19.5. SortedList

Put simply, a SortedList is a Hashtable which is sorted by the keys. Like a Hashtable, it is accessible by both its keys and values. The API documentation describes a SortedList as a hybrid between a Hashtable and an Array. It behaves like a Hashtable when accessed via a key. It behaves like an Array when accessed via its GetByIndex method.

If you do not need to access elements in a SortedList via its indices, then use a Hashtable rather than a SortedList because the sorting has a performance implication, especially if you do not need that functionality.

The program below demonstrates how a SortedList can be used. The output is interspersed with the code to show the outputs of the different sections.

 1: using System;
 2: using System.Collections;
 3:
 4: public class TestClass{
 5:   public static void Main(){
 6:
 7:     SortedList sl = new SortedList();
 8:     sl.Add("A", "apple");
 9:     sl.Add("E", "eggplant");
10:     sl.Add("B", "banana");
11:     sl.Add("C", "coconut");
12:     PrintCollection1(sl);
13:

Output:

Key:A Value:apple
Key:B Value:banana
Key:C Value:coconut
Key:E Value:eggplant

Notice that, unlike a Hashtable, the elements in a SortedList are always dynamically sorted by the key every time a new element is inserted.

14:     Console.WriteLine
15:       ("Index of key B:" +sl.IndexOfKey("B"));
16:     Console.WriteLine
17:       ("Index of value apple:"
           +sl.IndexOfValue("apple"));
18:

Output:

Index of key B: 1
Index of value apple: 0

The IndexOfKey and IndexOfValue methods are demonstrated above.

19:     sl.Add("D", "durian");
20:     PrintCollection1(sl);

Output:

Key:A Value:apple
Key:B Value:banana
Key:C Value:coconut
Key:D Value:durian
Key:E Value:eggplant

When a new key/value pair is added, the indices of all elements are updated as sorting is performed again.

21:   } // end main
22:
23:   // 1st way to do it
24:   public static void PrintCollection1 (SortedList s){
25:     IDictionaryEnumerator enumerator = s.GetEnumerator();
26:     while (enumerator.MoveNext())
27:       Console.WriteLine("Key:"  + enumerator.Key +
28:                        " Value:"+ enumerator.Value);
29:     Console.WriteLine();
30:   }
31:
32:   // 2nd way to do it
33:   public static void PrintCollection2 (SortedList s){
34:     for (int i=0; i<s.Count; i++)
35:       Console.WriteLine("Key:"  + s.GetKey(i) +
36:                        " Value:"+ s.GetByIndex(i));
37:     Console.WriteLine();
38:   }
39: }

Both PrintCollection1 and PrintCollection2 are functionally identical. Since the elements in a SortedList are indexed, you can use the GetByIndex and GetKey methods to retrieve a value or key by its index respectively (lines 35 – 36).

A SortedList has the following public properties – Count, Keys, Values; and the following methods – ContainsKey, ContainsValue, Remove. These properties and methods are used in exactly the same way as in a Hashtable class and are therefore not included in this example again.