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ToLookup 返回允许索引的数据结构。这是一种扩展方法。它生成一个 ILookup 实例，可以使用 foreach 循环对其进行索引或枚举。这些条目组合成每个键的分组。 - dotnetperls

string[] array = { "one", "two", "three" };
//create lookup using string length as key
var lookup = array.ToLookup(item => item.Length);

//join the values whose lengths are 3
Console.WriteLine(string.Join(",",lookup[3]));
//output: one,two

另一个例子：

int[] array = { 1,2,3,4,5,6,7,8 };
//generate lookup for odd even numbers (keys will be 0 and 1)
var lookup = array.ToLookup(item => item % 2);

//print even numbers after joining
Console.WriteLine(string.Join(",",lookup[0]));
//output: 2,4,6,8

//print odd numbers after joining
Console.WriteLine(string.Join(",",lookup[1]));
//output: 1,3,5,7