C# files are compiled using Microsoft compiler and .NET Core, you can use Windows Runtime API, but classes implemented in C# files aren't accessible from JS or Boo languages.
Note: when using API from Windows Runtime, it's advisable to wrap the code with NETFX_CORE define, because the API is only avaible when building to Windows Store Apps, and it's not available in Unity Editor.
#if NETFX_CORE
public string GetTemporaryFolder()
{
return Windows.Storage.ApplicationData.Current.TemporaryFolder.Path;
}
#endif