ASP.NET Core 2 中的依赖注入引发异常 [英] Dependency injection in ASP.NET Core 2 throws exception

问题描述

当我尝试在 Startup.cs 文件的 Configure 方法中使用自定义 DbContext 时收到以下异常.我在 2.0.0-preview1-005977 版本中使用 ASP.NET Core

I receive following exception when I try to use custom DbContext in Configure method in Startup.cs file. I use ASP.NET Core in version 2.0.0-preview1-005977

未处理的异常:System.Exception:无法为类型Communicator.Backend.Startup"上的方法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范围服务Communicator.Backend.Data.CommunicatorContext".

Unhandled Exception: System.Exception: Could not resolve a service of type 'Communicator.Backend.Data.CommunicatorContext' for the parameter 'dbContext' of method 'Configure' on type 'Communicator.Backend.Startup'. ---> System.InvalidOperationException: Cannot resolve scoped service 'Communicator.Backend.Data.CommunicatorContext' from root provider.

当我尝试接收其他实例时也会抛出此异常.

This exception is also thrown in case when I try to receive other instance.

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

Unhandled Exception: System.Exception: Could not resolve a service of type 'Communicator.Backend.Services.ILdapService' ...

这是我的 ConfigureServices 和 Configure 方法.

Here are my ConfigureServices and Configure methods.

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);
}

推荐答案

引用文档

服务可在启动

ASP.NET Core 依赖注入提供应用程序服务应用程序的启动.您可以通过包括以下内容来请求这些服务适当的接口作为 Startup 类的参数构造函数或其 Configure 或 ConfigureServices 方法之一.

ASP.NET Core dependency injection provides application services during an application's startup. You can request these services by including the appropriate interface as a parameter on your Startup class's constructor or one of its Configure or ConfigureServices methods.

按顺序查看Startup类中的每个方法他们被称为，可能会要求以下服务参数:

Looking at each method in the Startup class in the order in which they are called, the following services may be requested as parameters:

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

• In the constructor: IHostingEnvironment, ILoggerFactory
• In the ConfigureServices method: IServiceCollection
• In the Configure method: IApplicationBuilder, IHostingEnvironment, ILoggerFactory, IApplicationLifetime

您正在尝试解决启动期间不可用的服务，

You are trying to resolve services that are not available during startup,

...CommunicatorContext dbContext, ILdapService ldapService) {

这会给你你得到的错误.如果您需要访问实现，则需要执行以下操作之一:

which will give you the errors you are getting. If you need access to the implementations then you need to do one of the following:

1. 修改 ConfigureServices 方法并从服务集合访问它们.即

1. Modify the ConfigureServices method and access them there from the service collection. i.e.

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();
}

修改 ConfigureServices 方法以返回 IServiceProvider，Configure 方法获取一个 IServiceProvider，然后在那里解析您的依赖项.即

Modify the ConfigureServices method to return IServiceProvider, Configure method to take a IServiceProvider and then resolve your dependencies there. i.e.

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);
}

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