ToLookup C# with Example

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 
Console.WriteLine(string.Join(",",lookup[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 
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