YouTip

C# Collections

In C#, a Hashtable is a collection that stores data in the form of key-value pairs (key=>value), where both the key and the value can be any object.

Each item in a Hashtable has a key=>value pair. The key is used to access the item in the collection.

Hashtables organize and access data based on a hashing algorithm, providing efficient lookup, insertion, and deletion operations.

Hashtable is a non-generic collection located in the System.Collections namespace. If a generic version is needed, you can use Dictionary<TKey, TValue>.

Basic Structure:

• Key (Key): Used to identify each data item. It must be unique and cannot be null.
• Value (Value): The data associated with the key. It can be null.

Characteristics:

• Key-Value Pair Storage: Each key is unique and associated with a value.
• Hashing Algorithm: Locates data storage positions using the key's hash code. The average time complexity for lookup and insertion operations is O(1).
• Dynamic Capacity Adjustment: When the number of elements exceeds the capacity, the table dynamically expands and reallocates storage.
• Unordered Storage: The storage order of key-value pairs is unrelated to the insertion order and is determined by the hash codes.

The following table lists some common properties of the Hashtable class:

Property Name	Type	Description
Count	int	Gets the number of key-value pairs contained in the Hashtable.
IsReadOnly	bool	Indicates whether the Hashtable is read-only.
IsFixedSize	bool	Indicates whether the Hashtable has a fixed size.
IsSynchronized	bool	Indicates whether the Hashtable is thread-safe.
Keys	ICollection	Gets a collection containing all the keys in the Hashtable.
Values	ICollection	Gets a collection containing all the values in the Hashtable.
SyncRoot	object	Gets an object that can be used to synchronize access to the Hashtable.

The following table lists some common methods of the Hashtable class:

Method Name	Return Type	Description
Addition and Removal
Add(object key, object value)	void	Adds the specified key-value pair to the Hashtable.
Clear()	void	Removes all elements from the Hashtable.
Remove(object key)	void	Removes the element with the specified key.
Querying and Accessing
Contains(object key)	bool	Determines whether the Hashtable contains a specific key.
ContainsKey(object key)	bool	Determines whether the Hashtable contains a specific key (equivalent to Contains).
ContainsValue(object value)	bool	Determines whether the Hashtable contains a specific value.
Copying and Enumeration
CopyTo(Array array, int index)	void	Copies the Hashtable elements to an Array, starting at the specified index.
GetEnumerator()	IDictionaryEnumerator	Returns an enumerator for the Hashtable, used to iterate through the key-value pairs.

The following example demonstrates the concept of a Hashtable:

Example

using System;
using System.Collections;

namespace CollectionsApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            Hashtable ht = new Hashtable();

            ht.Add("001", "Zara Ali");
            ht.Add("002", "Abida Rehman");
            ht.Add("003", "Joe Holzner");
            ht.Add("004", "Mausam Benazir Nur");
            ht.Add("005", "M. Amlan");
            ht.Add("006", "M. Arif");
            ht.Add("007", "Ritesh Saikia");

            if (ht.ContainsValue("Nuha Ali"))
            {
                Console.WriteLine("This student name is already in the list");
            }
            else
            {
                ht.Add("008", "Nuha Ali");
            }

            // Get the collection of keys
            ICollection key = ht.Keys;

            foreach (string k in key)
            {
                Console.WriteLine(k + ": " + ht);
            }

            Console.ReadKey();
        }
    }
}

When the above code is compiled and executed, it produces the following result:

007: Ritesh Saikia
004: Mausam Benazir Nur
005: M. Amlan
008: Nuha Ali
002: Abida Rehman
003: Joe Holzner
001: Zara Ali
006: M. Arif

Comparison with Other Collections

Feature	Hashtable	Dictionary<TKey, TValue>
Type Safety	No (non-generic, stores object type)	Yes (generic, strongly typed constraints)
Performance	Slower (requires boxing and unboxing operations)	Faster (no boxing, generic directly supports types)
Key Uniqueness	Yes	Yes
Value Can Be null	Yes	Yes
Ordering	None	None
Thread Safety	Not thread-safe by default	Not thread-safe by default

• Hashtable is an efficient key-value pair collection, suitable for scenarios requiring fast storage and retrieval of key-value pairs.
• Its non-generic design makes it inferior to generic collections (like Dictionary<TKey, TValue>) in terms of type safety and performance.
• In modern C# development, Dictionary<TKey, TValue> is more commonly used, but Hashtable is still suitable for simple applications or scenarios requiring compatibility with legacy code.

C# Collections