Hashtable

This C# class optimizes lookups. It computes a hash of each key you add, and then uses this hash code to look up the element very quickly.

This is an older .NET Framework type. It is slower than the generic Dictionary type. But if an old program uses Hashtable, it is helpful to know how to use this type.

ContainsKey

To start, we create a Hashtable. You will want to call ContainsKey on your Hashtable with the key contents. This method returns true if the key is found.

using System;
using System.Collections;

// Create a Hashtable.
Hashtable hashtable = new Hashtable();
hashtable.Add("bird", 1);
hashtable.Add("cat", 2);
hashtable.Add("dog", 10);

// See if the Hashtable contains this key.
bool result = hashtable.ContainsKey("bird");
Console.WriteLine(result);

// Test the Contains method.
result = hashtable.Contains("bird");
Console.WriteLine(result);

// Get value of cat key.
int value = (int)hashtable["bird"];
Console.WriteLine(value);
True
True
1

Foreach

The example adds 3 integer keys, with one string value each, to the Hashtable. You can loop through the Hashtables by using DictionaryEntry in a foreach-loop.

using System;
using System.Collections;

Hashtable hashtable = new Hashtable();
hashtable[1] = "One";
hashtable[2] = "Two";
hashtable[13] = "Thirteen";

foreach (DictionaryEntry entry in hashtable)
{
    Console.WriteLine("{0}, {1}", entry.Key, entry.Value);
}
13, Thirteen
2, Two
1, One

Multiple types

The example here adds string keys and int keys. Each of the key-value pairs has different types. You can put them all in the same Hashtable.

using System;
using System.Collections;

class Program
{
    static Hashtable GetHashtable()
    {
        Hashtable hashtable = new Hashtable();

        hashtable.Add(300, "Carrot");
        hashtable.Add("Area", 1000);
        return hashtable;
    }

    static void Main()
    {
        Hashtable hashtable = GetHashtable();

        string value1 = (string)hashtable[300];
        Console.WriteLine(value1);

        int value2 = (int)hashtable["Area"];
        Console.WriteLine(value2);
    }
}
Carrot
1000

Cast

You can use the as-operator to attempt to cast an object to a specific reference type. If the cast does not succeed, the result will be null.

using System;
using System.Collections;
using System.IO;

Hashtable hashtable = new Hashtable();
hashtable.Add(400, "Blazer");

// This cast will succeed.
string value = hashtable[400] as string;
if (value != null)
{
    Console.WriteLine(value);
}

// This cast won't succeed, but won't throw.
StreamReader reader = hashtable[400] as StreamReader;
if (reader != null)
{
    Console.WriteLine("Unexpected");
}

// You can get the object and test it.
object value2 = hashtable[400];
if (value2 is string)
{
    Console.Write("is string: ");
    Console.WriteLine(value2);
}
Blazer
is string: Blazer

Keys, values

We can loop over keys and values, or store them in an ArrayList. This example shows all the keys, then all the values, and then stores the keys in an ArrayList.

using System;
using System.Collections;

Hashtable hashtable = new Hashtable();
hashtable.Add(400, "Blaze");
hashtable.Add(500, "Fiery");
hashtable.Add(600, "Fire");
hashtable.Add(800, "Immolate");

// Display the keys.
foreach (int key in hashtable.Keys)
{
    Console.WriteLine(key);
}

// Display the values.
foreach (string value in hashtable.Values)
{
    Console.WriteLine(value);
}

// Put keys in an ArrayList.
ArrayList arrayList = new ArrayList(hashtable.Keys);
foreach (int key in arrayList)
{
    Console.WriteLine(key);
}
800       (First loop)
600
500
400
Immolate  (Second loop)
Fire
Fiery
Blaze
800       (Third loop)
600
500
400

Count, Clear

You can count the elements in a Hashtable with the Count property. The example also shows using the Clear method to erase all the Hashtable contents.

using System;
using System.Collections;

// Add four elements to Hashtable.
Hashtable hashtable = new Hashtable();
hashtable.Add(1, "x");
hashtable.Add(2, "y");
hashtable.Add(3, "z");
hashtable.Add(10, "q");

// Get Count of Hashtable.
int count = hashtable.Count;
Console.WriteLine(count);

// Clear the Hashtable.
hashtable.Clear();

// Get Count of Hashtable again.
Console.WriteLine(hashtable.Count);
4
0

Benchmark

We test the Hashtable collection against the Dictionary. The benchmark first populates an equivalent version of each collection.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;

const int _max = 1000000;
// Set up collections.
Hashtable hashtable = new Hashtable();
for (int i = 0; i <= 30000; i++)
{
    hashtable[i.ToString("00000")] = i;
}

var dictionary = new Dictionary<string, int>();
for (int i = 0; i <= 30000; i++)
{
    dictionary.Add(i.ToString("00000"), i);
}

// Version 1: use Hashtable.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!hashtable.ContainsKey("09999") || !hashtable.ContainsKey("30000"))
    {
        return;
    }
}
s1.Stop();

// Version 2: use Dictionary.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!dictionary.ContainsKey("09999") || !dictionary.ContainsKey("30000"))
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
66.99 ns    Hashtable
51.22 ns    Dictionary

Keys and values, notes

The Keys and Values public accessors return a collection of the keys and values in the Hashtable at the time they are accessed.

Hashtable is an older collection that is obsoleted by the Dictionary collection. Knowing how to use it is critical when maintaining older programs. These programs are important.