ASP.NET Core 2中的依赖注入会引发异常 [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：无法解析服务类型'Communicator.Backend.Startup'上方法'Configure'的参数'dbContext'类型为'Communicator.Backend.Data.CommunicatorContext'。 ---> System.InvalidOperationException：无法从根提供者解析作用域服务'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.

此异常

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

这是我的 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);
}

推荐答案

报价文档

启动中可用的服务

ASP.NET Core依赖项注入在$期间提供应用程序服务b $ b应用程序的启动。您可以通过在$$$$$构造函数或其中的配置之一中将
作为适当的接口作为参数添加到$$$$ Start $ 类中，以请求这些服务。 或 ConfigureServices 方法。

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.

查看 Startup 类，按照它们调用
的顺序，可能需要以下服务作为
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<$ c中$ c>配置方法： 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) {

这将为您提供错误信息。如果需要访问实现，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，然后从服务集合中访问它们。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 方法返回IServiceProvider，配置方法获取 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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