'Could not load file or assembly 'netstandard, Version=2.0.0.0, ...'. Reference assemblies should not be loaded for execution

Question

Goal: From a .NET 4.7 console app, using reflection with Assembly.GetType(), I am trying extract the Type of a netstandard 2.0 class from Assembly X. Then I want to create an instance of this Type with Activator.CreateInstance().

What I am trying to do: However, this assembly X has a dependency to netstandard 2.0. To be able to get the Type, netstandard dependency has to be loaded into the AppDomain. That's why when the AppDomain is requesting the netstandard assembly through the AssemblyResolve event, I simply load the dll like this :

var netStandardDllPath = @"C:\Users\xxx\.nuget\packages\NETStandard.Library.2.0.0-preview1-25301-01\build\netstandard2.0\ref\netstandard.dll";

return Assembly.LoadFrom(netStandardDllPath);

Which throws:

System.BadImageFormatException: 'Could not load file or assembly 'file:///C:\Users\vincent.lerouvillois.nuget\packages\NETStandard.Library.2.0.0-preview1-25301-01\build\netstandard2.0\ref\netstandard.dll' or one of its dependencies. Reference assemblies should not be loaded for execution. They can only be loaded in the Reflection-only loader context. (Exception from HRESULT: 0x80131058)'

Inner Exception: BadImageFormatException: Cannot load a reference assembly for execution.

What I know: I know that they want us to load the DLL with Assembly.ReflectionOnlyLoadFrom. But doing that will prevent me from instanciate the type with Activator.CreateInstance(). See Microsoft official post

Also, I tried referencing the Nuget packages NETStandard.Library 2.0.0-preview1-25301-01 and NETStandard.Library.NETFramework 2.0.0-preview1-25305-02 in my console app so it would have the netstandard 2.0 libraries referenced, but it didn't change anything.

Question: Does anyone would know if there is a proper way to load that dll without error, or maybe if this is a bug, or else? Or why this kind of dll is not able to load for execution?

Answer 1

The netstandard.dll you are trying to load is a reference assembly that which cannot be loaded for runtime on .NET Framework as pointed out by others. However if you need to resolve that dependency you will need to runtime version that maps to the framework you are trying to run on.

For .NET Standard support we are including them as part of the msbuild extensions that ship with VS so you will want to get the version of netstandard.dll from there. Depending on which version of VS2017 you have installed it should be somewhere like C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\MSBuild\Microsoft\Microsoft.NET.Build.Extensions\net461\lib\netstandard.dll or from the .NET Core 2.0 SDK you can find it C:\Program Files\dotnet\sdk\2.0.0\Microsoft\Microsoft.NET.Build.Extensions\net461\lib\netstandard.dll

Try using one of those versions in your scenario.