How-To: Manually Resolving Dependencies with CommunityToolkit.Mvvm

While making gradual changes to an existing app's codebase, you may find it necessary to access the DI container to manually resolve dependencies. For instance, if you are overhauling a view model to separate its logic into services, you may need to resolve the service without using constructor injection. The CommunityToolkit.Mvvm package provides a static Ioc.Default property that exposes the DI container used by the application.

This tutorial will walk you through how to set up this feature and use it to manually resolve dependencies.

Warning

This approach to resolving dependencies is not recommended, and should serve primarily as a stopgap measure while you refactor your codebase to use constructor injection.

Step-by-step

Important

This guide assumes you used the template wizard or dotnet new unoapp to create your solution. If not, it is recommended that you follow the Creating an application with Uno.Extensions documentation to create an application from the template.

1. Add CommunityToolkit.Mvvm to your project

Add Mvvm to the <UnoFeatures> property in the Class Library (.csproj) file. This will add the CommunityToolkit.Mvvm package to your project.
```
<UnoFeatures>
    Material;
    Extensions;
+   Mvvm;
    Toolkit;
</UnoFeatures>
```

2. Register services with the DI container

Register services with the DI container as you normally would. For instance, you can use the Configure method on IHostBuilder to register services with the DI container:

private IHost Host { get; set; }

protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    var appBuilder = this.CreateBuilder(args)
        .Configure(hostBuilder => {
            hostBuilder.ConfigureServices(services =>
                services
                    .AddSingleton<IPrimaryService, PrimaryService>()
                    .AddSingleton<ISecondaryService, SecondaryService>()
            );
        });

    Host = appBuilder.Build();
}

3. Configure the DI container to use the CommunityToolkit.Mvvm service provider

Observe that the built IHost instance is available to the App.cs file. It is stored in a property on the App class:
```
private IHost Host { get; set; }
```
Because the IHost instance is available to the App.cs file, you can get the IHost.Services collection and configure the service provider to use it.

To do so, add the following line to the OnLaunched method:

protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    ...
    Ioc.Default.ConfigureServices(Host.Services);
}

4. Resolve services from the DI container

You can now resolve services from the DI container using the Ioc.Default property. For instance, you can resolve the IPrimaryService service in a view model that is not registered with the DI container.

Do this by calling GetService or GetRequiredService method on the singleton provider instance:

public class MainViewModel : ObservableRecipient
{
    private readonly IPrimaryService? primaryService;

    private readonly ISecondaryService secondaryService;

    public MainViewModel()
    {
        // Get the IPrimaryService instance if available; otherwise returns null.
        primaryService = Ioc.Default.GetService<IPrimaryService>();

        // Get the ISecondaryService instance if available; otherwise throws an exception.
        secondaryService = Ioc.Default.GetRequiredService<ISecondaryService>();
    }
}