为什么我的作用域依赖项在单个作用域中实例化了两次？

Question

假设我有一个 Singleton，它具有一个 Scoped 依赖项（并且永远由 Singleton 持有）。

然后我创建一个作用域并从容器中获取 Singleton 和 Scoped 实例。我希望 Scoped 在单个范围内只实例化一次。

请查看简化的代码示例：

namespace TrivialConsole
{
    using Microsoft.Extensions.DependencyInjection;
    using System;

    class Program
    {
        static void Main(string[] args)
        {
            var services = new ServiceCollection();
            services.AddScoped<Scoped>();
            services.AddSingleton<Singleton>();

            var provider = services.BuildServiceProvider();
            using (var rootScope = provider.CreateScope())
            {
                var scopeProvider = rootScope.ServiceProvider;
                var singleton = scopeProvider.GetRequiredService<Singleton>();
                var scoped = scopeProvider.GetRequiredService<Scoped>();
            }
        }
    }

    public class Singleton
    {
        private static int instances = 0;

        private readonly Scoped scoped;

        public Singleton(Scoped scoped)
        {
            instances += 1;
            this.scoped = scoped;
            Console.WriteLine($"singleton {instances}");
        }
    }

    public class Scoped
    {
        private static int instances = 0;

        public Scoped()
        {
            instances += 1;
            Console.WriteLine($"scoped {instances}");
        }
    }
}

结果：

scoped 1
singleton 1
scoped 2

预期结果 ->

scoped 1
singleton 1

Answer 1

根据文档，在此框架中不应将作用域类型注入到单例中。

Do not resolve a scoped service from a singleton and be careful not to do so indirectly, for example, through a transient service. It may cause the service to have incorrect state when processing subsequent requests. It's fine to:

Resolve a singleton service from a scoped or transient service.

Resolve a scoped service from another scoped or transient service.

Reference

为什么我的作用域依赖项在单个作用域中实例化了两次？

Why is my scoped dependency instantiated twice in single scope?

dependency-injection

.net-5