ASP.NET Core 2中的依赖注入会引发异常

Question

当我尝试Configure在Startup.cs文件中的方法中使用自定义DbContext时,我收到以下异常.我在版本2.0.0-preview1-005977中使用ASP.NET Core

未处理的异常:System.Exception:无法为类型为"Communicator.Backend.Startup"的方法"Configure"的参数"dbContext"解析类型为"Communicator.Backend.Data.CommunicatorContext"的服务.---> System.InvalidOperationException:无法从根提供程序解析作用域服务"Communicator.Backend.Data.CommunicatorContext".

当我尝试接收其他实例时,也会抛出此异常.

未处理的异常:System.Exception:无法解析类型为"Communicator.Backend.Services.ILdapService"的服务 ...

这是我ConfigureServices和Configure方法.

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<CommunicatorContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
    services.AddCookieAuthentication();
    services.Configure<LdapConfig>(Configuration.GetSection("Ldap"));
    services.AddScoped<ILdapService, LdapService>();
    services.AddMvc();
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, CommunicatorContext dbContext, ILdapService ldapService)
{
    app.UseAuthentication();
    app.UseWebSockets();
    app.Use(async (context, next) =>
    {
        if (context.Request.Path == "/ws")
        {
            if (context.WebSockets.IsWebSocketRequest)
            {
                WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
                await Echo(context, webSocket);
            }
            else
            {
                context.Response.StatusCode = 400;
            }
        }
        else
        {
            await next();
        }
    });
    app.UseMvc();
    DbInitializer.Initialize(dbContext, ldapService);
}

Answer 1

引用文档

启动中可用的服务

ASP.NET Core依赖注入在应用程序启动期间提供应用程序服务.您可以通过在Startup类的构造函数或其中一个Configure或多个ConfigureServices方法中包含适当的接口作为参数来请求这些服务.

Startup按照调用它们的顺序查看类中的每个方法,可以请求以下服务作为参数:

• 在构造函数中:IHostingEnvironment,ILoggerFactory
• 在ConfigureServices方法中:IServiceCollection
• 在该Configure方法中:IApplicationBuilder,IHostingEnvironment, ILoggerFactory,IApplicationLifetime

您正在尝试解决启动期间不可用的服务,

...CommunicatorContext dbContext, ILdapService ldapService) {

这会给你带来的错误.如果您需要访问实现,则需要执行以下操作之一:

1. 修改ConfigureServices方法并从服务集合中访问它们.即

   public IServiceProvider ConfigureServices(IServiceCollection services) {
       services.AddDbContext<CommunicatorContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
       services.AddCookieAuthentication();
       services.Configure<LdapConfig>(Configuration.GetSection("Ldap"));
       services.AddScoped<ILdapService, LdapService>();
       services.AddMvc();
   
       // Build the intermediate service provider
       var serviceProvider = services.BuildServiceProvider();
   
       //resolve implementations
       var dbContext = serviceProvider.GetService<CommunicatorContext>();
       var ldapService = serviceProvider.GetService<ILdapService>();
       DbInitializer.Initialize(dbContext, ldapService);
   
       //return the provider
       return serviceProvider();
   }
   

2. 修改ConfigureServices方法以返回IServiceProvider,Configure获取a IServiceProvider然后在那里解析依赖关系的方法.即

   public IServiceProvider ConfigureServices(IServiceCollection services) {
       services.AddDbContext<CommunicatorContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
       services.AddCookieAuthentication();
       services.Configure<LdapConfig>(Configuration.GetSection("Ldap"));
       services.AddScoped<ILdapService, LdapService>();
       services.AddMvc();
   
       // Build the intermediate service provider then return it
       return services.BuildServiceProvider();
   }
   
   public void Configure(IApplicationBuilder app, IHostingEnvironment env, 
                         ILoggerFactory loggerFactory, IServiceProvider serviceProvider) {
   
       //...Other code removed for brevity
   
       app.UseMvc();
   
       //resolve dependencies
       var dbContext = serviceProvider.GetService<CommunicatorContext>();
       var ldapService = serviceProvider.GetService<ILdapService>();
       DbInitializer.Initialize(dbContext, ldapService);
   }
   

Answer 2

来自NKosi的解决方案是有效的,因为通过调用services.BuildServiceProvider()自己没有参数,你没有通过validateScopes.因为禁用此验证,所以不会抛出异常.然而,这并不意味着问题不存在.

EF Core DbContext注册了scoped生活方式.在ASP本机DI容器范围连接到实例IServiceProvider.通常,当您使用DbContextfrom Controller时没有问题,因为ASP IServiceProvider为每个请求创建新范围(新实例),然后使用它来解析此请求中的所有内容.但是,在应用程序启动期间,您没有请求范围.您的实例IServiceProvider不是作用域的(换句话说,在根作用域中).这意味着您应该自己创建范围.你可以这样做:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    var scopeFactory = app.ApplicationServices.GetRequiredService<IServiceScopeFactory>();
    using (var scope = scopeFactory.CreateScope())
    {
        var db = scope.ServiceProvider.GetRequiredService<CommunicatorContext>();
        var ldapService = scope.ServiceProvider.GetRequiredService<ILdapService>();
        // rest of your code
    }
    // rest of Configure setup
}

该ConfigureServices方法可以保持不变.

编辑

您的解决方案将在2.0.0 RTM中运行,无需任何更改,因为将在RTM作用域服务提供程序中为Configure方法https://github.com/aspnet/Hosting/pull/1106创建.

Answer 3

在ASP.NET Core 2.0及更新版本中,您可以简单地将所需的作用域服务注入Configure构造函数中,就像您最初尝试的那样:

public void Configure(
    IApplicationBuilder app,
    IHostingEnvironment env,
    ILoggerFactory loggerFactory,
    CommunicatorContext dbContext,
    ILdapService ldapService)
{
  // ...
}

由于#1106的改进,这要容易得多.

Answer 4

.UseDefaultServiceProvider(options => 
            options.ValidateScopes = false)

之后在Program.cs中添加它 .UseStartup<Startup>()

适合我

文档在这里

Answer 5

另外，您可以在您的Configure方法中创建服务范围：

var scopeFactory = ApplicationServices.GetService<IServiceScopeFactory>();
using (var scope = scopeFactory.CreateScope())
{
    var dbContext = scope.ServiceProvider.GetService<CommunicatorDbContext>();
    DbInitializer.Initializer(dbContext, ldapService);
}

尽管如Slack所述，但不要这样做；-)