Improve host version selection semantics

[richlander]

Various versioning scenarios remain too hard with .NET. We should find a way to resolve the.

We designed some solutions recently: https://github.com/dotnet/designs/blob/main/accepted/2022/version-selection.md. We should decide which of these to implement next.

We were recently discussing some related problems:

• dotnet test has complicated versioning semantics. We need to do some combination of documenting and resolving that.
• It's hard to test an unmodified app on multiple runtime versions.
• Virtual environment providers publish multiple (many!) .NET SDK versions for global.json compatibility. We need to help them deliver a simpler experience.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[richlander]

Here's a related conversation:

Tried some dirty tricks and the following seems to work. Make your test project have the lowest TFM you want to work with - so for example net6.0. Then run it with dotnet test /p:RollForward=disable /p:RuntimeFrameworkVersion=6.0.2 - this will force the test to run on the specified version of NETCore.App. Note that if you want to run against a different version you have to rebuild the test (changing the command line will not force a rebuild) so typically dotnet clean and then again. This works across major versions, meaning this works as well: dotnet test /p:RollForward=disable /p:RuntimeFrameworkVersion=7.0.0-preview.5.22301.12.

It's pretty easy to conclude that this is not a great solution. Hence "dirty tricks".

Related to: dotnet/sdk#11613

[ghost]

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Issue Details

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Various versioning scenarios remain too hard with .NET. We should find a way to resolve the.

We designed some solutions recently: https://github.com/dotnet/designs/blob/main/accepted/2022/version-selection.md. We should decide which of these to implement next.

We were recently discussing some related problems:

• dotnet test has complicated versioning semantics. We need to do some combination of documenting and resolving that.
• It's hard to test an unmodified app on multiple runtime versions.
• Virtual environment providers publish multiple (many!) .NET SDK versions for global.json compatibility. We need to help them deliver a simpler experience.

Author:	richlander
Assignees:	-
Labels:	area-Host, untriaged
Milestone:	-

[vitek-karas]

Just want to make it clear that the "dirty tricks" as per #71286 (comment) actually rebuild the app - so even cleanup of that experience would not help with the requirement to test unmodified app on multiple runtimes.

[richlander]

Ah. Poor wording on my part. I meant unmodified in the sense of source (primarily the project file). Not having to re-build would also be nice, but not what I meant.