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comvb6com-interop.net-standard-2.0

How do I expose a .netstandard2.0 library with COM for use in VB6?

I have a dotnet core library, a framework 4.7.2 library and a vb6 application.

I want to write a common library for them all to access and so choose .netstandard2.0

I tried a the 4.7.2 framework wrapper library between .netstandard2.0 library and vb6.

However I ran into assembly binding problems

Looking at the docs I see

In .NET Core, the process for exposing your .NET objects to COM has been significantly streamlined in comparison to .NET Framework.

However no mention .netstandard2.0

I decided to try following the docs anyway even though my project is using .netstandard2.0

I got up to the instructions on Generating the COM Host in which case the output files ProjectName.dll, ProjectName.deps.json, ProjectName.runtimeconfig.json and ProjectName.comhost.dll should build.

However the ProjectName.comhost.dll and ProjectName.runtimeconfig.json do not create.

I see in this dotnet standard issue that Microsoft plans on having tooling support in "Preview 4"

I am running VS 16.4.5

[Update]

I decided to try making a .net core wrapper library and enabling it for com.

I was able to add my .netstandard to the wrapper library via a nuget package (I build the .netstandard library using azure devops)

When I build my wrapper library the .dll, .deps.json, .pdb, .runtimeconfig.dev.json and .runtimeconfig.json files are created in a bin\Debug\netcoreapp3.1 folder.

However none of the .netstandard library files appear in the bin\debug folder.

I copied the .netstandard library and the .netcore wrapper libraries to the same folder and ran 

regsvr32 MyCoreComWrapper.comhost.dll

However no .tlb file is created which I need to be able to use from VB6

I note the following in the docs

Unlike in .NET Framework, there is no support in .NET Core for generating a COM Type Library (TLB) from a .NET Core assembly. The guidance is to either manually write an IDL file or a C/C++ header for the native declarations of the COM interfaces.

I found some information on github but would love a step by step guide to making the .tlb

I thought about using latebinding instead but am unsure of how to use it with a com library.

[Update]

I put a sample project on GitHub including some VB6 files. With VB6 referencing the .tlb referenced with the framework library.

When I try to run that I get 

Could not load file or assembly 'Microsoft.EntityFrameworkCore, Version=3.1.2.0, 
Culture=neutral, PublicKeyToken=adb9793829ddae60' or one of its dependencies. The system cannot find the file specified.

So I copied all the files from my framework test project to my vb6 folder, rebuilt and ran.

Then I got the error

Could not load file or assembly 'Microsoft.Extensions.DependencyInjection.Abstractions, Version=3.1.0.0, 
Culture=neutral, PublicKeyToken=adb9793829ddae60' or one of its dependencies. The system cannot find the file specified.

I see the file Microsoft.Extensions.DependencyInjection.dll is present with File version 3.100.220.6706

Solution

  • The issue is due to assembly binding resolution that fails when ran from VB6 (IDE or compiled .exe file).

    Here are the steps to solve it:

    • Compile the VB project, for example, let's assume the compiled file is Project1.exe.
    • Copy all .NET assemblies (including x86 and x64 directories, and languages directory if localized version is important) aside the compiled VB6 file
    • Now run Project1.exe, you will get an error like this:

    enter image description here

    The error is clearly a mismatch between the version of your assemblies aside the Project1.exe file and the version of referenced assemblies (not references you've created yourself but reference embedded in these assemblies... ). You don't see that when you start a .NET program because resolution is a very complex process that depends on a lot of parameters (and it's not getting any better with .NET Core, Framework, Standard, nugets, etc.).

    To futher check it's a mismatch error, you can also use the Fuslogvw.exe (Assembly Binding Log Viewer) tool from the SDK.

    Now we know it's an assembly version mismatch issue, what you can do is create a file named Project1.exe.config aside Project1.exe and add assembly binding redirects to it.

    The easiest way to configure it is to redirect all possible versions to the ones present in the directory that contains your program, so in your case (and as of today, as all these can evolve...), it would be something like this, possibly for every assembly you reference directly or indirectly:

    <configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      ...
      <dependentAssembly>
        <assemblyIdentity name="Microsoft.Extensions.DependencyInjection.Abstractions" publicKeyToken="adb9793829ddae60" />
        <!-- 3.1.2.0 is the version of the assembly you ship -->
        <bindingRedirect oldVersion="0.0.0.0-65535.65535.65535.65535" newVersion="3.1.2.0" />
      </dependentAssembly>
      ...
    </assemblyBinding>
  </runtime>
</configuration>

    Unfortunately, there are many satellite assemblies, and it's a bit tedious to create all redirects with correct information, so I've created a tool that creates a .config file with the redirects configured automatically for all .NET assemblies in a given directory: https://github.com/smourier/BindingRedirectGenerator.

    If you want it to work for the VB6 IDE too, you'll have to use the same procedure in a VB6.exe.config file aside VB6.exe.