MediatR 和 SimpleInjector 的依赖范围问题

Question

我一直在使用 Entity Framework 进行数据访问的 WinForms 应用程序中使用 MediatR 库来试验中介模式和 CQRS。该应用程序用于批次制造工厂，允许用户查看活动批次和已完成批次的列表，并在必要时更新批次信息。每个批次都有大量与之相关的信息，例如质量和过程测量。根据这些文章，读写数据被组织成查询和命令：

Meanwhile... on the query side of my architecture

CQRS with MediatR and AutoMapper

这是查询和查询处理程序的一个简单示例。 DataContext 使用 SimpleInjector 注入查询处理程序。

public class GetAllBatchesQuery: IRequest<IEnumerable<Batch>> { }

public class GetAllBatchesQueryHandler :
    IRequestHandler<GetAllBatchesQuery, IEnumerable<Batch>>
{
    private readonly DataContext _context;

    public GetAllBatchesQueryHandler(DataContext context)
    {
        _context= context;
    }

    public IEnumerable<Batch> Handle(GetAllBatchesQueryrequest)
    {
        return _db.Batches.ToList();
    }
}

演示者将按如下方式调用它：

var batches = mediator.Send(new GetAllBatchesQuery());

我运行遇到的问题是 DbContext 的生命周期。理想情况下，我想为每个隔离事务使用一个实例，在这种情况下，它包括以下内容：

正在从数据库中检索批次列表
检索批次的质量指标列表（这些存储在不同的数据库中并通过存储过程访问）
批量更新，可能包括更新数据库中的多个实体

这将引导我走向 DbContext 的范围内或短暂的生活方式。但是，当使用 transient 生活方式时，SimpleInjector 会引发以下错误，该错误是在注册类型时抛出的，如下所示：

container.Register<DataContext>();

An unhandled exception of type 'SimpleInjector.DiagnosticVerificationException' occurred in SimpleInjector.dll

Additional information: The configuration is invalid. The following diagnostic warnings were reported:

-[Disposable Transient Component] DataContext is registered as transient, but implements IDisposable.

在 SimpleInjector 网站上研究这个问题揭示了以下内容 note：

Warning: Transient instances are not tracked by the container. This means that Simple Injector will not dispose transient instances.

这让我走上了为 DataContext 使用 Lifetime Scope 生活方式的道路。为此，我为我的查询创建了一个新的装饰器 class 并将其注册如下：

public class LifetimeScopeDecorator<TRequest, TResponse> :
    IRequestHandler<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly IRequestHandler<TRequest, TResponse> _decorated;
    private readonly Container _container;

    public LifetimeScopeDecorator(
        IRequestHandler<TRequest, TResponse> decorated,
        Container container)
    {
        _decorated = decorated;
        _container = container;
    }

    public TResponse Handle(TRequest message)
    {
        using (_container.BeginLifetimeScope())
        {
            var result = _decorated.Handle(message);
            return result;
        }
    }
}

...

container.RegisterDecorator(
    typeof(IRequestHandler<,>),
    typeof(ExecutionContextScopeDecorator<,>));

但是，进行该更改会导致不同的异常，这次在以下行抛出：

var batches = mediator.Send(new GetAllBatchesQuery());

An unhandled exception of type 'System.InvalidOperationException' occurred in MediatR.dll

Additional information: Handler was not found for request of type MediatorTest.GetAllBatchesQuery.

Container or service locator not configured properly or handlers not registered with your container.

调试并查看 MediatR 代码后，似乎在调用 mediator.Send(...) 方法时，通过调用 [=19] 创建了 GetAllBatchesQueryHandler class 的新实例=].但是，由于此时DataContext不在执行范围内，可能没有正确初始化，导致异常。

我相信我了解问题的根本原因，但不知道如何有效地解决它。为了帮助更好地说明这个问题，我开发了以下最小示例。任何实现 IDisposable 的 class 都会导致与 DataContext.

