C# | Add key and value into StringDictionary

Last Updated : 11 Jul, 2025

StringDictionary.Add(String, String) method is used to add an entry with the specified key and value into the StringDictionary. Syntax:

public virtual void Add (string key, string value);

Parameters:

key: It is the key of the entry which is to be added.
value: It is the value of the entry which is to be added. The value can be null.

Exceptions:

ArgumentNullException : If the key is null.
ArgumentException : It is an entry with the same key already exists in the StringDictionary.
NotSupportedException : If the StringDictionary is read-only.

Below programs illustrate the use of StringDictionary.Add(String, String) method: Example 1:

CSHARP

// C# code to add key and value
// into the StringDictionary
using System;
using System.Collections;
using System.Collections.Specialized;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a StringDictionary named myDict
        StringDictionary myDict = new StringDictionary();

        // Adding key and value into the StringDictionary
        myDict.Add("A", "Apple");
        myDict.Add("B", "Banana");
        myDict.Add("C", "Cat");
        myDict.Add("D", "Dog");
        myDict.Add("E", "Elephant");

        // Displaying the keys and values in StringDictionary
        foreach(DictionaryEntry dic in myDict)
        {
            Console.WriteLine(dic.Key + "  " + dic.Value);
        }
    }
}

Output:

d  Dog
b  Banana
c  Cat
e  Elephant
a  Apple

Example 2:

CSHARP

// C# code to add key and value
// into the StringDictionary
using System;
using System.Collections;
using System.Collections.Specialized;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a StringDictionary named myDict
        StringDictionary myDict = new StringDictionary();

        // Adding key and value into the StringDictionary
        myDict.Add("A", "Apple");
        myDict.Add("B", "Banana");
        myDict.Add("C", "Cat");
        myDict.Add("D", "Dog");

        // It should raise "ArgumentException"
        // as an entry with the same key
        // already exists in the StringDictionary.
        myDict.Add("C", "Code");

        // Displaying the keys and values in StringDictionary
        foreach(DictionaryEntry dic in myDict)
        {
            Console.WriteLine(dic.Key + "  " + dic.Value);
        }
    }
}

Runtime Error:

Unhandled Exception: System.ArgumentException: Item has already been added. Key in dictionary: 'c' Key being added: 'c'

Note:

The key is handled in a case-insensitive manner i.e, it is translated to lowercase before it is added to the string dictionary.
This method is an O(1) operation.

Reference: