没有使用泛型扩展方法的类型推断

Question

我有以下方法:

public static TEventInvocatorParameters Until
    <TEventInvocatorParameters, TEventArgs>(this TEventInvocatorParameters p,
                                            Func<TEventArgs, bool> breakCond)
    where TEventInvocatorParameters : EventInvocatorParameters<TEventArgs>
    where TEventArgs : EventArgs
{
    p.BreakCondition = breakCond;
    return p;
}

而这堂课

public class EventInvocatorParameters<T>
    where T : EventArgs
{
    public Func<T, bool> BreakCondition { get; set; }
    // Other properties used below omitted for brevity.
}

现在,我有以下问题:

1. 此扩展方法甚至可以显示所有类型string.
2. 我不能写new EventInvocatorParameters<EventArgs>(EventABC).Until(e => false);它告诉我"方法的类型参数......不能从用法中推断出来."

我不能使用像这样的泛型类型参数吗？你会如何解决这个问题？
重点:我需要这两个通用参数,因为我需要返回调用此扩展方法的相同类型.

---

更广泛的图片(没有必要回答问题!):
我正在尝试创建一个流畅的界面来调用事件.基础是这个静态类:

public static class Fire
{
   public static void Event<TEventArgs>(
       ConfiguredEventInvocatorParameters<TEventArgs> parameters)
    where TEventArgs : EventArgs
    {
        if (parameters.EventHandler == null)
        {
            return;
        }

        var sender = parameters.Sender;
        var eventArgs = parameters.EventArgs;
        var breakCondition = parameters.BreakCondition;

        foreach (EventHandler<TEventArgs> @delegate in 
                 parameters.EventHandler.GetInvocationList())
        {
            try
            {
                @delegate(sender, eventArgs);
                if (breakCondition(eventArgs))
                {
                    break;
                }
            }
            catch (Exception e)
            {
                var exceptionHandler = parameters.ExceptionHandler;
                if (!exceptionHandler(e))
                {
                    throw;
                }
            }
        }
    }
}

要确保只能使用完全配置的参数调用此方法,它只接受ConfiguredEventInvocatorParameters<T>来自以下内容的方法EventInvocatorParameters<T>:

public class ConfiguredEventInvocatorParameters<T>
    : EventInvocatorParameters<T>
    where T : EventArgs
{
    public ConfiguredEventInvocatorParameters(
        EventInvocatorParameters<T> parameters, object sender, T eventArgs)
        : base(parameters)
    {
        EventArgs = eventArgs;
        Sender = sender;
    }

    public T EventArgs { get; private set; }
    public object Sender { get; private set; }

}

以下是有效的电话:

Fire.Event(EventName.With(sender, eventArgs));
Fire.Event(EventName.With(sender, eventArgs).Until(e => e.Cancel));
Fire.Event(EventName.Until(e => e.Cancel).With(sender, eventArgs));

以下内容无效:

// no sender or eventArgs have been specified, i.e. missing call to With(...)
Fire.Event(EventName.Until(e => e.Cancel));

为了使这项工作,存在名为的扩展方法With,它接受a EventHandler<TEventArgs或a TEventInvocatorParameters并返回a ConfiguredEventInvocatorParameters<TEventArgs>.With现在之后的所有调用也需要返回类型ConfiguredEventInvocatorParameters<TEventArgs>,否则有效调用的第二个示例(Until最后一个)将不起作用.
如果您对API有任何想法,请告诉我.但是,我想避免以下三件事:

• 如果尚未完全配置参数,则仅在运行时失败
• 创建反向语法,如 EventName.With(...).Until(...).Fire()
• 使用臭名昭着的Do方法开始:Fire(EventName).With(...).Until(...).Do();

Answer 1

泛型方法的类型推断故意并没有使从限制任何扣减.相反,从参数和形式参数中进行推导,然后根据约束检查推导出的类型参数.

有关约束和方法签名的一些设计问题的详细讨论,包括几十个人告诉我,我认为现有设计是明智的是错误的,请参阅我关于这个主题的文章:

http://blogs.msdn.com/b/ericlippert/archive/2009/12/10/constraints-are-not-part-of-the-signature.aspx

Answer 2

对于任何感兴趣的人，目前，我使用通用类层次结构解决了最初的问题（流畅的事件调用 API）。这基本上就是 Hightechrider 对类固醇的回答。

public abstract class EventInvocatorParametersBase
    <TEventInvocatorParameters, TEventArgs>
    where TEventArgs : EventArgs
    where TEventInvocatorParameters :
        EventInvocatorParametersBase<TEventInvocatorParameters, TEventArgs>

{
    protected EventInvocatorParametersBase(
        EventHandler<TEventArgs> eventHandler,
        Func<Exception, bool> exceptionHandler,
        Func<TEventArgs, bool> breakCondition)
    {
        EventHandler = eventHandler;
        ExceptionHandler = exceptionHandler;
        BreakCondition = breakCondition;
    }

    protected EventInvocatorParametersBase(
        EventHandler<TEventArgs> eventHandler)
        : this(eventHandler, e => false, e => false)
    {
    }

    public Func<TEventArgs, bool> BreakCondition { get; set; }
    public EventHandler<TEventArgs> EventHandler { get; set; }
    public Func<Exception, bool> ExceptionHandler { get; set; }

    public TEventInvocatorParameters Until(
        Func<TEventArgs, bool> breakCondition)
    {
        BreakCondition = breakCondition;
        return (TEventInvocatorParameters)this;
    }

    public TEventInvocatorParameters WithExceptionHandler(
        Func<Exception, bool> exceptionHandler)
    {
        ExceptionHandler = exceptionHandler;
        return (TEventInvocatorParameters)this;
    }

    public ConfiguredEventInvocatorParameters<TEventArgs> With(
        object sender, 
        TEventArgs eventArgs)
    {
        return new ConfiguredEventInvocatorParameters<TEventArgs>(
            EventHandler, ExceptionHandler, BreakCondition,
            sender, eventArgs);
    }
}

public class EventInvocatorParameters<T> :
    EventInvocatorParametersBase<EventInvocatorParameters<T>, T>
    where T : EventArgs
{
    public EventInvocatorParameters(EventHandler<T> eventHandler)
        : base(eventHandler)
    {
    }
}

public class ConfiguredEventInvocatorParameters<T> :
    EventInvocatorParametersBase<ConfiguredEventInvocatorParameters<T>, T>
    where T : EventArgs
{
    public ConfiguredEventInvocatorParameters(
        EventHandler<T> eventHandler,
        Func<Exception, bool> exceptionHandler,
        Func<T, bool> breakCondition, object sender,
        T eventArgs)
        : base(eventHandler, exceptionHandler, breakCondition)
    {
        EventArgs = eventArgs;
        Sender = sender;
    }

    public ConfiguredEventInvocatorParameters(EventHandler<T> eventHandler,
                                              object sender,
                                              T eventArgs)
        : this(eventHandler, e => false, e => false, sender, eventArgs)
    {
    }

    public T EventArgs { get; private set; }
    public object Sender { get; private set; }
}

public static class EventExtensions
{
    public static EventInvocatorParameters<TEventArgs> Until<TEventArgs>(
        this EventHandler<TEventArgs> eventHandler,
        Func<TEventArgs, bool> breakCondition)
        where TEventArgs : EventArgs
    {
        return new EventInvocatorParameters<TEventArgs>(eventHandler).
            Until(breakCondition);
    }

    public static EventInvocatorParameters<TEventArgs> 
        WithExceptionHandler<TEventArgs>(
            this EventHandler<TEventArgs> eventHandler,
            Func<Exception, bool> exceptionHandler)
        where TEventArgs : EventArgs
    {
        return
            new EventInvocatorParameters<TEventArgs>(eventHandler).
                WithExceptionHandler(exceptionHandler);
    }

    public static ConfiguredEventInvocatorParameters<TEventArgs>
        With<TEventArgs>(
            this EventHandler<TEventArgs> eventHandler, object sender,
            TEventArgs eventArgs)
        where TEventArgs : EventArgs
    {
        return new ConfiguredEventInvocatorParameters<TEventArgs>(
            eventHandler, sender, eventArgs);
    }
}

这允许您编写如下代码：

Fire.Event(EventName.WithExceptionHandler(e => false)
                    .Until(e => false).With(this, EventArgs.Empty));
Fire.Event(EventName.With(this, EventArgs.Empty));
Fire.Event(EventName.WithExceptionHandler(e => false)
                    .With(this, EventArgs.Empty).Until(e => false));
Fire.Event(EventName.With(this, EventArgs.Empty)
                    .WithExceptionHandler(e => false).Until(e => false));

但它不允许您编写此内容，因为并未提供所有必要的信息（eventArgs 和发件人）：

Fire.Event(EventName.Until(e => false));
Fire.Event(EventName);