IEnumerable foreach，对最后一个元素做一些不同的事情

问题描述

我有一个 IEnumerable<T> .我要为集合的每个项目(最后一个项目除外)做一件事情，我想对它做其他事情.我该如何整齐地编写代码?在伪代码中

I have an IEnumerable<T>. I want to do one thing for each item of the collection, except the last item, to which I want to do something else. How can I code this neatly? In Pseudocode

foreach (var item in collection)
{
    if ( final )
    {
        g(item)
    }
    else
    {
        f(item)
    }
}

因此，如果我的IEnumerable是Enumerable.Range(1,4)，我会执行f(1)f(2)f(3)g(4).注意如果我的IEnumerable恰好是长度1，我想要g(1).

So if my IEnumerable were Enumerable.Range(1,4) I'd do f(1) f(2) f(3) g(4). NB. If my IEnumerable happens to be length 1, I want g(1).

我的IEnumerable恰好有点cr脚，使Count()与遍历整个事情一样昂贵.

My IEnumerable happens to be kind of crappy, making Count() as expensive as looping over the whole thing.

推荐答案

由于您提到IEnumerable[<T>](不是IList[<T>]等)，我们不能依赖计数等:因此，我很想展开:

Since you mention IEnumerable[<T>] (not IList[<T>] etc), we can't rely on counts etc: so I would be tempted to unroll the foreach:

using(var iter = source.GetEnumerator()) {
    if(iter.MoveNext()) {
        T last = iter.Current;
        while(iter.MoveNext()) {
            // here, "last" is a non-final value; do something with "last"
            last = iter.Current;
        }
        // here, "last" is the FINAL one; do something else with "last"
    }
}

请注意，以上内容在技术上仅对IEnuemerable<T>有效；对于非泛型，您需要:

Note the above is technically only valid for IEnuemerable<T>; for non-generic, you'd need:

var iter = source.GetEnumerator();
using(iter as IDisposable) {
    if(iter.MoveNext()) {
        SomeType last = (SomeType) iter.Current;
        while(iter.MoveNext()) {
            // here, "last" is a non-final value; do something with "last"
            last = (SomeType) iter.Current;
        }
        // here, "last" is the FINAL one; do something else with "last"
    }
}

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