The great majority of apps built for Windows 8.1/Windows Phone 8.1 work on Windows 10 as-is – no changes required what so ever. But what if you want to leverage the new APIs provided by Windows 10 such as the inking API while still supporting the Windows 8.1 version of your app? Or you might be among the few unfortunate ones who have been using some API deprecated on Windows 10; UserInformation class no longer works on Windows 10 but you have to use the User class instead. How to do that without duplicating the code base and having two completely separate app projects to maintain? In this article I’ll describe two approaches to do that.
Shared code and assets in portable project
The first approach is to include all the shared code (in practice that can be almost all of your code) to a separate portable project in your Windows 8.1 solution. First you need to create the project: Right click your solution in the solution explorer, hover on Add and select New Project…
Use Class Library as the project type, name it and hit OK.
Drag all the code and asset files you want to share between both the Windows 8.1 and Windows 10 app to the newly created Class Library project.
Note that if you have a solution that supports both Windows 8.1 and Windows Phone 8.1, you have to have at least a partial main page (the page you navigate to in the start-up) in the original Windows 8.1 and Windows Phone 8.1 projects. This due to the fact that you can’t add a reference to your Class Library project in the Shared (Windows 8.1/Windows Phone 8.1) project where your App class lives. And without the reference you can’t make your app to navigate to a page defined in your Class Library project in the start-up. Makes sense? Ok, cool, let’s carry on…
Now that we have the code moved to the Class Library project, we must add it as a reference to the other projects so that we can access the classes and assets. Right click the References under the projects in the solution explorer and select Add Reference…
On Projects tab you should now find the Class Library project. Check the checkbox and click OK.
Now fix any minor problems you may have and once your app builds and runs it is time to move on to work on the Windows 10 solution. Create a new Universal Windows 10 application project and add the Class Library project containing the shared code to the Windows 10 solution as an existing project:
Add the Class Library project as a reference to your main Windows 10 project (as explained before), make your main project to use the shared code and you’re all set! Fine – I realize it’s not often this simple and you need to do some tweaking to get all the other dependencies working and so on, but these are the first steps to take.
If you now want to extend the app on Windows 10 by utilizing the cool new APIs, you need to add that specific code to the main project. You can’t, of course, access the code in the main project from the shared code (for many reasons, one being that this would create a circular dependency), but one solution is to define interfaces in the shared code and providing the implementations from the main project. See my example, namely IUserInformationHelper interface in the Class Library, Windows 10 UserInformationHelper implementation and App.xaml.cs where the implementation is provided.
- Allows management of the shared code as a single project
- Other dependencies (Nuget packages and such) may cause problems e.g. if they aren’t as universal and work only on Windows 8.1 and not on Windows 10
- You cannot use conditional preprocessing blocks in the shared code (#if) to target a specific platform since the compilation symbols are also shared
Shared code and asset files as links
Another way of sharing code between solutions is adding the code and asset files as links. Using links you don’t have to change your existing solution. Simply create a new – in this case – Windows 10 application project and start adding the files from your existing Windows 8.1 solution. Right click your new project in the solution explorer, hover on Add and select Existing Item… Then browse the Windows 8.1 solution folder containing the files you want to add, select the files and click Add As Link:
The files are now shown in your solution explorer. However, they are not physically in your new project but exist in the Windows 8.1 application project folder. Any changes you make to these files will also appear in both projects.
While adding the files individually can be tedious, the benefit here is that you can take advantage of conditional preprocessing blocks in C# code:
#if WINDOWS_UWP // Have your Windows 10 specific code here #else // Have your Windows 8.1 specific code here #endif
- Conditional preprocessing blocks and compilation symbols can be used
- Dependencies to additional libraries and Nuget packages are easier to maintain
- Adding platform specific features, e.g. new Windows 10 APIs, is trivial
- Adding/removing shared code and asset files needs to be done in both solutions separately
An example for using the both approaches featured in this article can be found here in GitHub.