How to Implement Hash Table in C# - Simple Guide

In C#, you can implement a hash table using the built-in Dictionary<TKey, TValue> class which stores key-value pairs with fast lookup. Alternatively, you can create a custom hash table by using an array of buckets and handling collisions with chaining or open addressing.

Syntax

The basic syntax to declare a hash table using the built-in Dictionary<TKey, TValue> is:

• Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>(); creates a new dictionary.
• dict.Add(key, value); adds a key-value pair.
• dict[key] accesses the value by key.

For a custom implementation, you define an array of buckets and implement methods to add, find, and remove items using a hash function.

csharp

Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("apple", 5);
int value = dict["apple"];

Example

This example shows how to use the built-in Dictionary<string, int> as a hash table to store and retrieve values by keys.

csharp

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Dictionary<string, int> hashTable = new Dictionary<string, int>();

        // Add key-value pairs
        hashTable.Add("apple", 10);
        hashTable.Add("banana", 20);
        hashTable.Add("orange", 30);

        // Access value by key
        Console.WriteLine("Value for 'apple': " + hashTable["apple"]);

        // Check if key exists
        if (hashTable.ContainsKey("banana"))
        {
            Console.WriteLine("Value for 'banana': " + hashTable["banana"]);
        }

        // Iterate over all key-value pairs
        foreach (var pair in hashTable)
        {
            Console.WriteLine(pair.Key + ": " + pair.Value);
        }
    }
}

Output

Value for 'apple': 10
Value for 'banana': 20
apple: 10
banana: 20
orange: 30

Common Pitfalls

Common mistakes when using hash tables in C# include:

• Trying to add a key that already exists causes an exception. Use ContainsKey to check before adding.
• Accessing a key that does not exist throws a KeyNotFoundException. Use TryGetValue to safely get values.
• Using mutable objects as keys can cause unexpected behavior because their hash code may change.

csharp

/* Wrong way: Adding duplicate key throws exception */
Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("key1", 1);
dict.Add("key1", 2); // Throws exception

/* Right way: Check before adding */
if (!dict.ContainsKey("key1"))
{
    dict.Add("key1", 2);
}

/* Safe access using TryGetValue */
if (dict.TryGetValue("key2", out int value))
{
    Console.WriteLine(value);
}
else
{
    Console.WriteLine("Key not found");
}

Quick Reference

Here is a quick summary of common Dictionary<TKey, TValue> methods:

Method	Description
Add(key, value)	Adds a new key-value pair; throws if key exists.
ContainsKey(key)	Checks if the key exists.
TryGetValue(key, out value)	Tries to get value safely without exception.
Remove(key)	Removes the key-value pair if it exists.
Clear()	Removes all key-value pairs.

Key Takeaways

Use C#'s built-in Dictionary for efficient hash table implementation.

Always check if a key exists before adding to avoid exceptions.

Use TryGetValue to safely access values without risking exceptions.

Avoid using mutable objects as keys to prevent hash code issues.

Dictionary provides fast lookup, insertion, and removal by key.