We already have this detection in telemetry, it would be nice to re-use the class in some form (e.g. linking it because this is mstest specific feature).

https://github.com/dotnet/sdk/blob/main/src/Cli/dotnet/Telemetry/CIEnvironmentDetectorForTelemetry.cs

Azure DevOps Reporter extension could also use it if it added detection specific to each CI provider (it needs to know if it runs on azdo), same for the colored outputter that uses ansi, but does not render progress that will be needed for this PR #5535

The logic is taken from dotnet and documented here: https://learn.microsoft.com/en-us/dotnet/core/tools/telemetry#continuous-integration-detection

How would the actual usage look like? I think we can assume that most users just use single CI and they don't way to specify it. How would inverse usage (run just locally) look like?

One more usage for TF reporter is the coloring in CI with simplified ANSI, in TerminalTest reporter. Linking to this comment from there.

How would the actual usage look like? I think we can assume that most users just use single CI and they don't way to specify it. How would inverse usage (run just locally) look like?

This is supported by ConditionMode in ConditionBaseAttribute already (that's how you can include/exclude OS in OSConditionAttribute today)

OSConditionAttribute requires the OS to be specified right? For CI I would expect to not provide anything by default, because I am probably running on just 1 CI system, so which one exactly it is, is not relevant until I am trying to run on 2 or more, and have different rules for each.

@nohwnd Yes we can start by not requiring to specify any specific CI system. So the attribute just requires ConditionMode.

Later on, we can add extra constructor overloads (or properties) to the attribute for specific CI systems. In most cases, one wouldn't need to special case a specific CI system. But in some cases, one may need to do so. For example, Aspire runs tests on both GitHubActions and Azure Pipelines. And here is a case where they need to special case GitHub Actions:

https://github.com/dotnet/aspire/blob/4e112fd2e97fbb19ed09a6eeb79e158e2354b1b2/tests/Aspire.TestUtilities/RequiresPlaywrightAttribute.cs#L21