C# 4 and .NET 4

254 ❘ ChaPTer 10 cOllectiOns
following example, a list of Racer objects is fi lled. Because List < T > implements IEnumerable < T > ,
the ToLookup() method can be invoked on the racers list. This method requires a delegate of type
Func < TSource, TKey > that defi nes the selector of the key. Here the racers are selected based on the
country by using the Lambda expression r = > r.Country . The foreach loop accesses only the racers
from Australia by using the indexer.
You can read more about extension methods in Chapter 11, “ Language Integrated
Query. ” Lambda expressions are explained in Chapter 8.
var racers = new List < Racer > ();
racers.Add(new Racer("Jacques", "Villeneuve", "Canada", 11));
racers.Add(new Racer("Alan", "Jones", "Australia", 12));
racers.Add(new Racer("Jackie", "Stewart", "United Kingdom", 27));
racers.Add(new Racer("James", "Hunt", "United Kingdom", 10));
racers.Add(new Racer("Jack", "Brabham", "Australia", 14));
var lookupRacers = racers.ToLookup(r = > r.Country);
foreach (Racer r in lookupRacers["Australia"])
{
Console.WriteLine(r);
}
The output shows the racers from Australia:
Alan Jones
Jack Brabham
sorted dictionary
code snippet LookupSample/Program.cs
SortedDictionary < TKey, TValue > is a binary search tree where the items are sorted based on the key.
The key type must implement the interface IComparable < TKey > . If the key type is not sortable, you can
also create a comparer implementing IComparer < TKey > and assign the comparer as a constructor argument
of the sorted dictionary.
Earlier in this chapter you read about SortedList < TKey, TValue > . SortedDictionary < TKey, TValue >
and SortedList < TKey, TValue > have similar functionalities. But because SortedList < TKey, TValue > is
implemented as a list that is based on an array and SortedDictionary < TKey, TValue > is implemented as
a dictionary, the classes have different characteristics:
➤ SortedList < TKey, TValue > uses less memory than SortedDictionary < TKey, TValue > .
➤
➤
SortedDictionary < TKey, TValue > has faster insertion and removal of elements.
When populating the collection with already sorted data, SortedList < TKey, TValue > is faster, if
capacity changes are not needed.
SortedList consumes less memory than SortedDictionary . SortedDictionary is
faster with inserts and with the removal of unsorted data.
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