C# Hashtable

C# collections

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

Each item in the hash table has akey=>valueYes, the key is used to access items in the collection.

Hash tables organize and access data based on hash algorithms, providing efficient lookup, insertion, and deletion operations.

Hashtableis a non-generic collection, located in the namespaceSystem.Collectionsin. If a generic version is needed, you can useDictionary<TKey, TValue>。

Basic structure:

Key（Key): used to identify each data item, must be unique and cannot benull。
Value（Value): the data corresponding to the key, which can benull。

Features:

Key-value pair storage: each key is unique and associated with a value.
Hash algorithm: locates data storage positions via the key's hash code, with an average time complexity of O(1) for lookup and insertion operations.
Dynamically adjust capacity: when the number of elements in the hash table exceeds capacity, the table dynamically expands and reallocates storage.
Unordered storage: the storage order of key-value pairs is independent of insertion order and is determined by the hash code.

Methods and properties of the Hashtable class

The table below listsHashtableSome commonly used ones of the classProperty：

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

The table below listsHashtableSome commonly used ones of the classMethods：

Method Name	Return Type	Description
Adding and removing
`Add(object key, object value)`	`void`	Adds the specified key-value pair to the hash table.
`Clear()`	`void`	Clear the hash table.
`Remove(object key)`	`void`	Removes the key-value pair with the specified key.
Query and access
`Contains(object key)`	`bool`	Determines whether the hash table contains the specified key.
`ContainsKey(object key)`	`bool`	Determines whether the hash table contains the specified key (equivalent to`Contains`）。
`ContainsValue(object value)`	`bool`	Determines whether the hash table contains the specified value.
Copying and enumerating
`CopyTo(Array array, int index)`	`void`	Copies the elements of the hash table to an array, starting at the specified index.
`GetEnumerator()`	`IDictionaryEnumerator`	Returns an enumerator for the hash table, used to iterate through key-value pairs.

Example

The following example demonstrates the concept of the 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[k]);
}
Console.ReadKey();
}
}
}

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

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

Features	`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 operations, generics directly support types)
Uniqueness of keys	Yes	Yes
Whether the value can be`null`	Yes	Yes
Sorting	None	None
Thread Safety	Not thread-safe by default	Not thread-safe by default

HashtableIs an efficient key-value pair collection, suitable for scenarios requiring fast storage and retrieval of key-value pairs.
non-genericIts design makes it inferior to generic collections in terms of type safety and performance (such asDictionary<TKey, TValue>）。
In modern C# development,Dictionary<TKey, TValue>More commonly used, butHashtableIt is still suitable for simple applications or scenarios compatible with legacy code.

C# collections

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