如何问“是否存在一个完全满足条件的元素"?在LINQ吗? [英] How to ask "Is there exactly one element satisfying condition" in LINQ?

问题描述

一个简单的问题，以编程方式询问此序列中是否确实有一个元素满足X条件?"的首选方法是什么?使用Linq吗?

Quick question, what's the preferable way to programmaticaly ask "Is there exactly one element in this sequence that satisfies X condition?" using Linq?

即

// Pretend that the .OneAndOnlyOne() method exists
int[] sequence = new int[] { 1, 1, 2, 3, 5, 8 };
Assert.IsTrue(sequence.OneAndOnlyOne(x => x == 2);
Assert.IsFalse(sequence.OneAndOnlyOne(x => x == 1);

类似的事情可以通过以下方式完成:

something like this can be done with:

sequence.SingleOrDefault(x => x == 2) != null;

但这有点笨拙.

我想我可以滚动自己的扩展方法，但是这似乎是我代码中的常见模式，我想确保有一个很好的干净方法来做到这一点.有没有使用内置的LINQ方法的方法?

I suppose I could roll my own extension method, but this seems to be a common pattern in my code and I want to make sure there's a good clean way to do it. Is there a way using the built-in LINQ methods?

推荐答案

您可以这样做:

bool onlyOne = source.Where(/*condition*/).Take(2).Count() == 1

在多次匹配的情况下，这将防止count不必要地枚举较大的序列.

Which will prevent count from enumerating a large sequence unnecessarily in the event of multiple matches.

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