C# | Add the specified key and value into the ListDictionary

ListDictionary.Add(Object, Object) method is used to add an entry with the specified key and value into the ListDictionary.

Syntax:

public void Add (object key, object value);

Parameters:

key : The key of the entry to add.
value : The value of the entry to add. 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 ListDictionary.

Below given are some examples to understand the implementation in a better way:

Example 1:

// C# code to add an entry with
// the specified key and value
// into the ListDictionary
using System;
using System.Collections;
using System.Collections.Specialized;

class GFG {

    // Driver code
    public static void Main()
    {

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

        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");

        // Displaying the total number of elements in myDict
        Console.WriteLine("Total number of elements in myDict are : "
                                                      + myDict.Count);

        // Displaying the elements in ListDictionary myDict
        foreach(DictionaryEntry de in myDict)
        {
            Console.WriteLine(de.Key + " " + de.Value);
        }
    }
}

Output:

Total number of elements in myDict are : 6
Australia Canberra
Belgium Brussels
Netherlands Amsterdam
China Beijing
Russia Moscow
India New Delhi

Example 2:

// C# code to add an entry with
// the specified key and value
// into the ListDictionary
using System;
using System.Collections;
using System.Collections.Specialized;

class GFG {

    // Driver code
    public static void Main()
    {

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

        myDict.Add("Australia", "Canberra");
        myDict.Add("Belgium", "Brussels");

        // This should raise "ArgumentNullException"
        // as key is null
        myDict.Add(null, "Amsterdam");

        myDict.Add("China", "Beijing");
        myDict.Add("Russia", "Moscow");
        myDict.Add("India", "New Delhi");

        // Displaying the total number of elements in myDict
        Console.WriteLine("Total number of elements in myDict are : "
                                                     + myDict.Count);

        // Displaying the elements in ListDictionary myDict
        foreach(DictionaryEntry de in myDict)
        {
            Console.WriteLine(de.Key + " " + de.Value);
        }
    }
}

Output:

Unhandled Exception:
System.ArgumentNullException: Key cannot be null.
Parameter name: key

Note:

• An object that has no correlation between its state and its hash code value should typically not be used as the key. For example, String objects are better than StringBuilder objects for use as keys.
• This method is an O(n) operation, where n is Count.

Reference:

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