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ToLookup returns a data structure that allows indexing. It is an extension method. It produces an ILookup instance that can be indexed or enumerated using a foreach-loop. The entries are combined into groupings at each key. - 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
//output: one,two

Another Example:

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
//output: 2,4,6,8

//print odd numbers after joining
//output: 1,3,5,7

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