Add design document for issuing a warning when targeting netstandard1.x

INDEX.md

@@ -85,6 +85,7 @@ Use update-index to regenerate it:
 | 2023 | [Experimental APIs](accepted/2023/preview-apis/preview-apis.md) | [Immo Landwerth](https://github.com/terrjobst) |
 | 2023 | [Multi-threading on a browser](accepted/2023/wasm-browser-threads.md) | [Pavel Savara](https://github.com/pavelsavara) |
 | 2023 | [net8.0-browser TFM for applications running in the browser](accepted/2023/net8.0-browser-tfm.md) | [Javier Calvarro](https://github.com/javiercn) |
+| 2024 | [.NET Standard Targeting Recommendations](accepted/2024/net-standard-recommendation.md) | [Immo Landwerth](https://github.com/terrajobst), [Viktor Hofer](https://github.com/ViktorHofer) |
 
 ## Drafts
 

accepted/2024/net-standard-recommendation.md

@@ -0,0 +1,128 @@
+# .NET Standard Targeting Recommendations
+
+**Owner** [Immo Landwerth](https://github.com/terrajobst) | [Viktor Hofer](https://github.com/ViktorHofer)
+
+We have already said that there is [no new version of .NET
+Standard][net-standard-future] and that the replacement moving forward is to
+just target the platform-neutral `netX.Y` [base framework][tfm] (with `x.y >=
+5.0`) of .NET Core.
+
+For customers that need to author code that still needs to be consumed by .NET
+Framework, the recommendation is to continue using `netstandard2.0`. There is
+very little reason to target .NET Standard 2.1 because you lose .NET Framework
+while only gaining very little few additional APIs. And if you don't care about
+.NET Framework support, then just target .NET Core directly; at least then you
+get access to a lot more functionality, in exchange for the reduced reach.
+
+There is basically no reason to target .NET Standard 1.x any more as all .NET
+implementations that are still supported, can reference .NET Standard 2.0
+libraries.
+
+However, we have found that many customers still target .NET Standard 1.x. We
+found out the reasons boil down to one of two things:
+
+1. Targeting the lowest possible versions is considered better
+2. It was like and there is no reason to change
+
+The first argument has merit in a world where .NET Standard continues to produce
+new versions. Since that's no longer the case you might as well update to a
+later version that allows you to use more functionality. We found the biggest
+jump in productivity is with .NET Standard 2.0 because it also supports
+[referencing .NET Framework libraries][netfx-compate-mode], which is useful in
+cases where you modernize existing applications and have some mixture until you
+can upgrade all the projects.
+
+The second reason is always valid; however, in cases when you're actively
+maintaining a code base it makes sense to upgrade to avoid missing out on
+features that could have made your life easier and you didn't realize exist
+because code completion simply didn't advertise them.
+
+## Scenarios and User Experience
+
+### Building a project targeting .NET Standard
+
+Jackie is assigned a work item to add a feature that requires writing code in
+the shared business logic project. That project happens to target .NET Standard
+1.4. She doesn't know why because the project was built before her time on the
+team.  The fist time she compiles in Visual Studio she sees a warning:
+
+> warning NETSDK1234: Contoso.BusinessLogic.csproj: Targeting .NET Standard
+> prior to 2.0 is no longer recommended. See \<URL\> for more details.
+
+Following the link, she finds a page that inform her that all supported .NET
+implementations allow using .NET Standard 2.0 or higher. She asks one of her
+colleagues and she remembers that they used .NET Standard 1.4 because at some
+point her team also maintained a Windows Phone app (which, to everybody's
+disappointment no longer exists).
+
+Jack decides to upgrade the project to .NET Standard 2.0. She later realizes
+that this was a good idea because it enables her to use many of the
+`Microsoft.Extensions` libraries that make her life a lot easier, specifically
+configuration and DI.
+
+## Requirements
+
+### Goals
+
+* Issue a warning when *building* a project targeting `netstandard1.x` with the
+  .NET 9 SDK
+
+### Non-Goals
+
+* Issue a warning when building a project targeting `netstandard2.x`
+* Issue a warning when consuming a library that was built for .NET Standard 1.x.
+* Issue a warning building with a .NET SDK prior to 9.0
+
+## Stakeholders and Reviewers
+
+* Libraries team
+* SDK team
+* Build team
+* NuGet team
+
+## Design
+
+The .NET SDK will check the `TargetFramework` property. If it uses
+`netstandardX.Y` with `X.Y < 2.0` it will produce the following warning:
+
+> Targeting .NET Standard prior to 2.0 is no longer recommended. See \<URL\> for
+> more details.
+
+* The warning should have a diagnostic ID (prefixed `NETSDK` followed by a four
+  digit number).
+* The warning should be suppressable via the `NoWarn` property.
+* The URL should be point to the documented ID [here][sdk-errors] (not sure why
+  it says `sdk-errors` -- I don't believe we have a section for warnings).
+
+## Q & A
+
+### Didn't you promise that .NET Standard is supported forever?
+
+We promised that future .NET implementations will continue to support
+*consuming* libraries built for .NET Standard, all the way back to 1.0. That is
+different from promising that we'll support *producing* .NET Standard binaries
+forever; that would only make sense if .NET Standard is still the best way to
+build reusable libraries. And [since .NET 5 and the platform-specific
+flavors][tfm] of .NET we think there is now a better way to do so.
+
+The original promise around consumption still stands: existing libraries built
+for .NET Standard 1.0 can still be consumed and we have no plans on changing
+that. So any existing investment in building a library that can be consumed in
+as many places as possible still carries forward.
+
+### Can I continue to build libraries targeting .NET Standard 1.x?
+
+Yes. There *may* come a time where the .NET SDK stops supporting building .NET
+Standard libraries, but we currently have no plans on doing so as the targeting
+packs for .NET Standard 1.x aren't bundled with the SDK and are only downloaded
+on demand already; thus, the benefit to removing the support for building .NET
+Standard 1.x seems marginal.
+
+We believe our ecosystem is better off targeting at least .NET Standard 2.0
+which is why we want to issue a warning, but the motivation isn't in us being
+able to remove the support for building.
+
+[net-standard-future]: https://devblogs.microsoft.com/dotnet/the-future-of-net-standard/
+[tfm]: https://github.com/dotnet/designs/blob/main/accepted/2020/net5/net5.md
+[netfx-compate-mode]: https://learn.microsoft.com/en-us/dotnet/core/porting/#net-framework-compatibility-mode
+[sdk-errors]: https://learn.microsoft.com/en-us/dotnet/core/tools/sdk-errors/