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ToLookup 返回允許索引的資料結構。這是一種擴充套件方法。它生成一個 ILookup 例項，可以使用 foreach 迴圈對其進行索引或列舉。這些條目組合成每個鍵的分組。 - 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

另一個例子：

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