LINQ ToLookup Query Operator

Creates a Lookup<TKey, TElement> from an IEnumerable<T> according to specified key selector and element selector functions.

class Package
             {
                 public string Company { get; set; }
                 public double Weight { get; set; }
                 public long TrackingNumber { get; set; }
             }

             public static void ToLookupEx1()
             {
                 // Create a list of Packages.
               List<Package> packages =
                     new List<Package>
                        { new Package { Company = "Coho Vineyard",
                             Weight = 25.2, TrackingNumber = 89453312L },
                            new Package { Company = "Lucerne Publishing",
                             Weight = 18.7, TrackingNumber = 89112755L },
                            new Package { Company = "Wingtip Toys",
                             Weight = 6.0, TrackingNumber = 299456122L },
                            new Package { Company = "Contoso Pharmaceuticals",
                             Weight = 9.3, TrackingNumber = 670053128L },
                            new Package { Company = "Wide World Importers",
                             Weight = 33.8, TrackingNumber = 4665518773L } };

                 // Create a Lookup to organize the packages.
                 // Use the first character of Company as the key value.
                 // Select Company appended to TrackingNumber
                 // as the element values of the Lookup.
                 ILookup<char, string> lookup =
                     packages
                      .ToLookup(p => Convert.ToChar(p.Company.Substring(0, 1)),
                              p => p.Company + " " + p.TrackingNumber);

                 // Iterate through each IGrouping in the Lookup.
                 foreach (IGrouping<char, string> packageGroup in lookup)
                 {
                     // Print the key value of the IGrouping.
                   Console.WriteLine(packageGroup.Key);
                     // Iterate through each value in the
                     // IGrouping and print its value.
                     foreach (string str in packageGroup)
                       Console.WriteLine("    {0}", str);
                }
            }

            /*
             This code produces the following output:

             C
                 Coho Vineyard 89453312
                 Contoso Pharmaceuticals 670053128
             L
                 Lucerne Publishing 89112755
             W
                 Wingtip Toys 299456122
                 Wide World Importers 4665518773
             */