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C# | Dictionary.Add() Method

Last Updated : 11 Jul, 2025
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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:

R

rupesh_rao
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