PlatformProperty.cs

// This is the root Class1.cs file
public partial class Class1
{
      public string Platform
      {
          get => impl_GetPlatform();
      }
}

// This goes in PlatformWin/Class1.cs
public partial class Class1
{
     string impl_GetPlatform() => "win";
}


// This goes in PlatformOsx/Class1.cs
public partial class Class1
{
     string impl_GetPlatform() => "osx";
}