Basics #

NuGet Packages: Microsoft.Extensions.DependencyInjection.Abstractions or Microsoft.Extensions.DependencyInjection

“Manual” approach (i.e. without IHostBuilder):

// Create service collection for registering all services
IServiceCollection serviceCollection = new ServiceCollection();

serviceCollection.AddSingleton<ITest, Test1>();

// Create service provider to resolve/build objects.
IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();

var testService = serviceProvider.GetRequiredService<ITest>();
testService.DoSomething();

Overwriting (Multiple Singletons for same Interface) #

The last wins:

IServiceCollection serviceCollection = ...

serviceCollection.AddSingleton<ITest, Test1>();
serviceCollection.AddSingleton<ITest, Test2>();

IServiceProvider serviceProvider = ...

var testService = serviceProvider.GetRequiredService<ITest>(); // --> Test2