Insert an element into the ArrayList at the specified index in C#

The ArrayList.Insert() method in C# allows you to insert an element at a specific index position in an ArrayList. This method shifts all existing elements from the specified index to the right, making room for the new element.

Syntax

Following is the syntax for the Insert() method −

arrayList.Insert(index, value);

Parameters

• index ? The zero-based index at which the new element should be inserted.
• value ? The object to insert into the ArrayList.

Using Insert() with String Elements

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList list = new ArrayList();
      list.Add("One");
      list.Add("Two");
      list.Add("Three");
      list.Add("Four");
      list.Add("Five");
      list.Add("Six");
      list.Add("Seven");
      list.Add("Eight");
      
      Console.WriteLine("ArrayList elements...");
      foreach(string str in list) {
         Console.WriteLine(str);
      }
      
      Console.WriteLine("ArrayList is read-only? = " + list.IsReadOnly);
      Console.WriteLine("Does the element Six in the ArrayList? = " + list.Contains("Six"));
      
      list.Insert(4, "Twelve");
      Console.WriteLine("ArrayList elements...UPDATED");
      foreach(string str in list) {
         Console.WriteLine(str);
      }
   }
}

The output of the above code is −

ArrayList elements...
One
Two
Three
Four
Five
Six
Seven
Eight
ArrayList is read-only? = False
Does the element Six in the ArrayList? = True
ArrayList elements...UPDATED
One
Two
Three
Four
Twelve
Five
Six
Seven
Eight

Using Insert() with Integer Elements

Example

using System;
using System.Collections;

public class Demo {
   public static void Main() {
      ArrayList arrList = new ArrayList();
      arrList.Add(100);
      arrList.Add(200);
      arrList.Add(300);
      arrList.Add(400);
      arrList.Add(500);
      
      Console.WriteLine("Display elements...");
      IEnumerator demoEnum = arrList.GetEnumerator();
      while (demoEnum.MoveNext()) {
         Object ob = demoEnum.Current;
         Console.WriteLine(ob);
      }
      
      arrList.Insert(4, 1000);
      Console.WriteLine("Display elements...UPDATED");
      demoEnum = arrList.GetEnumerator();
      while (demoEnum.MoveNext()) {
         Object ob = demoEnum.Current;
         Console.WriteLine(ob);
      }
   }
}

The output of the above code is −

Display elements...
100
200
300
400
500
Display elements...UPDATED
100
200
300
400
1000
500

How It Works

When you call Insert(index, value), the ArrayList automatically shifts all elements at the specified index and beyond one position to the right. This operation increases the Count property of the ArrayList by 1. If the index is equal to Count, the element is added to the end of the ArrayList, similar to using the Add() method.

Conclusion

The ArrayList.Insert() method provides a convenient way to add elements at specific positions within an ArrayList. It automatically handles the shifting of existing elements and maintains the ArrayList's capacity, making it useful for dynamic list manipulation in legacy C# code.