Microsoft.Extensions.DependencyInjection的Singleton依赖注入关系，只是对于同一个ServiceProvider是单例的

在Microsoft.Extensions.DependencyInjection中，Singleton代表单例依赖注入，但是这个单例也只是对于相同的ServiceProvider对象而言。

假设我们的项目中有接口IPeople和实现类People，它们之间的依赖注入关系是Singleton的。

新建一个.NET Core控制台项目，代码如下所示：

using Microsoft.Extensions.DependencyInjection;
using System;

namespace NetCoreDISingletonDemos
{
    interface IPeople
    {

    }

    class People : IPeople
    {

    }

    class Program
    {
        static void Main(string[] args)
        {
            IServiceCollection services = new ServiceCollection();
            services.AddSingleton<IPeople, People>();//注册接口IPeople和类People的关系为Singleton

            IPeople peopleA = null;
            IPeople peopleB = null;

            ServiceProvider serviceProvider = null;

            //下面测试在同一个ServiceProvider中获取IPeople对象两次，赋值给peopleA和peopleB变量
            using (serviceProvider = services.BuildServiceProvider())
            {
                peopleA = serviceProvider.GetService<IPeople>();
                peopleB = serviceProvider.GetService<IPeople>();

                Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//True
            }

            peopleA = null;
            peopleB = null;

            //下面测试在不同的ServiceProvider中获取IPeople对象两次，赋值给peopleA和peopleB变量
            ServiceProvider serviceProviderA = null;
            ServiceProvider serviceProviderB = null;

            using (serviceProviderA = services.BuildServiceProvider())
            {
                peopleA = serviceProviderA.GetService<IPeople>();
            }

            using (serviceProviderB = services.BuildServiceProvider())
            {
                peopleB = serviceProviderB.GetService<IPeople>();
            }

            Console.WriteLine("serviceProviderA==serviceProviderB?{0}", (serviceProviderA == serviceProviderB).ToString());//False
            Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//False

            peopleA = null;
            peopleB = null;

            //下面测试用同一个ServiceProvider创建的不同IServiceScope，来获取IPeople对象两次，赋值给peopleA和peopleB变量
            using (serviceProvider = services.BuildServiceProvider())
            {
                IServiceScope serviceScopeA = null;
                IServiceScope serviceScopeB = null;

                using (serviceScopeA = serviceProvider.CreateScope())
                {
                    peopleA = serviceScopeA.ServiceProvider.GetService<IPeople>();
                }

                using (serviceScopeB = serviceProvider.CreateScope())
                {
                    peopleB = serviceScopeB.ServiceProvider.GetService<IPeople>();
                }

                Console.WriteLine("serviceScopeA.ServiceProvider==serviceScopeB.ServiceProvider?{0}", (serviceScopeA.ServiceProvider == serviceScopeB.ServiceProvider).ToString());//False
                Console.WriteLine("peopleA==peopleB?{0}", (peopleA == peopleB).ToString());//True
            }

            Console.WriteLine("Press any key to end...");
            Console.ReadKey();
        }
    }
}

执行结果如下所示：

可以看到：

• 我们使用同一个ServiceProvider获取到的两个IPeople对象是相同的。
• 我们使用不同的ServiceProvider获取到的两个IPeople对象是不同的。
• 我们使用同一个ServiceProvider创建的不同IServiceScope，获取到的两个IPeople对象是相同的。

关于Microsoft.Extensions.DependencyInjection的Singleton依赖注入关系，可以参考这里。