How to cook Azure Mobile Apps (node.js): Xamarin and .NET Standard

In the previous post we discussed how to setup Mobile Apps in node.js, and it’s time to start developing a simple client application. I will use Xamarin and C# programming language, because Xamarin is the coolest technology right now that allows me to develop applications for all modern devices.

My application is not very complex, so, I will use Xamarin forms to develop it just once for all platforms not digging to any platform specific features.

You can create your application on Mac or Windows using Visual Studio for Mac or Visual Studio 2017 (2015). In my case I am sitting at Kelowna airport with my Windows computer, so, I will use Visual Studio 2017 Update 3.

Pay attention that Visual Studio allows you to create Xamarin Forms application using Shared Project or PCL templates. It’s strange, but Shared Project is the default option, and I would like to recommend changing it to PCL. Later, you will be able to use .NET Standard rather than PCL just moving your code to a .NET Standard library project.

Visual Studio creates 4 projects in one solution and three of these projects contain platform-specific code for supported platforms (iOS, Android and UWP), but the last one is exactly Xamarin.Forms based portable class library. Almost all time we will work with this library, but on the first step we need to add a reference to Mobile Apps SDK for each of our projects. Therefore, I would recommend to select Manage NuGet Packages for the solution rather than for a particular project and find Microsoft.Azure.Mobile.Client library:

The process is not complicated, but there is a surprise: we will not be able to install the latest version of the library exactly to Xamarin.Forms project:

The latest version supports .NET Standard (at least 1.4) and we can easy add it to our Android, iOS or UWP applications (because Xamarin added support for it some time ago), but you cannot add it to your PCL (Xamarin.Forms part). Of course, you can say that I could avoid this problem using Shared Project approach. Yes, I could do it as well as I could create lots of “spaghetti” #if #else code and got a problem with IntelliSense and so on, but I want to stay with “a separate library” approach and there are several ways to fix the problem:

· I will start with some suicidal ways like: you can use REST API directly. It’s really great way to increase your income if anybody pays you for number of code lines, but in my case, it’s not true;

· Some guys like to recommend using the previous version of the library. Talking about “previous” they tell about 3.0.1 that was published couple versions ago. I am not sure what there is changed since 3.0.1, but I believe that they didn’t publish new couple versions “just because”. That’s why I would not recommend using this approach as well;

· Finally, you can migrate your library to .NET Standard;

Let’s stay with the last approach and see, how we can make the migration.

It’s funny, but you could do the migration process just clicking a button. Using Visual Studio 2017 Update 3 I could not find any button. If you visit PCL project settings, you still will be able to select a profile for PCL and you will see a message about .NET Standard benefits, but the button is disappeared. Probably, it was a reason to do that. At the same time, I am sure that yesterday I could see the button in Visual Studio for Mac.

Because we don’t have any button, we need to create one more project based on .NET Standard library template manually:

Opening project settings you can select any .NET Standard version for the project. Just pay attention that Visual Studio 2017 Update 3 will not install .NET Standard 2.0 automatically and you will need to install it manually from Microsoft web site, but Update 4 installer should include 2.0 version.

But don’t be in a hurry because .NET Standard 2.0 is just released, and it will be supported by Xamarin in next major update only. In fact, the problem even is not Xamarin, but in UWP platform that works based .NET Standard 1.4. You can find this information looking at the following page: https://docs.microsoft.com/en-us/dotnet/standard/net-standard

Microsoft is going to support .NET Framework 2.0 for UWP projects since Fall Creators Update, but it doesn’t mean that you will be able to target .NET Standard 2.0 and earlier UWP releases at the same time.

Therefore, if you want to avoid any previews, alfas and insider releases at this time, you need to stay with .NET Standard 1.4, at least for next couple months.

Once the project is created, we can move all our files from Xamarin.Forms PCL project to .NET Standard library project and remove initial Class1.cs. Thanks to Galaxy you can use Drag and Drop to copy all your files and folders. So, if you already have lots of files in the PCL, it should not be challenging.

Now, it’s time to remove your PCL, and you can rename your .NET Standard library using “old” PCL name. It will not help you to avoid problems with dependencies for your iOS, Android and UWP projects, and you should target your .net standard library from each of them.

The last thing that we need to do is adding reference to Microsoft.Azure.Mobile.Client and Xamarin.Forms libraries. The first package will not generate any problems this time, but I cannot say the same about the second one. Xamarin.Forms 2.3.4 still is not .NET Standard library.

Of course, the next release (2.3.5) already supports .NET Standard, but it’s still in preview. At the same time, if you want to test new features, you can check “Include prereleases” box in NuGet package manager and use a prerelease for 2.3.5 or even 2.4.0.

Both releases support .NET Standard, but the second one supports even .NET Standard 2.0. But if you want to use .NET Standard 2.0 for all projects, including UWP, you will need to install Visual Studio 2017 Update 4 Preview, the latest Windows 10 Insider Preview and the latest UWP SDK insider preview. I can use both options on my computer, but today we are working with components that are in production right now. So, we need to think about how to use the current Xamarin.Forms release.

It’s not the biggest problem because .NET Standard supports type forwarding and other stuff to target old libraries and we can use Xamarin.Forms, but we need to make some modifications in our project file: just open your .csproj for editing and add <PackageTargetFallback>portable-net45+win8+wpa81+wp8</PackageTargetFallback> right after TargetFramework:

It will tell the compiler that if it cannot add a reference due to a problem with the target platform, it’s OK and it should be ignored.

That’s all. Now, we can recompile our solution and even start initial projects using real devices or emulators.

Of course, once a new version of Xamarin.Forms is released, we will be able to use it as a .NET Standard library, but the previous approach will work fine for any other PCL assembly. Counting that there are huge number of existing PCLs, it’s important to know how to use them.

One more thing. If you still targeting Silverlight (WP) using Xamarin, you may need to add a special library to enable compatibility for it:

Next time we are going to start using our Mobile Apps and we will discuss lazy loading and 50 records limit.