Xamarin.Forms constructor injection with unity

2018-03-24T00:00:00+01:00

A simple and straight forwared approach to get dependency injection, especially constructor injection, to work in Xamarin.Forms via Unity.

Project setup

Open Visual Studio for Mac and create a Multiplatform -> Blank Forms App. Choose any name check Android and iOS for the target platforms, choose for Shared Code the Portable Class Library and check use XAML for user interface files. Click next for the next step and now lets jump right into it.

Install unity container

One simple and easy way to use constructor injection in Xamarin.Forms is via unity. Do a right click on packages in your solution folder of the shared project and select Add packages and type Unity. NuGet will show you the unity package from author Microsoft.Practices.Unity, choose this one and select Add Package.

Create the service to be injected

You have to create a interface and the corresponding service. Let’s add a simple DataService to demonstrate the injection. First, create the interface IDataService and add the method GetCities() to it.

public interface IDataService
{
    IEnumerable<string> GetCities();
}

Now let us create the DataService class with it’s implemented interface.

public class DataService : IDataService
{
    public IEnumerable<string> GetCities()
    {
        return new List<string>{
          "Karlsruhe",
          "Split",
          "Dubrovnik"
        };
    }
}

Inject the service via construction injection

To consume the data fetched by the dataservice we have to inject the service in a viewmodel to demonstrate it. Create the CityListViewModel with a uninitialized private field of type IDataService. Now inject the service in the constructor. With this implementaion we will get a NullReferenceException because the DataService was not instantiated.

public class CityListViewModel
{
    private readonly IDataService dataservice;

    public CityListViewModel(IDataService dataService)
    {
        this.dataservice = dataService;
    }

    public IEnumerable<string> AllCities => dataservice.GetCities();
}

Setup the unity container

To prevent throwing NullReferenceException from last step, we have to setup a container to be able to use constructor injection in our shared code files. A good place to do this, is right after Xamarin.Forms initializes it’s components. The initialization takes place at App.xaml.cs file. We just have to instatiate a new UnityContainer and register our types, next we have to tell the ServiceLocator to use our newly created container as the current LocatorProvicer. Finally we have to call our method from App.xaml.css constructor. And thats it!

private void RegisterService()
{
    var unityContainer = new UnityContainer();
    unityContainer.RegisterType<IDataService, DataService>();
    // register any service you like with the scheme interface, service
    ServiceLocator.SetLocatorProvider(() => 
        new UnityServiceLocator(unityContainer));
}