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.