How to set OS condition for TargetFramework in MSBuild - xamarin.forms

Question

I have few projects in my solution - few for mobile (Xamarin.Forms) and one xUnit. xUnit is build with .NET5 and references mobile project.
There are 2 pipelines on Azure DevOps to build mobile app. One is for Android and runs on Windows, another for iOS and runs on OSX.
xUnit project build started to fail after I added new NuGet to mobile project (Rg.Plugins.Popup).
Initially, setting the TargetFramework for xUnit project looked like this
<PropertyGroup>
<TargetFramework>net5.0</TargetFramework>
<IsPackable>false</IsPackable>
</PropertyGroup>
That setup worked on OSX, but did not work on Windows with error message Error NETSDK1136: The target platform must be set to Windows (usually by including '-windows' in the TargetFramework property) when using Windows Forms or WPF, or referencing projects or packages that do so.
Changing it to
<PropertyGroup>
<TargetFramework>net5.0-windows</TargetFramework>
<IsPackable>false</IsPackable>
</PropertyGroup>
fixes Windows problem, but it starts failing on OSX (with same error message, which is weird, as we are no longer on Windows).
I tried few other options, but could not make it to succeed on both machines.
<PropertyGroup>
<TargetFramework Condition=" '$(OS)' == 'Windows_NT' ">net5.0-windows</TargetFramework>
<TargetFramework Condition=" '$(OS)' != 'Windows_NT' ">net5.0</TargetFramework>
<IsPackable>false</IsPackable>
</PropertyGroup>
<PropertyGroup>
<TargetFrameworks>net5.0;net5.0-windows</TargetFrameworks>
<IsPackable>false</IsPackable>
</PropertyGroup>
I also tried with Choose, When and Otherwise, but it looks like you cannot wrap TargetFramework into that at all.
Am I doing it completely wrong or it it just some coincidence that correct code does not work in my case?

Answer 1

Try to place Condition inside PropertyGroup not TargetFramework .
Try the code below
<PropertyGroup Condition=" '$(OS)' != 'Windows_NT' ">
<TargetFramework>netstandard2.0</TargetFramework>
</PropertyGroup>
<PropertyGroup Condition=" '$(OS)' == 'Windows_NT' ">
<TargetFrameworks>netstandard2.0;net45</TargetFrameworks>
</PropertyGroup>