C# 3.0 Cookbook

Recipe 9.10 Sorting a Hashtable's Keys and/or Values

Problem

You want to sort the keys and/or values contained in a Hashtable in order to display the entire Hashtable to the user sorted in either ascending or descending order.

Solution

Use the Keys and Values properties of a Hashtable object to obtain an ICollection of its key and value objects. The methods shown here return an ArrayList of objects containing the keys or values of a Hashtable :

using System; using System.Collections; // Return an ArrayList of Hashtable keys public static ArrayList GetKeys(Hashtable table) { return (new ArrayList(table.Keys)); } // Return an ArrayList of Hashtable values public static ArrayList GetValues(Hashtable table) { return (new ArrayList(table.Values)); }

The following code creates a Hashtable object and displays first keys, and then values, sorted in ascending and descending order:

public static void TestSortKeyValues( ) { // Define a hashtable object Hashtable hash = new Hashtable( ); hash.Add(2, "two"); hash.Add(1, "one"); hash.Add(5, "five"); hash.Add(4, "four"); hash.Add(3, "three"); // Get all the keys in the hashtable and sort them ArrayList keys = GetKeys(hash); keys.Sort( ); // Display sorted key list foreach (object obj in keys) Console.WriteLine("Key: " + obj + " Value: " + hash[obj]); // Reverse the sorted key list Console.WriteLine( ); keys.Reverse( ); // Display reversed key list foreach (object obj in keys) Console.WriteLine("Key: " + obj + " Value: " + hash[obj]); // Get all the values in the hashtable and sort them Console.WriteLine( ); Console.WriteLine( ); ArrayList values = GetValues(hash); values.Sort( ); foreach (object obj in values) Console.WriteLine("Value: " + obj); // Reverse the sorted value list Console.WriteLine( ); values.Reverse( ); foreach (object obj in values) Console.WriteLine("Value: " + obj); }

The key/value pairs are displayed as shown:

Key: 1 Value: one Key: 2 Value: two Key: 3 Value: three Key: 4 Value: four Key: 5 Value: five Key: 5 Value: five Key: 4 Value: four Key: 3 Value: three Key: 2 Value: two Key: 1 Value: one Value: five Notice that the values are sorted alphabetically Value: four Value: one Value: three Value: two Value: two Value: three Value: one Value: four Value: five

Discussion

The Hashtable object exposes two useful properties for obtaining a collection of its keys or values. The Keys property returns an ICollection containing all the keys currently in the Hashtable . The Values property returns the same for all values currently contained in the Hashtable .

The GetKeys method uses the Keys property. Once the ICollection of keys is returned through this property, a new ArrayList is created to hold the keys. This ArrayList is then returned to the caller. The GetValues method works in a similar manner except that it uses the Values property.

The ICollection object returned either from the Keys or Values properties of a Hashtable object are direct references to the key and value collections within the Hashtable . This means that if the keys and/or values change in a Hashtable , the key and value collections will be altered accordingly .

See Also

See the "Hashtable Class" and "ArrayList Class" topics in the MSDN documentation.