How to Create Dictionary in C# - Simple Guide

In C#, you create a dictionary using the Dictionary<TKey, TValue> class by specifying key and value types. You can initialize it with new Dictionary<TKey, TValue>() and add items using the Add method or indexer.

Syntax

The basic syntax to create a dictionary in C# is:

• Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>(); creates an empty dictionary.
• TKey is the type of keys (like strings or integers).
• TValue is the type of values stored.

You add items with dict.Add(key, value); or dict[key] = value;.

csharp

Dictionary<string, int> ages = new Dictionary<string, int>();

Example

This example shows how to create a dictionary, add items, and print them.

csharp

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        Dictionary<string, int> ages = new Dictionary<string, int>();
        ages.Add("Alice", 30);
        ages["Bob"] = 25;

        foreach (var person in ages)
        {
            Console.WriteLine($"{person.Key} is {person.Value} years old.");
        }
    }
}

Output

Alice is 30 years old.
Bob is 25 years old.

Common Pitfalls

Common mistakes when creating dictionaries include:

• Adding a key that already exists causes an exception.
• Using dict[key] = value; will overwrite existing values without error.
• Trying to access a key that does not exist throws a KeyNotFoundException.

To avoid errors, check if a key exists with ContainsKey before adding or accessing.

csharp

var dict = new Dictionary<string, int>();
dict.Add("key1", 1);
// dict.Add("key1", 2); // This throws an exception

dict["key1"] = 3; // This overwrites the value

if (dict.ContainsKey("key2"))
{
    Console.WriteLine(dict["key2"]);
}
else
{
    Console.WriteLine("Key not found.");
}

Output

Key not found.

Quick Reference

Operation	Code Example	Description
Create empty dictionary	var dict = new Dictionary();	Creates an empty dictionary with string keys and int values.
Add item	dict.Add("key", 100);	Adds a new key-value pair; throws if key exists.
Add or update item	dict["key"] = 100;	Adds or updates the value for the key.
Check key exists	dict.ContainsKey("key")	Returns true if the key is in the dictionary.
Access value	var val = dict["key"];	Gets the value for the key; throws if key missing.

Key Takeaways

Use Dictionary to create dictionaries with specific key and value types.

Add items with Add() or indexer syntax; Add() throws if key exists, indexer overwrites.

Check for keys with ContainsKey() to avoid exceptions when accessing values.

Accessing a missing key throws KeyNotFoundException, so always verify key presence.

Dictionaries store unique keys and allow fast lookup of values by key.