ILookup＆LT; TKEY的，TVal＆GT;与IGrouping＆LT; TKEY的，TVal＆GT;

问题描述

我一直有麻烦阐述和，并很好奇，如果我理解正确了。 LINQ通过产生的 IGrouping 项目序列同时也给了我一个 ToLookup 扩展方法加剧了问题。所以，感觉就像他们一样，直到我看了更加紧密。

  VAR Q1 =
    从N N
    n组由n.MyKey为G
    选择克;
// Q1是IEnumerable的＆LT; IGrouping＆LT; TKEY的，TVal＆GT;＆GT;
 

这是等同于：

 无功Q2 = N.GroupBy（N =＆GT; n.MyKey，N =＆GT; N）;
// Q2是IEnumerable的＆LT; IGrouping＆LT; TKEY的，TVal＆GT;＆GT;
 

这看起来很像：

  VAR Q3 = N.ToLookup（N =＆GT; n.MyKey，N =＆GT; N）;
// Q3是ILookup＆LT; TKEY的，TVal＆GT;
 

我在下面的类比是否正确？

1. 的 IGrouping＆LT; TKEY的，TVal＆GT; 是一个单组（即键控顺序），类似于 KeyValuePair＆LT; TKEY的，TVal＆GT; 其中值实际上是元素的序列（而不是一个单一的元素）


2. 的的IEnumerable＆LT; IGrouping＆LT; TKEY的，TVal＆GT;＆GT; 就是其中的一个序列（类似于你会得到什么了一个的IDictionary＆LT迭代时; TKEY的，TVal＆GT;


3. 的 ILookup＆LT; TKEY的，TVal＆GT; 更像是一个的IDictionary＆LT; TKEY的，TVal＆GT; 其中值实际上元素序列

解决方案

是的，所有这些都是正确的。

和 ILookup＆LT; TKEY的，TValue＆GT; 还延伸的IEnumerable＆LT; IGrouping＆LT; TKEY的，TValue＆GT;＆GT; ，所以你可以在所有的键/收集对迭代以及（或代替）只看了特别的关键字。

我基本上认为 ILookup＆LT的; TKEY的，TValue＆GT; 作为是像的IDictionary＆LT; TKEY的，IEnumerable的＆LT; TValue＆GT;＆GT; 。

请记住 ToLookup 是现在就做操作（立即执行），而一个 GROUPBY 是推迟。碰巧的是，与拉LINQ的作品，当你开始拉的方式 IGrouping 从a的结果S GROUPBY ，它有反正读取所有数据（因为你无法通过切换中途组），而在其他实现它可能是能够产生流的结果。 （它在推LINQ。我期望的LINQ to事件是相同的）

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>>

Which is equivalent to:

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

Which looks a lot like:

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

Am I correct in the following analogies?

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

解决方案

Yes, all of those are correct.

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.

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

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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