A hashtable is a collection that uses key/value pairs and the key is subsequently used to create a hash code which is used in the organization of the key/value pairs.

There are certain points one must remember when dealing with hash tables:

  • Keys cannot have a null value, but values can.

  • Keys have to be unique in a hashtable. You cannot have duplicate keys.

  • DictionaryEntry is used to store the contents of a hashtable (key/value pairs).

  • You can insert and store in a hashtable elements that are either the same or different types.

  • A hashtable is as large as the number of elements contained within it.

Creating a hashtable

Import the System.Collections namespace:

using System.Collections;

Create a hashtable:

Hashtable nameOfHashtable = new Hashtable();

Add elements to a hashtable

We use the Add method in order to insert elements into a hashtable:

Hashtable hashTableExample = new Hashtable();

hashTableExample.Add("LL", "Lower Level");
hashTableExample.Add("S1", "Suite Level 1");
hashTableExample.Add("P1", "Penthouse Level 1");

Remove elements from a hashtable

We use the Remove method in order to remove a single element at a time from the hashtable using the key.


We use the Clear method in order to clear the entire hashtable:


Check if a key exists

The Contains and ContainsKey methods can be used to check whether a key exists:

hashTableExample.Contains("LL"); //True
hashTableExample.ContainsKey("S1"); //False

Check if a value exists

The ContainsValue method can be used to check whether a value exists:

hashTableExample.ContainsValue("Lower Level"); //True

Output elements of a hashtable

In order to output the elements of a hashtable we use a foreach loop and loop over the hashtable as a dictionary:

foreach(DictionaryEntry element in hashTableExample)
    Console.WriteLine("{0} and {1} ", element.Key, element.Value);
/* P1 and Penthouse Level 1 
 * S1 and Suite Level 1 
 * LL and Lower Level