为什么.NET Core DI容器不注入ILogger？

我正在尝试在基于 .NET Core 2.1 的 C# 控制台应用程序中登录并运行。

我将以下代码添加到我的 DI 声明中：

var sc = new ServiceCollection();

sc.AddLogging(builder =>
{
    builder.AddFilter("Microsoft", LogLevel.Warning);
    builder.AddFilter("System", LogLevel.Warning);
    builder.AddFilter("Program", LogLevel.Warning);
    builder.AddConsole();
    builder.AddEventLog();
});

我正在尝试使用接口注入服务Microsoft.Extensions.Logging.ILogger在服务的构造函数中，但我收到以下错误：

未处理的异常：System.InvalidOperationException：尝试激活“MyService”时无法解析类型“Microsoft.Extensions.Logging.ILogger”的服务。

    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateArgumentCallSites(Type serviceType, Type implementationType, CallSiteChain callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateConstructorCallSite(ResultCache lifetime, Type serviceType, Type implementationType, CallSiteChain callSiteChain)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateExact(ServiceDescriptor descriptor, Type serviceType, CallSiteChain callSiteChain, Int32 slot)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateExact(Type serviceType, CallSiteChain callSiteChain)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateCallSite(Type serviceType, CallSiteChain callSiteChain)
    at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateArgumentCallSites(Type serviceType, Type implementationType, CallSiteChain callSiteChain, ParameterInfo[] parameters, Boolean throwIfCallSiteNotFound)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateConstructorCallSite(ResultCache lifetime, Type serviceType, Type implementationType, CallSiteChain callSiteChain)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateExact(ServiceDescriptor descriptor, Type serviceType, CallSiteChain callSiteChain, Int32 slot)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.TryCreateEnumerable(Type serviceType, CallSiteChain callSiteChain)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteFactory.CreateCallSite(Type serviceType, CallSiteChain callSiteChain)
    at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.CreateServiceAccessor(Type serviceType)
    at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
    at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
    at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
    at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider)
    at Program.Main(String[] args) in Program.cs:line 27

我在这里误解了什么吗？不应该是AddLogging方法足以公开 DI 容器中的服务吗？

依赖注入系统未注册ILogger。相反，它注册ILogger<T>。如果您需要记录器的实例，则需要接受ILogger<MyService>.

这背后的原因是通用参数用于构建记录器的类别名称——所有记录器都需要的东西。对于非通用记录器，不一定有一个好的默认名称。如果你really想要一个非通用的ILogger，您可以像这样注册一个（随意更改名称）：

services.AddSingleton(sp => sp.GetRequiredService<ILoggerFactory>().CreateLogger("DefaultLogger"));

或者，您可以接受ILoggerFactory在你的构造函数中，然后动态创建你自己的记录器。