Microsoft.Extensions.DependencyInjection在构造方法注入中，可以注入IServiceProvider接口实例

我们在使用Microsoft.Extensions.DependencyInjection进行构造方法依赖注入时，可以注入IServiceProvider接口的实例，然后可以用这个实例来获取其它依赖注入的类。

下面的示例代码中，我们定义了IPeople接口和实现类People，是一组依赖注入关系。还定义了IPeopleService接口和实现类PeopleService，也是一组依赖注入关系，在PeopleService类的构造方法中首先注入了IPeople接口示例，还注入了IServiceProvider的接口实例，然后在PeopleService类CreatePeople方法中，来使用IServiceProvider接口的实例来获取其它依赖注入的类。

using Microsoft.Extensions.DependencyInjection;namespace Net8DependencyInjection
{interface IPeople{string? Name{get;set;}}class People : IPeople{private string? _name;public string? Name{get{return _name;}set{_name = value;}}}interface IPeopleService{IServiceProvider GetCurrentServiceProvider();IPeople? CreatePeople();}class PeopleService : IPeopleService, IDisposable{protected IServiceProvider _serviceProvider;protected IPeople _people;//因为PeopleService类由依赖注入DI创建，所以依赖注入DI可以给PeopleService类的构造方法注入IServiceProvider接口的实例，可以用它来获取其它依赖注入的类public PeopleService(IPeople people, IServiceProvider serviceProvider){_people = people;_serviceProvider = serviceProvider;}//用PeopleService类的构造方法注入的IServiceProvider接口实例，来获取其它依赖注入的类public IPeople? CreatePeople(){return _serviceProvider.GetService<IPeople>();}public void Dispose(){if (_serviceProvider is IDisposable){Console.WriteLine("Disposing _serviceProvider ...");IDisposable disposable = (IDisposable)_serviceProvider;disposable.Dispose();}}//返回构造方法中依赖注入的IServiceProvider接口实例public IServiceProvider GetCurrentServiceProvider(){return _serviceProvider;}}internal class Program{static void Main(string[] args){IServiceCollection services = new ServiceCollection();services.AddTransient<IPeople, People>(); //注册IPeople接口和People类的依赖注入关系services.AddTransient<IPeopleService, PeopleService>(); //注册IPeopleService接口和PeopleService类的依赖注入关系using (ServiceProvider rootServiceProvider = services.BuildServiceProvider()){IPeopleService? rootPeopleService = rootServiceProvider.GetService<IPeopleService>();IPeople? peopleInRoot = rootPeopleService?.CreatePeople();peopleInRoot!.Name = "Jack";Console.WriteLine("rootPeopleService.GetCurrentServiceProvider is {0}", rootPeopleService?.GetCurrentServiceProvider().GetType().ToString()); //rootPeopleService.GetCurrentServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScopeConsole.WriteLine("rootServiceProvider is {0}", rootServiceProvider.GetType().ToString()); //rootServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceProviderConsole.WriteLine("rootPeopleService.GetCurrentServiceProvider == rootServiceProvider is {0}", rootPeopleService?.GetCurrentServiceProvider() == rootServiceProvider); //rootPeopleService.GetCurrentServiceProvider == rootServiceProvider is False
Console.WriteLine();Console.WriteLine();Console.WriteLine();using (IServiceScope serviceScope = rootServiceProvider.CreateScope()){IPeopleService? scopePeopleService = serviceScope.ServiceProvider.GetService<IPeopleService>();IPeople? peopleInScope = scopePeopleService?.CreatePeople();peopleInScope!.Name = "Tom";Console.WriteLine("peopleInRoot == peopleInScope is {0}", peopleInRoot == peopleInScope); //peopleInRoot == peopleInScope is FalseConsole.WriteLine("scopePeopleService.GetCurrentServiceProvider is {0}", scopePeopleService?.GetCurrentServiceProvider().GetType().ToString()); //scopePeopleService.GetCurrentServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScopeConsole.WriteLine("serviceScope.ServiceProvider is {0}", serviceScope.ServiceProvider.GetType().ToString()); //serviceScope.ServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScopeConsole.WriteLine("scopePeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is {0}", scopePeopleService?.GetCurrentServiceProvider() == serviceScope.ServiceProvider); //scopePeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is TrueConsole.WriteLine("rootPeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is {0}", rootPeopleService?.GetCurrentServiceProvider() == serviceScope.ServiceProvider); //rootPeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is False
                }}Console.WriteLine("Press any key to end...");Console.ReadLine();}}
}

上面的代码运行后，输出的结果如下所示：

rootPeopleService.GetCurrentServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope
rootServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceProvider
rootPeopleService.GetCurrentServiceProvider == rootServiceProvider is FalsepeopleInRoot == peopleInScope is False
scopePeopleService.GetCurrentServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope
serviceScope.ServiceProvider is Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope
scopePeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is True
rootPeopleService.GetCurrentServiceProvider == serviceScope.ServiceProvider is False
Disposing _serviceProvider ...
Disposing _serviceProvider ...
Press any key to end...

下面的这篇微软官方文档，还提到Microsoft.Extensions.DependencyInjection在使用依赖注入调用类的构造方法创建实例时，构造方法中可以包含非依赖注入关系的参数，但是这种参数必须要有默认值，否则会报错：

Constructor injection behavior

Constructors can accept arguments that aren't provided by dependency injection, but the arguments must assign default values.