Switching Your Xamarin Project to Standard Class Projects
There are lots of differences between using a Standard Class/Portable Class Library (PCL) and Shared projects in a Xamarin solution. However, the most obvious one appears when you open any XAML file in a Shared project: In a Standard Class library you'll get IntelliSense support; in a Shared Project you won't get any IntelliSense support and virtually every element in your XAML file will be flagged as an error (though, fortunately, your solution will still compile).
Unless you have a compelling reason to go with a Shared project (for example: your version of Visual Studio doesn't support Standard Class Library projects), you'll want to use PCL or a Standard Class Library project ... and a Standard Class Library project is your best choice going forward. In fact, if your version of Visual Studio doesn't support Standard Class Library Projects and you want to work with Xamarin, it might be time to upgrade to a newer version of Visual Studio (remembering that, for example, Visual Studio 17 Community Edition is free).
If you're not sure which kind of project your Solution is currently using, first look at the icon beside your common project in Solution Explorer: If it's two overlapping diamonds, you have a Shared project (bad); if you have a simple box with "C#" inside of it then you have a Standard Class Library (good) or a PCL (not so good) project. To distinguish between a PCL and a Standard Class Library, open the project's properties and see if you have a Library tab on the left. If you do, you have a PCL project (as I said: not so good).
To convert your solution to using a Standard Class Library project, first right-click on your Solution node in Solution Explorer and use Add | New Project | .NET Standard | Class Library (.NET Standard) project to add a Standard Class library project your solution. Once the project is added, delete any default resources in the project (you won't need the Class1.cs file, for example). Then drag and drop any resources from your existing Shared/PCL project to your new Standard Class Library project. If you have an App.xaml file that marks the start point of your application in your old project make sure that you drag it to your new project.
Next, right-click on your new project and use Manage NuGet Packages to add the Xamarin.Forms package to your project. You'll need to add any other references or NuGet packages your original project was using.
Now do a rebuild on your new project. If you get some compile-time errors you haven't seen before, open your project's Properties and, on the Application tab, check that the Target framework dropdown list is set to the highest level (as I write this, that's .NET Standard 2.0). If it isn't, set the dropdown to the highest level and try another build. If you still have compile-time problems, then it's too early to move to a .NET Standard Class Library project and you'll have to live with your Shared or PCL project.
Now, the scary part: right click on your Shared or PCL project and pick Remove. Remind yourself that the project isn't gone, it's just not part of the solution. If it turns out you need something from it, you can use Add | Existing Item to pick up anything you've forgotten (you can also open the old project in Visual Studio to check any settings you might have missed).
If you don't yet have a XAML file (other than App.xaml) in your new project, right-click on your new project in Solution Explorer and select Add | New Item | Xamarin Forms | Content Page to add one. If you want this to be your start page, make sure this new Page's name matches the name in the App class's constructor in the App.xaml.cs file (you can either give your new XAML file a matching name or change the name in App.xaml.cs).
Finally, in the other projects in your solution, use Add | New Reference to add a project reference to your new Standard Class Library project and do a rebuild of your solution to flag any namespace issues that you have to clean up.
Posted by Peter Vogel on 08/06/2018 at 10:42 AM