The highlighted line shows how the `IWeatherForecastService` is registered in the test context's `Services` collection, which is just a standard [`IServiceCollection`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.iservicecollection), using the standard .NET Core dependency injection (DI) services method, [`AddSingleton`](https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.dependencyinjection.servicecollectionserviceextensions.addsingleton?view=dotnet-plat-ext-3.1#Microsoft_Extensions_DependencyInjection_ServiceCollectionServiceExtensions_AddSingleton__1_Microsoft_Extensions_DependencyInjection_IServiceCollection___0_).
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