有人可以向我解释为什么会出现此错误

Question

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    public abstract class Provider
    {
        public abstract string Name { get; }
    }

    public abstract class Provider<T> : Provider
        where T : BaseService
    {
        public T Service { get; set; }
    }

    public class FooProvider : Provider<FooService>
    {
        public override string Name => "FooProvider";
    }

    public class BarProvider : Provider<BarService>
    {
        public override string Name => "BarProvider";
    }

    public abstract class BaseService
    {
        public abstract string Name { get; }
    }

    public class FooService : BaseService
    {
        public override string Name => "FooService";
    }

    public class BarService : BaseService
    {
        public override string Name => "BarService";
    }


    class Program
    {
        public static List<BaseService> Services = new List<BaseService> { };
        public static List<Provider<BaseService>> Providers = new List<Provider<BaseService>> { };

        static void Main()
        {
            Console.WriteLine("Services");
            Services.Add(new FooService());
            Services.Add(new BarService());
            Services.ForEach(service => Console.WriteLine(service.Name));

            Console.WriteLine("Provider");
            // Error CS1503  Argument 1: cannot convert from 'ConsoleApp1.FooProvider' to 'ConsoleApp1.Provider<ConsoleApp1.BaseService>'    ConsoleApp1 C:\Projetos\Git\sbh\src\ms\sbh - src - ms - migracao\ConsoleApp1\Program.cs   56  Active
            Providers.Add(new FooProvider());

            //Error CS1503  Argument 1: cannot convert from 'ConsoleApp1.BarProvider' to 'ConsoleApp1.Provider<ConsoleApp1.BaseService>'    ConsoleApp1 C:\Projetos\Git\sbh\src\ms\sbh - src - ms - migracao\ConsoleApp1\Program.cs   61  Active
            Providers.Add(new BarProvider());

            Providers.ForEach(provider => Console.WriteLine($"provider {provider.Name} - service {provider.Service.Name}"));
        }
    }
}

如果您评论添加提供商的命令，您会发现错误不会发生。我不知道为什么会发生错误，因为我知道简单的服务列表和供应商列表不工作。

Answer 1

这是因为泛型的协变性。您的示例中的服务之所以有效，是因为 List<T>（或更准确地说 IEnumerable<T>）被标记为具有协变参数类型。

https://docs.microsoft.com/en-us/dotnet/standard/generics/covariance-and-contravariance#list-of-variant-generic-interface-and-delegate-types

这允许将 FooService 用作 BaseService。您的 Provider class 中的泛型未标有差异，因此不能以相同的方式使用。如果你想要这个功能，你需要创建一个变体通用接口。

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/covariance-contravariance/creating-variant-generic-interfaces

有人可以向我解释为什么会出现此错误

Someone can explain me why this error occurs I tried to create a provider to rosolver a list of service to make a connection from obj1 with service1

c#

generic-type-argument

.net-core