Run .xaml.cs file during runtime

Hello!
My app uses connection with server, which allows users to download certain files from it. Downloading, managing etc. works well, but how to open them when the app is opened? Just like we have some .xaml and .xaml.cs files that come with app and opening them is not a problem, I want to do the same with files I downloaded,
e.g. App came with HelloWorld.xaml and HelloWorld.xaml.cs files, I can open them easily.
Now I download file HelloWorld2.xaml.cs and want to use it the same time as HelloWorld.xaml (as it has nothing, because whole UI is ,,made'' in .xaml.cs file)
If something is unclear, just ask