How to Resolve Missing mscorlib 2.0.5.0 Reference in .NET Standard or .NET Core

This error occurs when you attempt to forcibly reference a PCL (Portable Class Library) project from a .NET Standard project.

This often happens when PackageTargetFallback is specified.

It seems to stem from the fact that while references to System.* assemblies have explicit versions specified in PCL, they are not explicitly specified on the .NET Standard side, but the exact cause is not entirely clear.

Although the root cause is unclear, you can resolve this by installing the following compatibility package from NuGet:

https://hatenablog-parts.com/embed?url=https%3A%2F%2Fwww.nuget.org%2Fpackages%2FMicrosoft.NETCore.Portable.Compatibility%2F

This appears to be a transitional fix, and concerns should be alleviated once .NET Standard 2.0 and .NET Core 2.0 are released.