.NET Core 2.0 中间件构造函数依赖注入(Middleware inject)报错问题解决方法

出现错误异常的示例代码

1) Startup.cs代码

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();
    }
}

2) EmailRepository的代码

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;
        }
    }
}

3) 处理异常中间件(Middleware)代码

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>();
    }
}

解决错误异常的3种方法

1)修改中间件代码解决

IEmailRepository在Startup类别中将其注册为范围服务。这意味着您不能将其作为构造函数参数注入，因为中的构造函数注入Middleware只能Singleton解析服务Middleware。应该将依赖项移动到这样的Invoke方法：

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);
    }
}

2) 通过IServiceProvider获取依赖实例

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

3) 关闭作用域嵌套验证

.NET Core 1.1中默认为关闭,.NET Core 2.0中默认打开的，关闭后就不会在运行时检查范围级别是否嵌套不正确。修改Program.cs文件的BuildWebHost方法，如下：

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

参考文档：https://github.com/aspnet/DependencyInjection/issues/578