ILookup<TKey, TVal>对比 IGrouping<TKey, TVal> [英] ILookup&lt;TKey, TVal&gt; vs. IGrouping&lt;TKey, TVal&gt;

问题描述

我一直无法阐明 ILookup 和 IGrouping，并且很好奇我现在是否正确理解它.LINQ 通过生成 IGrouping 项的序列同时给我一个 ToLookup 扩展方法使问题复杂化.所以我感觉它们是一样的，直到我更仔细地观察.

I've been having trouble articulating the differences between ILookup<TKey, TVal> and IGrouping<TKey, TVal>, and am curious if I understand it correctly now. LINQ compounded the issue by producing sequences of IGrouping items while also giving me a ToLookup extension method. So it felt like they were the same until I looked more closely.

var q1 = 
    from n in N
    group n by n.MyKey into g
    select g;
// q1 is IEnumerable<IGrouping<TKey, TVal>>

相当于:

var q2 = N.GroupBy(n => n.MyKey, n => n);
// q2 is IEnumerable<IGrouping<TKey, TVal>>

看起来很像:

var q3 = N.ToLookup(n => n.MyKey, n => n);
// q3 is ILookup<TKey, TVal>

我在下面的类比中正确吗?

Am I correct in the following analogies?

1. An IGrouping 是单个组(即键控序列)，类似于 KeyValuePair，其中值实际上是一个序列元素(而不是单个元素)
2. An IEnumerable> 是一个序列(类似于迭代 IDictionary
3. ILookup 更像是一个 IDictionary，其中的值实际上是一个元素序列

1. An IGrouping<TKey, TVal> is a single group (i.e. a keyed sequence), analogous to KeyValuePair<TKey, TVal> where the value is actually a sequence of elements (rather than a single element)
2. An IEnumerable<IGrouping<TKey, TVal>> is a sequence of those (similar to what you get when iterating over an IDictionary<TKey, TVal>
3. An ILookup<TKey, TVal> is more like a IDictionary<TKey, TVal> where the value is actually a sequence of elements

推荐答案

是的，所有这些都是正确的.

Yes, all of those are correct.

和 ILookup 还扩展了 IEnumerable，因此您可以迭代所有键/​​集合对以及(或代替)只是查找特定的键.

And ILookup<TKey, TValue> also extends IEnumerable<IGrouping<TKey, TValue>> so you can iterate over all the key/collection pairs as well as (or instead of) just looking up particular keys.

我基本上认为 ILookup 就像 IDictionary>.

I basically think of ILookup<TKey,TValue> as being like IDictionary<TKey, IEnumerable<TValue>>.

请记住，ToLookup 是立即执行"操作(立即执行)，而 GroupBy 是延迟的.碰巧的是，按照pull LINQ"的工作方式，当您开始从 GroupBy 的结果中提取 IGrouping 时，它无论如何都必须读取所有数据(因为您不能在中途切换组)而在其他实现中它可能能够产生流结果.(它在 Push LINQ 中实现；我希望 LINQ to Events 是相同的.)

Bear in mind that ToLookup is a "do it now" operation (immediate execution) whereas a GroupBy is deferred. As it happens, with the way that "pull LINQ" works, when you start pulling IGroupings from the result of a GroupBy, it has to read all the data anyway (because you can't switch group midway through) whereas in other implementations it may be able to produce a streaming result. (It does in Push LINQ; I would expect LINQ to Events to be the same.)

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