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C# | Dictionary.Add() Method - GeeksforGeeks

C# | Dictionary.Add() Method

Last Updated : 11 Jul, 2025

Dictionary<TKey,TValue>.Add() Method

is used to add a specified key and value to the dictionary.

Syntax:

public void Add (TKey key, TValue value);

Parameters:

key: It is the key of the element to add. value: It is the value of the element to add. The value can be null for reference types.

Exceptions:

ArgumentNullException : If the key is null.
ArgumentException : If an element with the same key already exists in the Dictionary.

Below are the programs to illustrate the use of

Dictionary<TKey,TValue>.Add() Method

Example 1: csharp


 // C# code to add the specified key
// and value into the Dictionary
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

        // Create a new dictionary 
        // of strings, with string keys.
        Dictionary<string, string> myDict = 
           new Dictionary<string, string>();

        // Adding key/value pairs in myDict
        myDict.Add("Australia", "Canberra");
        myDict.Add("Belgium", "Brussels");
        myDict.Add("Netherlands", "Amsterdam");
        myDict.Add("China", "Beijing");
        myDict.Add("Russia", "Moscow");
        myDict.Add("India", "New Delhi");

        // To get count of key/value
        // pairs in myDict
        Console.WriteLine("Total key/value pairs in"+
                    " myDict are : " + myDict.Count);

        // Displaying the key/value
        // pairs in myDict
        Console.WriteLine("The key/value pairs"+
                           " in myDict are : ");

        foreach(KeyValuePair<string, string> kvp in myDict)
        {
            Console.WriteLine("Key = {0}, Value = {1}",
                              kvp.Key, kvp.Value);
        }
    }
}

Output:

Total key/value pairs in myDict are : 6
The key/value pairs in myDict are : 
Key = Australia, Value = Canberra
Key = Belgium, Value = Brussels
Key = Netherlands, Value = Amsterdam
Key = China, Value = Beijing
Key = Russia, Value = Moscow
Key = India, Value = New Delhi

Example 2: csharp


 // C# code to add the specified 
// key and value into the Dictionary
using System;
using System.Collections.Generic;

class GFG {

    // Driver code
    public static void Main()
    {

        // Create a new dictionary of 
        // strings, with string keys.
        Dictionary<string, string> myDict = 
          new Dictionary<string, string>();

        // Adding key/value pairs in myDict
        myDict.Add("Australia", "Canberra");
        myDict.Add("Belgium", "Brussels");
        myDict.Add("Netherlands", "Amsterdam");
        myDict.Add("China", "Beijing");
        myDict.Add("Russia", "Moscow");
        myDict.Add("India", "New Delhi");

        // The Add method throws an
        // exception if the new key is
        // already in the dictionary.
        try {

            myDict.Add("Russia", "Moscow");
        }

        catch (ArgumentException) {

            Console.WriteLine("An element with Key "+
                     "= \"Russia\" already exists.");
        }
    }
}

Output:

An element with Key = "Russia" already exists.

Note:

A key cannot be null, but a value can be if TValue is a reference type.
If Count is less than the capacity, this method approaches an O(1) operation.
If the capacity must be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.

Reference:

https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.dictionary-2.add?view=netframework-4.7.2

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