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I have a simple .Net Core Console application referencing a .Net Framework 4.6.2 project in the same solution. When I build, I get the following error:

error CS0012: The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

Am I trying to do something that is not allowed?

Is there some way to reference a .Net Framework 4.6 (or Mono) assembly (or P/Invoke it) from a .Net Core application?

Using Visual Studio 2017, .Net Core 1.1

VMAtm
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Verax
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2 Answers2

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In .NET Core 1.x, you are not allowed to reference a .NET Framework 4.6 (or Mono) assembly. .NET Core 1.x and .NET Framework are 2 different runtimes, and their types don't mix - like the build error message says.

Instead, what you can do is to recompile that .NET Framework 4.6 assembly targeting ".NET Standard". Read more about .NET Standard. Compiling a library for .NET Standard allows that library to run on .NET Framework, Mono, or .NET Core. Another option is to multi-target that .NET Framework 4.6 assembly, so it builds both for .NET Framework 4.6 and for .NET Core 1.1. You should only do this if the APIs your library requires are not supported by .NET Standard. This is roughly the same as @VMAtm's answer, but you can do it with a single .csproj in VS 2017.

The good news is, what you are trying to do will be supported on .NET Core 2.0, which is slated for release later this year.

Eric Erhardt
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    I don't see any mention in the link you provided about referencing .Net Framework assemblies from .Net core. Could you please specifically point it out if I'm overlooking it? Also, is there any way, currently, to P/Invoke (or equivalent) the .Net Framework assembly from the .Net Core application. (assuming I only have a binary for the .Net Framework assembly)? – Verax Mar 10 '17 at 17:37
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    The .NET Standard link above links to the [introducing net standard blog](https://blogs.msdn.microsoft.com/dotnet/2016/09/26/introducing-net-standard/). In that post `this compatibility shim provides a bridge that enables you to convert your libraries to .NET Standard without having to give up referencing existing libraries that haven’t been converted yet.` No, there is no way to load/run that .NET Framework assembly on .NET Core 1.1, as there is no "compatibility shim" in .NET Core 1.1. – Eric Erhardt Mar 10 '17 at 22:46
  • "The good news is, what you are trying to do will be supported on .NET Core 2.0, which is slated for release later this year." I don't see anything in the Roadmap claiming that this capability will be added in 2.0. Can you cite a source? EDIT: Nevermind, I see it now. – Verax Mar 10 '17 at 23:43
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    @Verax Did Core 2.0 resolve your issue? I did go through the Roadmap but was not clear on it claiming this capability included in Core 2.0. Could you please point it out, as well? – nam Mar 27 '19 at 16:38
  • @nam I never followed through on it, so I'm not sure what's currently possible. – Verax Mar 28 '19 at 00:02
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    @Verax It seems, you still can't do it even in latest release to date: `.NET Core 2.2.1` as explained [here](https://stackoverflow.com/a/55176090/1232087). – nam Mar 28 '19 at 13:45
  • What if that project contains, say, Entity Framework 6 code? Can you still recompile it into ".NET Standard" and have it work with say, a .NET Framework web app as well as say, a .NET Core V2 Azure Function? Because I'm pretty sure you still can't do this in .NET Core version 2, which means .NET Core is pretty much useless for working with almost all existing .NET software. – nmit026 May 29 '19 at 00:21
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.Net Core and .Net 4.0 has different core libraries. The type System.Object from old .Net libraries is defined in mscorlib, which clearly hasn't been referenced.

If you do reference that library, you probably will get the name resolving error, because the same types will be defined in the same namespaces and with same type names in different libraries. So yes, this is not allowed.

Edit: However, there is a common approach known as Shared project, you can read about it here. Also this quaestion may be useful for you:

.net core classlibrary calling .net framework class library

Also you may try to create a portable class library, which can be referenced either from .Net Core and .Net 4.6 library.

Portable class libraries are great when your common code is platform-independent as well as for reusable libraries where the platform-specific code can be factored out.

Edit #2: As @EricErhardt mentioned, you can recompile your .Net 4.6 library with different targeting, which may be not an option if you don't have a source code.

Community
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VMAtm
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    I'm not actually interested in sharing code (although my test may have implied such). I want to call a .Net Framework assembly from a .Net Core application. I'm still looking for an answer to this question: Is there some way to reference a .Net Framework 4.6 (or Mono) assembly (or P/Invoke it) from a .Net Core application? – Verax Mar 10 '17 at 16:54
  • Did you saw the article I've linked? – VMAtm Mar 10 '17 at 16:59
  • Yes, of course I saw it, and their solution was to share code, which in not what I'm interested in. – Verax Mar 10 '17 at 17:21