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

The ListDictionary class in C# provides a simple dictionary implementation using a singly linked list. It is optimized for small collections (typically fewer than 10 items) and belongs to the System.Collections.Specialized namespace. To add key-value pairs, use the Add() method.

Syntax

Following is the syntax for adding key-value pairs to a ListDictionary −

ListDictionary.Add(object key, object value);

Parameters

• key − The key to add to the ListDictionary. Cannot be null.

• value − The value associated with the key. Can be null.

Using Add() Method with String Values

The following example demonstrates adding string key-value pairs to a ListDictionary −

using System;
using System.Collections;
using System.Collections.Specialized;

public class Demo {
    public static void Main(){
        ListDictionary dict = new ListDictionary();
        dict.Add("1", "One");
        dict.Add("2", "Two");
        dict.Add("3", "Three");
        dict.Add("4", "Four");
        dict.Add("5", "Five");
        
        Console.WriteLine("ListDictionary key-value pairs...");
        IDictionaryEnumerator demoEnum = dict.GetEnumerator();
        while (demoEnum.MoveNext())
            Console.WriteLine("Key = " + demoEnum.Key + ", Value = " + demoEnum.Value);
    }
}

The output of the above code is −

ListDictionary key-value pairs...
Key = 1, Value = One
Key = 2, Value = Two
Key = 3, Value = Three
Key = 4, Value = Four
Key = 5, Value = Five

Using Add() Method with Integer Values

The following example shows adding string keys with integer values and displaying only the keys −

using System;
using System.Collections;
using System.Collections.Specialized;

public class Demo {
    public static void Main(){
        ListDictionary listDict = new ListDictionary();
        listDict.Add("1", 100);
        listDict.Add("2", 200);
        listDict.Add("3", 300);
        listDict.Add("4", 400);
        listDict.Add("5", 500);
        listDict.Add("6", 600);
        listDict.Add("7", 700);
        listDict.Add("8", 800);
        listDict.Add("9", 900);
        listDict.Add("10", 1000);
        
        ICollection col = listDict.Keys;
        Console.WriteLine("Display all the keys...");
        foreach(String s in col){
            Console.WriteLine(s);
        }
    }
}

The output of the above code is −

Display all the keys...
1
2
3
4
5
6
7
8
9
10

Using Add() Method with Mixed Data Types

ListDictionary accepts any object type as both keys and values. Here's an example with mixed data types −

using System;
using System.Collections;
using System.Collections.Specialized;

public class Demo {
    public static void Main(){
        ListDictionary dict = new ListDictionary();
        dict.Add("name", "John");
        dict.Add("age", 25);
        dict.Add("salary", 50000.50);
        dict.Add("isActive", true);
        
        Console.WriteLine("Mixed data types in ListDictionary:");
        foreach(DictionaryEntry entry in dict){
            Console.WriteLine("Key: {0}, Value: {1}, Type: {2}", 
                entry.Key, entry.Value, entry.Value.GetType().Name);
        }
    }
}

The output of the above code is −

Mixed data types in ListDictionary:
Key: name, Value: John, Type: String
Key: age, Value: 25, Type: Int32
Key: salary, Value: 50000.5, Type: Double
Key: isActive, Value: True, Type: Boolean

Key Rules

• Keys must be unique − adding a duplicate key throws an ArgumentException.

• Keys cannot be null, but values can be null.

• ListDictionary is not thread-safe for concurrent operations.

• Best performance for collections with fewer than 10 items.

Conclusion

The Add() in ListDictionary allows you to insert key-value pairs into the collection. ListDictionary is ideal for small datasets due to its simple linked-list implementation, providing efficient operations when working with fewer than 10 items.