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我正在构建应用程序并尝试使用 MediatR 实现域事件。我在使用 MediatR 4.0.0 的 .NET Core 2.1（预览版，但我认为这不是问题）

我正在使用 eShopOnContainers/Ordering 服务的示例。他们使用 Autofac，但我更愿意坚持使用使用MediatR 扩展的本机 DI 容器

我的应用程序在启动时崩溃了。错误：

System.ArgumentException: '无法为服务类型实例化实现类型'MyProject.API.Application.Commands.IdentifierCommandHandler`2[T,R]'

'MediatR.IRequestHandler 2[MyProject.API.Application.Commands.IdentifiedCommand> 2[T,R],R]'。

我的 Nuget 包：

<ItemGroup>
    <PackageReference Include="MediatR" Version="4.0.1" />
    <PackageReference Include="MediatR.Extensions.Microsoft.DependencyInjection" Version="4.0.0" />
    <PackageReference Include="Microsoft.AspNetCore.App" Version="2.1.0-preview1-final" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="2.1.0-preview1-final" />
    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="2.1.0-preview1" />
    <PackageReference Include="Swashbuckle.AspNetCore" Version="2.2.0" />
  </ItemGroup>

尝试注册 IdentifiedCommandHandler 时，我的应用程序在启动时崩溃：

public class IdentifiedCommandHandler<T, R> : IRequestHandler<IdentifiedCommand<T, R>, R>
        where T : IRequest<R>
    {
        private readonly IMediator _mediator;
        private readonly IRequestManager _requestManager;

        public IdentifiedCommandHandler(IMediator mediator, IRequestManager requestManager)
        {
            _mediator = mediator;
            _requestManager = requestManager;
        }

        /// <summary>
        /// Creates the result value to return if a previous request was found
        /// </summary>
        /// <returns></returns>
        protected virtual R CreateResultForDuplicateRequest()
        {
            return default(R);
        }

        /// <summary>
        /// This method handles the command. It just ensures that no other request exists with the same ID, and if this is the case
        /// just enqueues the original inner command.
        /// </summary>
        /// <param name="message">IdentifiedCommand which contains both original command & request ID</param>
        /// <returns>Return value of inner command or default value if request same ID was found</returns>
        public async Task<R> Handle(IdentifiedCommand<T, R> message, CancellationToken cancellationToken)
        {
            var alreadyExists = await _requestManager.ExistAsync(message.Id);
            if (alreadyExists)
            {
                return CreateResultForDuplicateRequest();
            }
            else
            {
                await _requestManager.CreateRequestForCommandAsync<T>(message.Id);

                var result = await _mediator.Send(message.Command);

                return result;
            }
        }
    }

我将不胜感激。我不得不对其进行一些更改以使其与当前的 MediatR 兼容，因此我使用 IRequestHandler 而不是 IAsyncRequestHandler，因为它不再存在。

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