DllExport in a broker dll from VBA with reference to other .NET projects

Question

I want to use DllExport, in a dll which has a broker-function: routing some calls from external code (VBA) to other .NET dll's behind.

I got the DllExport working on a single dll: the dll is generated, and I can use it. But... only with code from that single dll. When I call code from another (in .NET referenced) dll, the solution still builds, I still can use the code from the dll with dllexport, but when calling a method that routes to the referenced dll, VBA gives me a 'Could not load file or assembly 'myseconddllname, version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of iets dependencies. The system cannot find the file specified.

I also tried to add DllExport to the second dll (which shouldn't be necessary, no objects/info from the second dll should be exposed to Unmanaged code). This didn't help.

Any ideas how to solve this?

Edit: registered an issue on github: Calling referenced .NET assembly

Answer 1

For others reference: with help from here and github-issue found the solution:

Having two projects:

1. ClassLibrary1 to call from VBA
2. ClassLibrary2 to be used from within ClassLibrary1 (without exposing something to VBA)

In ClassLibrary1 we've a static class UnmanagedExport:

static class UnmanagedExport
{
    static UnmanagedExport()
        => AppDomain.CurrentDomain.AssemblyResolve += CurrentDomain_AssemblyResolve;

    static Assembly CurrentDomain_AssemblyResolve(object sender, ResolveEventArgs args)
        => Assembly.LoadFrom($@"{new FileInfo(args.RequestingAssembly.Location).DirectoryName}\{args.Name.Split(',')[0]}.dll");

    [DllExport]
    [return: MarshalAs(UnmanagedType.IDispatch)]
    static object CreateBroker()
        => new Class1();
}

CreateBroker() creates an instance of Class1 (which is supposed to be the broker between VBA and ClassLibrary2). ClassLibrary1.Class1:

[ComVisible(true)]
[ClassInterface(ClassInterfaceType.AutoDual)]
public class Class1
{
    public void DoNothingOnClass2Typed()
        => new Class2().DoNothing();

    public void CreateClass2aFromClass2Typed()
        => new Class2().CreateClass2a();
}

Class2 (and Class2a) are both in ClassLibrary2 (which is referenced by ClassLibrary1):

namespace ClassLibrary2
{
    public class Class2
    {
        public void DoNothing()
        {
            return;
        }

        public void CreateClass2a()
        {
            var class2a = new Class2a();
        }
    }
}

From VBA this is called as:

Declare Function CreateBroker Lib "C:\source\repos\DllExportTest\ClassLibrary1\bin\Debug\ClassLibrary1.dll" () As Object


Public Sub RunTest()
  Dim class1 As Object

  Set class1 = CreateBroker()

  class1.DoNothingOnClass2Typed

  class1.CreateClass2aFromClass2Typed

  Set class1 = Nothing
End Sub

Works like a charm!

Note: ClassLibrary2 can also be a .NET Standard dll, so the solution is compatible with .NET Core also.