public static T RunSync<T>(Func<Task<T>> task)
    {
        var oldContext = SynchronizationContext.Current;
        var synch = new ExclusiveSynchronizationContext();
        SynchronizationContext.SetSynchronizationContext(synch);
        T ret = default(T);
        synch.Post(async _ =>
        {
            try
            {
                ret = await task();
            }
            catch (Exception e)
            {
                synch.InnerException = e;
                throw;
            }
            finally
            {
                synch.EndMessageLoop();
            }
        }, null);
        synch.BeginMessageLoop();
        SynchronizationContext.SetSynchronizationContext(oldContext);
        return ret;
    }

    private class ExclusiveSynchronizationContext : SynchronizationContext
    {
        private bool done;
        public Exception InnerException { get; set; }
        readonly AutoResetEvent workItemsWaiting = new AutoResetEvent(false);
        readonly Queue<Tuple<SendOrPostCallback, object>> items =
            new Queue<Tuple<SendOrPostCallback, object>>();

        public override void Send(SendOrPostCallback d, object state)
        {
            throw new NotSupportedException("We cannot send to our same thread");
        }

        public override void Post(SendOrPostCallback d, object state)
        {
            lock (items)
            {
                items.Enqueue(Tuple.Create(d, state));
            }
            workItemsWaiting.Set();
        }

        public void EndMessageLoop()
        {
            Post(_ => done = true, null);
        }

        public void BeginMessageLoop()
        {
            while (!done)
            {
                Tuple<SendOrPostCallback, object> task = null;
                lock (items)
                {
                    if (items.Count > 0)
                    {
                        task = items.Dequeue();
                    }
                }
                if (task != null)
                {
                    task.Item1(task.Item2);
                    if (InnerException != null) // the method threw an exeption
                    {
                        throw new AggregateException("AsyncHelpers.Run method threw an exception.", InnerException);
                    }
                }
                else
                {
                    workItemsWaiting.WaitOne();
                }
            }
        }

        public override SynchronizationContext CreateCopy()
        {
            return this;
        }
    }
}

Best Answer

Related Solutions

C# – Calling C# COM object

C# – How to run an async Task method synchronously

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