Dependency injection 依赖项注入容器未向构造函数提供contextAccessor参数 初始问题

我正在调用此

serviceProvider.GetRequiredService（）

，并收到以下错误。为什么依赖注入不负责提供非空的

上下文访问器

System.ArgumentNullException was unhandled by user code
  HResult=-2147467261
  Message=Value cannot be null.
Parameter name: contextAccessor
  ParamName=contextAccessor
  Source=Microsoft.AspNet.Identity
  StackTrace:
       at Microsoft.AspNet.Identity.SignInManager`1..ctor(UserManager`1 userManager, IHttpContextAccessor contextAccessor, IUserClaimsPrincipalFactory`1 claimsFactory, IOptions`1 optionsAccessor, ILogger`1 logger)
    --- End of stack trace from previous location where exception was thrown ---
       at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
       at Microsoft.Framework.DependencyInjection.ServiceLookup.Service.ConstructorCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Framework.DependencyInjection.ServiceProvider.ScopedCallSite.Invoke(ServiceProvider provider)
       at Microsoft.Framework.DependencyInjection.ServiceProvider.<>c__DisplayClass8_0.<RealizeService>b__0(ServiceProvider provider)
       at Microsoft.Framework.DependencyInjection.ServiceProvider.GetService(Type serviceType)
       at Microsoft.Framework.DependencyInjection.ServiceProviderExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
       at Microsoft.Framework.DependencyInjection.ServiceProviderExtensions.GetRequiredService[T](IServiceProvider provider)
       at ResourceOwnerPasswordFlow.Startup.<>c__DisplayClass6_0.<Configure>b__1(OpenIdConnectServerOptions options) in C:\Users\BigFont\Documents\GitHub\AspNet.Security.OpenIdConnect.Server\samples\ResourceOwnerPasswordFlow\Startup.cs:line 109
       at Microsoft.Framework.OptionsModel.ConfigureOptions`1.Configure(TOptions options, String name)
       at Microsoft.AspNet.Authentication.AuthenticationMiddleware`1..ctor(RequestDelegate next, IOptions`1 options, ILoggerFactory loggerFactory, IUrlEncoder encoder, ConfigureOptions`1 configureOptions)
       at AspNet.Security.OpenIdConnect.Server.OpenIdConnectServerMiddleware..ctor(RequestDelegate next, ILoggerFactory loggerFactory, IDistributedCache cache, IHtmlEncoder htmlEncoder, IUrlEncoder urlEncoder, IDataProtectionProvider dataProtectionProvider, IOptions`1 options, ConfigureOptions`1 configuration) in C:\Users\BigFont\Documents\GitHub\AspNet.Security.OpenIdConnect.Server\src\AspNet.Security.OpenIdConnect.Server\OpenIdConnectServerMiddleware.cs:line 38
  InnerException: 

serviceProvider.GetRequiredService>（）

因此，看起来您使用的是服务定位器反模式，如果必须这样做，这是可以的，但看起来您请求的类型是错误的 你不应该使用：

serviceProvider.GetRequiredService< SignInManager< ApplicationUser>>()

而且，

IHttpContextAccessor

确实被注入，而我不必显式地将其添加到服务容器中。然后我可以稍后使用它访问当前的

httpContext

，如下所示：

string host = contextAccessor.HttpContext.Request.Host.Value;

---

在我自己的代码中，我使用的是

IdentityUser

，而不是像

ApplicationUser

这样的扩展；相反，它来自文档中的一个代码示例。我不确定服务定位器是否是反模式的。这似乎是我们应该深思熟虑地使用的东西。

serviceProvider.GetRequiredService< SignInManager< ApplicationUser>>()

public RequestSiteResolver(
        ISiteRepository siteRepository,
        IConfiguration configuration,
        IHttpContextAccessor httpContextAccessor)
    {
        contextAccessor = httpContextAccessor;
        siteRepo = siteRepository;
        config = configuration;

    }

string host = contextAccessor.HttpContext.Request.Host.Value;