相同的问题

using System;
using System.Collections.Generic;
using System.Reflection;
using MediatR;
using SimpleInjector;
using SimpleInjector.Extensions.LifetimeScoping;

namespace MediatorTest
{
    public class GetRandomQuery : IRequest<int>
    {
        public int Min { get; set; }
        public int Max { get; set; }
    }

    public class GetRandomQueryHandler : IRequestHandler<GetRandomQuery, int>
    {
        private readonly RandomNumberGenerator _r;

        public GetRandomQueryHandler(RandomNumberGenerator r)
        {
            _r = r;
        }

        public int Handle(GetRandomQuery request)
        {
            return _r.Next(request.Min, request.Max);
        }
    }

    public class RandomNumberGenerator : IDisposable
    {
        private Random _random = new Random();

        public RandomNumberGenerator() { }

        public void Dispose() { }

        public int Next(int min, int max)
        {
            var result = _random.Next(min, max);
            return result;
        }
    }

    public class LifetimeScopeDecorator<TRequest, TResponse> :
        IRequestHandler<TRequest, TResponse>
        where TRequest : IRequest<TResponse>
    {
        private readonly IRequestHandler<TRequest, TResponse> _decorated;
        private readonly Container _container;

        public LifetimeScopeDecorator(
            IRequestHandler<TRequest, TResponse> decorated,
            Container container)
        {
            _decorated = decorated;
            _container = container;
        }

        public TResponse Handle(TRequest message)
        {
            using (_container.BeginLifetimeScope())
            {
                var result = _decorated.Handle(message);
                return result;
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var assemblies = GetAssemblies();

            var container = new Container();
            container.Options.DefaultScopedLifestyle = new LifetimeScopeLifestyle();
            container.RegisterSingleton<IMediator, Mediator>();
            container.Register<RandomNumberGenerator>(Lifestyle.Scoped);
            container.Register(typeof(IRequestHandler<,>), assemblies);
            container.RegisterSingleton(new SingleInstanceFactory(container.GetInstance));
            container.RegisterSingleton(new MultiInstanceFactory(container.GetAllInstances));
            container.RegisterDecorator(
                typeof(IRequestHandler<,>),
                typeof(LifetimeScopeDecorator<,>));

            container.Verify();

            var mediator = container.GetInstance<IMediator>();

            var value = mediator.Send(new GetRandomQuery() { Min = 1, Max = 100 });

            Console.WriteLine("Value = " + value);

            Console.ReadKey();
        }

        private static IEnumerable<Assembly> GetAssemblies()
        {
            yield return typeof(IMediator).GetTypeInfo().Assembly;
            yield return typeof(GetRandomQuery).GetTypeInfo().Assembly;
        }
    }
}

Answer 1

问题是您的装饰对象（及其 DbContext 依赖项）是在创建装饰器时创建的，并且那时没有活动范围（因为您稍后在时间）。您应该使用 here 中描述的 decoratee 工厂。换句话说，你的 LifetimeScopeDecorator 应该实现如下：

public class LifetimeScopeDecorator<TRequest, TResponse> :
    IRequestHandler<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly Func<IRequestHandler<TRequest, TResponse>> _decorateeFactory;
    private readonly Container _container;

    public LifetimeScopeDecorator(
        Func<IRequestHandler<TRequest, TResponse>> decorateeFactory,
        Container container)
    {
        _decorateeFactory = decorateeFactory;
        _container = container;
    }

    public TResponse Handle(TRequest message)
    {
        using (_container.BeginLifetimeScope())
        {
            var result = _decorateeFactory.Invoke().Handle(message);
            return result;
        }
    }
}

与原始实现的不同之处在于注入了 Func<IRequestHandler<TRequest, TResponse>> 而不是 IRequestHandler<TRequest, TResponse>。这允许 Simple Injector 在创建范围后推迟创建。

MediatR 和 SimpleInjector 的依赖范围问题

Dependency Scope Issues with MediatR and SimpleInjector

c#

entity-framework

mediator

simple-injector

mediatr