How to Add to Dictionary in C#: Syntax and Examples

In C#, you add items to a Dictionary using the Add(key, value) method or by assigning a value to a key using dictionary[key] = value. The Add method throws an error if the key exists, while the indexer updates or adds the key-value pair.

Syntax

There are two main ways to add items to a Dictionary<TKey, TValue> in C#:

• Add(key, value): Adds a new key-value pair. Throws an exception if the key already exists.
• dictionary[key] = value: Adds or updates the value for the given key.

csharp

dictionary.Add(key, value);
dictionary[key] = value;

Example

This example shows how to create a dictionary, add items using both Add and the indexer, and print the contents.

csharp

using System;
using System.Collections.Generic;

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

        // Add using Add method
        dictionary.Add("apple", 3);

        // Add or update using indexer
        dictionary["banana"] = 5;
        dictionary["apple"] = 4; // updates value for "apple"

        foreach (var item in dictionary)
        {
            Console.WriteLine($"{item.Key}: {item.Value}");
        }
    }
}

Output

apple: 4
banana: 5

Common Pitfalls

Using Add with a key that already exists causes an ArgumentException. To avoid this, check if the key exists with ContainsKey before adding, or use the indexer to update or add safely.

csharp

var dictionary = new Dictionary<string, int>();
dictionary.Add("key1", 1);

// Wrong: Adding same key again throws exception
// dictionary.Add("key1", 2); // Throws ArgumentException

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

// Or use indexer to add or update
dictionary["key1"] = 2;

Quick Reference

Method	Description	Behavior if Key Exists
Add(key, value)	Adds a new key-value pair	Throws ArgumentException
dictionary[key] = value	Adds or updates the key-value pair	Updates existing value or adds new key

Key Takeaways

Use dictionary.Add(key, value) to add new items but avoid duplicate keys.

Use dictionary[key] = value to add or update items safely.

Check dictionary.ContainsKey(key) before adding with Add to prevent errors.

Indexer syntax is simpler for adding or updating dictionary entries.