混合协方差与反方差

Question

目标：

迭代以下集合

var collection = new IImportTrigger<EventArgs>[]
{
    new FileSystemImportTrigger()
    , new TimerImportTrigger()
};

以这种方式

foreach (var trigger in collection)
{
    trigger.Import += trigger.OnImport;
}

，这就是我到目前为止所拥有的

public delegate void ImportTriggerEventHandler<in T>(object sender, T args) where T : EventArgs;

public interface IImportTrigger<out T> where T : EventArgs
{
    event ImportTriggerEventHandler<T> Import;
    void OnImport<T1>(object sender, T1 args) where T1 : EventArgs;
}

public class FileSystemImportTrigger : IImportTrigger<FileSystemEventArgs>
{
    public event ImportTriggerEventHandler<FileSystemEventArgs> Import;

    public void OnImport<T>(object sender, T args) where T : EventArgs {  }
}

public class TimerImportTrigger : IImportTrigger<ElapsedEventArgs>
{
    public event ImportTriggerEventHandler<ElapsedEventArgs> Import;

    public void OnImport<T>(object sender, T args) where T : EventArgs {  }
}

期望：

我想用一个泛型参数来定义IImportTrigger。

问题：

如果我将接口定义更改为以下内容(请注意，泛型参数T不再是协变的)。

public interface IImportTrigger<T> where T : EventArgs
{
    event ImportTriggerEventHandler<T> Import;
    void OnImport(object sender, T args);
}

因此

public class FileSystemImportTrigger : IImportTrigger<FileSystemEventArgs>
{
    public event ImportTriggerEventHandler<FileSystemEventArgs> Import;

    public void OnImport(object sender, FileSystemEventArgs args) { }
}

public class TimerImportTrigger : IImportTrigger<ElapsedEventArgs>
{

    public event ImportTriggerEventHandler<ElapsedEventArgs> Import;

    public void OnImport(object sender, ElapsedEventArgs args) { }
}

我将无法为我的集合创建一个公共类型

var collection = new IImportTrigger<EventArgs>[]
{
    new FileSystemImportTrigger()
    , new TimerImportTrigger()
};

因为泛型参数不再是输出安全的。

问题：

有什么办法可以完成我的方案吗？

Answer 1

通过将OnImport切换为非泛型，然后使用显式接口，然后再创建另一个不具有OnImport泛型子的非协变的派生接口，您可以实现它。

internal class Program
{
    private static void Main(string[] args)
    {
        var collection = new IImportTriggerBase<EventArgs>[]
        {
            new FileSystemImportTrigger()
            , new TimerImportTrigger()
        };

        foreach (var trigger in collection)
        {
            trigger.Import += trigger.OnImport;
        }
    }
}

public delegate void ImportTriggerEventHandler<in T>(object sender, T args) where T : EventArgs;

public interface IImportTriggerBase<out T> where T : EventArgs
{
    event ImportTriggerEventHandler<T> Import;
    void OnImport(object sender, EventArgs args);
}

public interface IImportTrigger<T> : IImportTriggerBase<T> where T : EventArgs
{
    void OnImport(object sender, T args);
}

public class FileSystemImportTrigger : IImportTrigger<FileSystemEventArgs>
{
    public event ImportTriggerEventHandler<FileSystemEventArgs> Import;

    public void OnImport(object sender, FileSystemEventArgs args) { }

    void IImportTriggerBase<FileSystemEventArgs>.OnImport(object sender, EventArgs args)
    {
        OnImport(sender, (FileSystemEventArgs)args);
    }
}

public class TimerImportTrigger : IImportTrigger<ElapsedEventArgs>
{
    public event ImportTriggerEventHandler<ElapsedEventArgs> Import;

    public void OnImport(object sender, ElapsedEventArgs args) { }

    void IImportTriggerBase<ElapsedEventArgs>.OnImport(object sender, EventArgs args)
    {
        OnImport(sender, (ElapsedEventArgs)args);
    }
}

但是，这确实为您提供了OnImport(object sender, EventArgs args)方法的额外遍历，该方法在IImportTrigger<T>上是可见的。

这是为了解决您的问题，如果我在哪里这样做，并且我假设正确的话，您只是希望派生类能够了解到Import正在被解雇的事实，并且您实际上不需要公开OnImport，我只想这样做。

internal class Program
{
    private static void Main(string[] args)
    {
        var collection = new IImportTrigger<EventArgs>[]
        {
            new FileSystemImportTrigger()
            , new TimerImportTrigger()
        };
    }
}

public delegate void ImportTriggerEventHandler<in T>(object sender, T args) where T : EventArgs;

public interface IImportTrigger<out T> where T : EventArgs
{
    event ImportTriggerEventHandler<T> Import;
}

public abstract class OnImportBase<T> : IImportTrigger<T> where T: EventArgs
{

    public event ImportTriggerEventHandler<T> Import;

    protected virtual void OnImport(object sender, T args)
    {
        var tmp = Import;
        if (tmp != null)
        {
            tmp(this, args);
        }
    }
}

public class FileSystemImportTrigger : OnImportBase<FileSystemEventArgs>
{
    protected override void OnImport(object sender, FileSystemEventArgs args)
    {
        DoSomeExtraStuffBeforeImport();
        base.OnImport(sender, args);
    }

    private void DoSomeExtraStuffBeforeImport()
    {
    }
}

public class TimerImportTrigger : OnImportBase<ElapsedEventArgs>
{
    protected override void OnImport(object sender, ElapsedEventArgs args)
    {
        base.OnImport(sender, args);
        DoSomeExtraStuffAfterImport();
    }

    private void DoSomeExtraStuffAfterImport()
    {
    }
}

这将消除事件订阅，并将其作为覆盖处理(这是.NET事件中的正常模式)。