无法从根提供程序.Net核心2解析作用域服务

Question

当我尝试运行我的应用程序时，我会得到错误。

InvalidOperationException: Cannot resolve 'API.Domain.Data.Repositories.IEmailRepository' from root provider because it requires scoped service 'API.Domain.Data.EmailRouterContext'.

奇怪的是，据我所知，这个EmailRepository和接口设置与其他所有存储库完全相同，但没有为它们抛出错误。只有当我尝试使用app.UseEmailingExceptionHandling()；行时，才会发生错误。这是我的一些Startup.cs文件。

public class Startup
{
    public IConfiguration Configuration { get; protected set; }
    private APIEnvironment _environment { get; set; }

    public Startup(IConfiguration configuration, IHostingEnvironment env)
    {
        Configuration = configuration;

        _environment = APIEnvironment.Development;
        if (env.IsProduction()) _environment = APIEnvironment.Production;
        if (env.IsStaging()) _environment = APIEnvironment.Staging;
    }

    public void ConfigureServices(IServiceCollection services)
    {
        var dataConnect = new DataConnect(_environment);

        services.AddDbContext<GeneralInfoContext>(opt => opt.UseSqlServer(dataConnect.GetConnectString(Database.GeneralInfo)));
        services.AddDbContext<EmailRouterContext>(opt => opt.UseSqlServer(dataConnect.GetConnectString(Database.EmailRouter)));

        services.AddWebEncoders();
        services.AddMvc();

        services.AddScoped<IGenInfoNoteRepository, GenInfoNoteRepository>();
        services.AddScoped<IEventLogRepository, EventLogRepository>();
        services.AddScoped<IStateRepository, StateRepository>();
        services.AddScoped<IEmailRepository, EmailRepository>();
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        loggerFactory.AddConsole();

        app.UseAuthentication();

        app.UseStatusCodePages();
        app.UseEmailingExceptionHandling();

        app.UseMvcWithDefaultRoute();
    }
}

这是EmailRepository

public interface IEmailRepository
{
    void SendEmail(Email email);
}

public class EmailRepository : IEmailRepository, IDisposable
{
    private bool disposed;
    private readonly EmailRouterContext edc;

    public EmailRepository(EmailRouterContext emailRouterContext)
    {
        edc = emailRouterContext;
    }

    public void SendEmail(Email email)
    {
        edc.EmailMessages.Add(new EmailMessages
        {
            DateAdded = DateTime.Now,
            FromAddress = email.FromAddress,
            MailFormat = email.Format,
            MessageBody = email.Body,
            SubjectLine = email.Subject,
            ToAddress = email.ToAddress
        });
        edc.SaveChanges();
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool disposing)
    {
        if (!disposed)
        {
            if (disposing)
                edc.Dispose();
            disposed = true;
        }
    }
}

最后是异常处理中间件。

public class ExceptionHandlingMiddleware
{
    private const string ErrorEmailAddress = "errors@ourdomain.com";
    private readonly IEmailRepository _emailRepository;

    private readonly RequestDelegate _next;

    public ExceptionHandlingMiddleware(RequestDelegate next, IEmailRepository emailRepository)
    {
        _next = next;
        _emailRepository = emailRepository;
    }

    public async Task Invoke(HttpContext context)
    {
        try
        {
            await _next.Invoke(context);
        }
        catch (Exception ex)
        {
            await HandleExceptionAsync(context, ex, _emailRepository);
        }
    }

    private static Task HandleExceptionAsync(HttpContext context, Exception exception,
        IEmailRepository emailRepository)
    {
        var code = HttpStatusCode.InternalServerError; // 500 if unexpected

        var email = new Email
        {
            Body = exception.Message,
            FromAddress = ErrorEmailAddress,
            Subject = "API Error",
            ToAddress = ErrorEmailAddress
        };

        emailRepository.SendEmail(email);

        context.Response.ContentType = "application/json";
        context.Response.StatusCode = (int) code;
        return context.Response.WriteAsync("An error occured.");
    }
}

public static class AppErrorHandlingExtensions
{
    public static IApplicationBuilder UseEmailingExceptionHandling(this IApplicationBuilder app)
    {
        if (app == null)
            throw new ArgumentNullException(nameof(app));
        return app.UseMiddleware<ExceptionHandlingMiddleware>();
    }
}

更新:我找到了一个链接https://github.com/aspnet/DependencyInjection/issues/578，它导致我从下面更改了Program.cs文件的BuildWebHost方法

public static IWebHost BuildWebHost(string[] args)
{
    return WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .Build();
}

到这个

public static IWebHost BuildWebHost(string[] args)
{
    return WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .UseDefaultServiceProvider(options =>
            options.ValidateScopes = false)
        .Build();
}

我不知道到底是怎么回事，但现在似乎起作用了。

Answer 1

您将IEmailRepository注册为Startup类中的一个作用域服务。这意味着您不能在Middleware中将其作为构造函数参数注入，因为只有Singleton服务可以通过Middleware中的构造函数来解析。应该将依赖项移动到Invoke方法，如下所示：

public ExceptionHandlingMiddleware(RequestDelegate next)
{
    _next = next;
}

public async Task Invoke(HttpContext context, IEmailRepository emailRepository)
{
    try
    {
        await _next.Invoke(context);
    }
    catch (Exception ex)
    {
        await HandleExceptionAsync(context, ex, emailRepository);
    }
}

Answer 2

获取作用域依赖实例的另一种方法是将服务提供者(IServiceProvider)注入中间件构造函数，在Invoke方法中创建scope，然后从作用域获取所需的服务：

using (var scope = _serviceProvider.CreateScope()) {
    var _emailRepository = scope.ServiceProvider.GetRequiredService<IEmailRepository>();

    //do your stuff....
}

有关更多细节，请查看方法主体中的解析服务( in asp.net核心依赖注入最佳实践技巧 )。

Answer 3

中间件始终是单例的，因此您不能在中间件的构造函数中将作用域依赖项作为构造函数依赖项。

中间件支持在Invoke方法上进行方法注入，因此您只需将IEmailRepository emailRepository作为参数添加到该方法中，它就会被注入到该方法中，并且可以作为作用域。

public async Task Invoke(HttpContext context, IEmailRepository emailRepository)
{

    ....
}