Detecting platform using IsOSPlatform() doesn't work in a Docker linux container

[replaysMike]

I'm trying to define the OS platform in my csproj as follows:

<PropertyGroup Condition="$([MSBuild]::IsOSPlatform('Windows'))">
    <DefineConstants>OS_WINDOWS</DefineConstants>
</PropertyGroup>
<PropertyGroup Condition="$([MSBuild]::IsOSPlatform('Linux'))">
    <DefineConstants>OS_LINUX</DefineConstants>
</PropertyGroup>

which would work fine normally, but when run in a docker linux container it thinks the OS is Windows. Are there any other options?

I've got libraries I'm trying to prevent from being included, but if this is more of a runtime issue then I may need to figure out alternatives.

RuntimeInformation.IsOSPlatform(OSPlatform.Linux) works correctly at runtime, but does not for msbuild. In my case I'm using PerformanceCounters when using Windows and not within a container, but I get TypeInitializer issues if it's even referenced at runtime.

#if OS_WINDOWS
if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) && !EnvironmentUtils.InDocker) {
  var performanceCounter = new PerformanceCounter();
  // no good
}
#endif

[KalleOlaviNiemitalo]

Condition="$([MSBuild]::IsOSPlatform('Windows'))" -- AFAIK this checks the platform on which you run MSBuild. So if you build on Windows but run on Linux, it won't match.

How do you set the target frameworks; do you have net6.0-windows there?

I'm surprised you get a TypeInitializerException. Which version of the System.Diagnostics.PerformanceCounter package are you using, and what is the inner exception?

[GangWang01]

As @KalleOlaviNiemitalo mentioned, OS_WINDOWS defined when building on Windows couldn't represent the OS which you run. RuntimeInformation.IsOSPlatform(OSPlatform.Linux) should be used to determine the OS you run.