注册两个继承相同接口的组件

Question

我是 autofac 的新手，我正在尝试理解这个概念。所以基本上浏览他们网站上的示例。我正在玩一些代码，但我不明白以下内容。

如果我如下注册两个类似的组件，如 todaywriter 和 yesterdaywriter 并解析我的容器，它只会写出最后注册的一，所以在这种情况下，它只会写出今天的日期而忽略昨天的日期。到底发生了什么？我无法注册从同一接口继承的 2 个组件？如果可以的话，我将如何显示它们。

class Program
{
    private static IContainer Container { get; set; }

    static void Main(string[] args)
    {
        var builder = new ContainerBuilder();
        builder.RegisterType<ConsoleOutput>().As<IOutput>();

        builder.RegisterType<yesterdayWriter>().As<IDateWriter>();
        builder.RegisterType<TodayWriter>().As<IDateWriter>();
        Container = builder.Build();

        WriteDate();
    }

    public static void WriteDate()
    {
        // Create the scope, resolve your IDateWriter,
        // use it, then dispose of the scope.
        using (var scope = Container.BeginLifetimeScope())
        {
            var writer = scope.Resolve<IDateWriter>();
            writer.WriteDate();
            Console.ReadLine();
        }
    }
}



// This implementation of the IOutput interface
// is actually how we write to the Console. Technically
// we could also implement IOutput to write to Debug
// or Trace... or anywhere else.
public class ConsoleOutput : IOutput
{
    public void Write(string content)
    {
        Console.WriteLine(content);
    }
}

// This interface helps decouple the concept of
// "writing output" from the Console class. We
// don't really "care" how the Write operation
// happens, just that we can write.
public interface IOutput
{
    void Write(string content);
}

// This interface decouples the notion of writing
// a date from the actual mechanism that performs
// the writing. Like with IOutput, the process
// is abstracted behind an interface.
public interface IDateWriter
{
    void WriteDate();
}

// This TodayWriter is where it all comes together.
// Notice it takes a constructor parameter of type
// IOutput - that lets the writer write to anywhere
// based on the implementation. Further, it implements
// WriteDate such that today's date is written out;
// you could have one that writes in a different format
// or a different date.
public class TodayWriter : IDateWriter
{
    private IOutput _output;
    public TodayWriter(IOutput output)
    {
        this._output = output;
    }

    public void WriteDate()
    {
        this._output.Write(DateTime.Today.ToShortDateString());
    }
}

// This TodayWriter is where it all comes together.
// Notice it takes a constructor parameter of type
// IOutput - that lets the writer write to anywhere
// based on the implementation. Further, it implements
// WriteDate such that today's date is written out;
// you could have one that writes in a different format
// or a different date.
public class yesterdayWriter : IDateWriter
{
    private IOutput _output;
    public yesterdayWriter(IOutput output)
    {
        this._output = output;
    }

    public void WriteDate()
    {
        this._output.Write(DateTime.Today.AddDays(-1).ToShortDateString());
    }
}

Answer 1

在您的代码示例中，您的容器包含 2 个与 IDateWriter 服务匹配的注册。当您解析 IDateWriter 服务时，Autofac 将为您提供最新的注册，在您的情况下为 TodayWriter。

如果您想解决所有 IDateWriter，您可以解决 IEnumerable<IDateWriter>。

foreach (var writer in scope.Resolve<IEnumerable<IDateWriter>>())
{
    writer.WriteDate();
}

如果你想走得更远，你可能想要一个聚合的IDateWriter。

例如：

public class AggregatedDateWriter : IDateWriter
{
    public AggregatedDateWriter(IEnumerable<IDateWriter> writers)
    {
        this._writers = writers; 
    }


    private readonly IEnumerable<IDateWriter> _writers;


    public void WriteDate()
    {
        foreach (IDateWriter writer in this._writers)
        {
            writer.WriteDate();
        }
    }
}

如果您尝试注册此类型，您将收到以下错误消息

Autofac.Core.DependencyResolutionException: Circular component dependency detected: ConsoleApplication75.AggregatedDateWriter

这是正常的，因为当 AggregatedDateWriter 被激活时，您尝试解析所有 IDateWriter。

为避免这种情况，您可以更改组件的注册方式。

builder.RegisterType<yesterdayWriter>()
        .Named<IDateWriter>("concrete");
builder.RegisterType<TodayWriter>()
        .Named<IDateWriter>("concrete");
builder.RegisterType<AggregatedDateWriter>()
        .As<IDateWriter>()
        .WithParameter((pi, c) => pi.Name == "writers", 
                        (pi, c) => c.ResolveNamed<IEnumerable<IDateWriter>>("concrete"));

WithParameter 方法告诉 Autofac 它应该如何处理组件的参数。如果您的 AggregatedDateWriter 构造函数有一个类型为 String 的 value 参数。您可以使用 .WithParameter("value", "anyString") 方法让 Autofac 在构造函数中使用此值。在这种情况下，第一个参数将查找参数名称 writers，第二个参数将告诉 Autofac 使用 c.ResolveNamed<IEnumerable<IDateWriter>>("concrete") 的结果作为其值。

如果您还想更进一步，您可以使用 Castle.Core 生成代理，它将自动为您生成 AggregatedDateWriter，并使用自定义 IRegistrationSource 注册此代理。

注册两个继承相同接口的组件

registering 2 components which inherits same interface

c#

autofac

ioc-container