C# Cookbook

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 ...