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.NET Standard 2

This document describes the plan for .NET Standard 2, which includes the definition of its API surface as well as the principles we use to extend and review those additions.


  • Driving force for consistency. We want to have an agreed upon set of required APIs that all .NET platforms have to implement in order to get access to the .NET library ecosystem.

  • Foundation for great cross-platform tooling. We want a simplified tooling experience in Visual Studio and the command line that allows customers to target the commonality of all .NET platforms by choosing a single version number.

Customer Scenarios

These are the scenarios we need to be able to support with .NET Standard 2:

  • Wide .NET Standard support. Stacey builds a .NET Standard-based library. She can consume the library from all current platforms.
  • Meet our ecosystem where it is. Paul can consume an mscorlib based assembly from a .NET Standard-based library without having to jump through hoops.
  • No PCL left behind. Dakota can consume a PCL based assembly from a .NET Standard-based library without having to jump through hoops.
  • Avoid cliffs. Ariel can extend a .NET Standard-based library to use components that aren’t available everywhere without having to cross-compile.


The scenarios above result in these requirements:

  • 100% source and binary compatible surface area for:
    • Classic .NET Framework and Xamarin assemblies (mscorlib and friends)
    • Existing portable class libraries (both, mscorlib as well as System.Runtime)
  • Binary compatibility refers to binding compat, but an understanding that behavioral compatibility may not be 100% possible.
  • We’ll not necessarily bring all the types that .NET Framework has, but that we’ll bring types in their entirety without subsetting members.
  • Focus on APIs that are available everywhere, but allow for platform specific extensions.

Assembly Unification

Given these requirements, .NET Standard will have to support the following assembly sets:

  • We'll have support for mscorlib and friends
  • We'll have support for System.Runtime and friends

Of course, we can't have two assemblies defining the same type; thus we have to pick one home for a given type and have the other assembly type forward to it. At first, it seems quite attractive to have the core assembly called mscorlib. We don't want to use System.Runtime as the goal of .NET Standard is to replace the current version of PCLs and contracts.

However, we can't call the core assembly mscorlib. The reason being that we have two conflicting requirements:

  1. We want to be able to consume existing binaries and have the APIs unify in .NET Standard, assuming the API is supported in .NET Standard.
  2. We also want to be able to model platform specific APIs without forcing all platform owners to provide the API.

Let's look at a concrete example. Assume I'm writing a class library that is targeting .NET Standard. Assume I'm only targeting Windows and thus want to use the registry. Since the registry is an extension, it sits on top of .NET Standard. Now I want to reuse a library that was compiled against the .NET Framework and only uses APIs .NET Standard has, plus the registry. This looks as follows:

Unfortunately, we can't unify mscorlib!RegistryKey with registry!RegistryKey as these have different identities.

Fortunately, this conflict can be easily resolved if we call the core assembly something else, such as netstandard.dll:

Legend: Dotted lines indicate type forwarding, solid lines indicate references

This frees up the name mscorlib in .NET Standard and we can introduce an assembly that simply type-forwards all its types to the appropriate location:

  • Required types go to netstandard.dll
  • Optional types go to <extension>.dll

Assembly Sets

This means we've logically three assembly sets in .NET Standard:

  1. The definition of the .NET Standard netstandard.dll
  2. Type forwarders for the classic assemblies such as mscorlib
  3. Type forwarders for the PCL contract assemblies such as System.Runtime

The developer will never explicitly have to interact with any of these sets. The netstandard.dll will always be referenced. The build system will automatically reference the type forwarding assemblies if the developer references a non-.NET Standard class library.

Please note that the above sets do not include the extensions, such as the registry. That is intentional. The .NET Standard only includes APIs that are considered required, and thus must be supported by every platform.

Extensions exist outside of the .NET Standard and must be manually referenced. This ensures that developers are aware if they start using functionality that might limit the set of platforms their library can run on. We expect the extensions to be provided as NuGet packages.

You might wonder how .NET Standard is self-contained if it has type forwarders to assemblies that aren't part of .NET Standard, i.e. are forwarding to extensions such as the registry. The answer is that the SDK that makes up the .NET Standard is in fact not self-contained and points to extension assemblies. Technically, dangling type forwarders aren't problematic and we have used them in the past. If the developer references an assembly like above and starts consuming an API that returns a type that lives in an extension, such as RegistryKey, the compiler will emit an error message like this:

CS0012: The type 'RegistryKey' is defined in an assembly that is not referenced. You must add a reference to assembly 'Microsoft.Win32.Registry, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089'

Also, since Visual Studio 2015 Update 3 the Roslyn team added a quick fix that will suggest missing NuGet packages when a type cannot be resolved. This will also handle cases like the one above and thus allow the developer to fix the issue right from the editor without having to know which NuGet package they need. The key point is that developer will have to take an explicit action to make the project less portable.

We'll provide another document that goes into more detail on how the user experience in Visual Studio can be improved to make developers aware of portability issues.

.NET Framework 4.6.1 supporting .NET Standard 2.0

.NET Framework 4.6.1 has the highest adoption, which makes it the most attractive version of .NET Framework to target.

By following normal versioning rules one would expect that .NET Standard 2.0 would only be supported by a newer version of .NET Framework, given that .NET Framework 4.6.1 only implements .NET Standard 1.4.

This would mean that the libraries compiled against .NET Standard 2.0 would not run on the vast majority of .NET Framework installations.

On the other hand, .NET Standard 2.0 adds many APIs that .NET Framework 4.6.1 already supports. The delta looks as follows:

  • .NET Standard 2.0 adds 14,994 APIs that .NET Framework 4.6.1 already supports
  • .NET Standard 2.0 only has 43 APIs that .NET Framework 4.6.1 doesn't support

Originally, we planned to simply elide those APIs from .NET Standard 2.0 in order to make it easier to understand what will work on .NET Framework 4.6.1.

However, we got a lot of feedback around this. You told us that this decision makes it really hard to reason about .NET Standard versioning rules. Thus, we decided to simplify this:

  • .NET Standard 2.0 will be a strict superset of .NET Standard 1.6. In other words, no breaking changes will happen between .NET Standard 2.0 and 1.x.
  • .NET Framework 4.6.1 will allow referencing binaries that are compiled against .NET Standard 2.0.

Considering the number of APIs that .NET Framework 4.6.1 will not support is low and that these are all brand-new APIs with low adoption we believe this is a much better trade-off.

You can use API Port to scan a given application to make sure no code in your application depends on these APIs.