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Showing content from https://www.geeksforgeeks.org/c-sharp/c-sharp-add-key-and-value-into-stringdictionary/ below:

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:

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

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