拉姆达为默认参数 [英] lambda as default argument

问题描述

我一直在寻找一个前面回答质疑获取下一个n个元素枚举没有发现任何满足和酿造我自己的。我想出了

I was looking for an anwer to question Get next N elements from enumerable didn't find any satisfying and brewed my own. What I came up with was

IEnumerable<T> Chunk<T, R>(IEnumerable<R> src, int n, Func<IEnumerable<R>, T> action){
  IEnumerable<R> head;
  IEnumerable<R> tail = src;
  while (tail.Any())
  {
    head = tail.Take(n);
    tail = tail.Skip(n);
    yield return action(head);
  }
}

我真的想虽然是有动作有 T =>的默认;吨，但我无法弄清楚如何使之成为默认参数。签名的IEnumerable< T>大块< T，R>（IEnumerable的< R> SRC，诠释N，Func键< IEnumerable的< R>中T>行动= T => T）给出了一个语法错误

What I would really like though, is to have action have a default of t=>t, but I can't figure out how to make that a default argument. The signature IEnumerable<T> Chunk<T, R>(IEnumerable<R> src, int n, Func<IEnumerable<R>, T> action = t=>t) gives a syntax error.

我的问题是，我该怎么做。

My question is, how do I do this?

我想这是相同的Specifying~~V lambda函数作为默认参数但对于C＃，而不是C ++

I suppose this is identical to Specifying a lambda function as default argument but for C# instead of C++

作为一个方面说明，我知道这不作任何语法差别，但它会读如果我切换更容易 T 和<$ C $ ç>研究？

As a side note, I know it doesn't make any syntactical difference, but would it be easier to read if I switched T and R?

推荐答案

默认值必须是常量，而对于委托唯一不变的值是空引用。

Default values have to be constants, and the only constant value for a delegate is a null reference.

我建议你使用重载来代替。需要注意的是 T => ŧ将是无效的，无论如何，除非你知道，有一个从的IEnumerable℃的转换; R> 到 T ，我没有看到任何地方。

I suggest you use overloading instead. Note that t => t wouldn't be valid anyway, unless you know that there's a conversion from IEnumerable<R> to T, which I don't see anywhere.

除了lambda表达式的有效性问题，你的 的可以使用空合并运营商：

Aside from the lambda expression validity problem, you could use the null coalescing operator:

IEnumerable<T> Chunk<T, R>(IEnumerable<R> src, int n,
                           Func<IEnumerable<R>, T> action = null)
{
    action = action ?? (t => t);
    ...
}

...但是这将是排序滥用它，你将无法告诉是否空是的实际的从谁想到他们传递一个非空值呼叫者，并希望你提出的 ArgumentNullException

... but that would be sort of abusing it, and you wouldn't be able to tell whether the null was actually from a caller who thought they were passing a non-null value, and would prefer you to raise an ArgumentNullException.

编辑：此外，注意你的方法是根本问题 - 遍历块将为了跳过适量评价原始序列几次（一旦每块）。它可能会更好写一个热切的返回之前读取每个块的方法。我们在 MoreLINQ 类似的方法，叫做的 批量 。需要注意的是批量已经完全超负荷这里所说的 - 在这种情况下， T => ŧ的作品，因为过载较少类型参数，所以我们知道身份的转换是好的。

Additionally, note that your method is fundamentally problematic - iterating over the chunks will evaluate the original sequence several times (once per chunk) in order to skip the right amount. It would probably be better to write a method which eagerly read each chunk before returning it. We have a similar method in MoreLINQ, called Batch. Note that Batch has exactly the overload mentioned here - and in this case, the t => t works, because the overload has fewer type parameters, so we know the identity conversion is okay.

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