C# LINQ 优化

发布于 2024-10-30 14:50:04 字数 433 浏览 0 评论 0原文

优化以下类型的语句的正确方法是什么：

IEnumerable<T> sequence = BuildSequence();
// 'BuildSequence' makes some tough actions and uses 'yield return'
// to form the resulting sequence.

现在，如果我愿意只采用一些第一个元素，我可以使用类似的东西：

sequence.Take(5);

所以，如果我的序列来自 BuildSequence 实际上包含数千个元素，我显然不希望构建所有元素，因为我只需要其中 5 个。

LINQ 是否可以优化此类操作，还是我必须自己发明一些东西？

原文

What would be the proper way of optimizing the following kind of statements:

IEnumerable<T> sequence = BuildSequence();
// 'BuildSequence' makes some tough actions and uses 'yield return'
// to form the resulting sequence.

Now, if I'm willing to take only some of the first elements, I could use something like:

sequence.Take(5);

And so, if my sequence from BuildSequence actually contains several thousands of elements, I obviously don't want all of them to be constructed, because I would only need 5 of them.

Does LINQ optimize this kind of operations or I would have to invent something myself?

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明媚如初 2024-11-06 14:50:04

迭代器块（yield return）为您处理这个问题；迭代器块是一个流 API；工作仅在迭代器（或 Dispose()）上的每次 MoveNext() 调用期间发生。由于 Take() also 不会读取整个流，因此保留了此行为。

但请注意，某些操作需要在本地缓冲数据 - 尤其是 GroupBy 和 OrderBy；但只要您使用非缓冲操作，就可以了。

例如：

static IEnumerable<int> ReadInts() {
    var rand = new Random();
    while(true) yield return rand.Next();
}
...
int count = ReadInts().Take(10).Count(); // 10 - it doesn't loop forever

The iterator block (yield return) handles this for you; an iterator block is a streaming API; work only happens during each MoveNext() call on the iterator (or on Dispose()). Because Take() also doesn't read the entire stream, this behaviour is preserved.

Note, however, that some operations need to buffer the data locally - GroupBy and OrderBy most notably; but as long as you use non-buffering operations, you are fine.

For example:

static IEnumerable<int> ReadInts() {
    var rand = new Random();
    while(true) yield return rand.Next();
}
...
int count = ReadInts().Take(10).Count(); // 10 - it doesn't loop forever

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