Gets or Sets the element at the specified index in the List in C#

In C#, you can get or set elements at a specific index in a List<T> using the indexer property. This allows you to access and modify list elements using square bracket notation, similar to arrays.

Syntax

Following is the syntax for accessing elements by index −

// Getting an element
T element = list[index];

// Setting an element
list[index] = value;

Getting Elements by Index

The following example demonstrates how to retrieve elements from a List<string> using their index positions −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(string[] args) {
      List<string> list = new List<string>();
      list.Add("100");
      list.Add("200");
      list.Add("300");
      list.Add("400");
      list.Add("500");
      list.Add("600");
      list.Add("700");
      list.Add("800");
      list.Add("900");
      list.Add("1000");

      Console.WriteLine("Element at index 0 = " + list[0]);
      Console.WriteLine("Element at index 1 = " + list[1]);
      Console.WriteLine("Element at index 2 = " + list[2]);
      Console.WriteLine("Element at index 3 = " + list[3]);
      Console.WriteLine("Element at index 4 = " + list[4]);
      Console.WriteLine("Element at index 5 = " + list[5]);
      Console.WriteLine("Element at index 6 = " + list[6]);
      Console.WriteLine("Element at index 7 = " + list[7]);
   }
}

The output of the above code is −

Element at index 0 = 100
Element at index 1 = 200
Element at index 2 = 300
Element at index 3 = 400
Element at index 4 = 500
Element at index 5 = 600
Element at index 6 = 700
Element at index 7 = 800

Setting Elements by Index

You can also modify existing elements by assigning new values to specific index positions −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(string[] args) {
      List<string> list = new List<string>();
      list.Add("100");
      list.Add("200");
      list.Add("300");
      list.Add("400");
      list.Add("500");

      Console.WriteLine("Original element at index 3 = " + list[3]);
      
      // Setting a new value at index 3
      list[3] = "450";
      
      Console.WriteLine("Updated element at index 3 = " + list[3]);
      
      // Multiple updates
      list[0] = "150";
      list[4] = "550";
      
      Console.WriteLine("After multiple updates:");
      for (int i = 0; i < list.Count; i++) {
         Console.WriteLine("Element at index " + i + " = " + list[i]);
      }
   }
}

The output of the above code is −

Original element at index 3 = 400
Updated element at index 3 = 450
After multiple updates:
Element at index 0 = 150
Element at index 1 = 200
Element at index 2 = 300
Element at index 3 = 450
Element at index 4 = 550

Index Bounds and Error Handling

It's important to ensure that the index is within the valid range (0 to Count - 1). Accessing an invalid index throws an ArgumentOutOfRangeException −

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(string[] args) {
      List<int> numbers = new List<int> { 10, 20, 30 };
      
      Console.WriteLine("List count: " + numbers.Count);
      Console.WriteLine("Valid index range: 0 to " + (numbers.Count - 1));
      
      // Safe access
      if (2 < numbers.Count) {
         Console.WriteLine("Element at index 2: " + numbers[2]);
      }
      
      // Demonstrate bounds checking
      try {
         Console.WriteLine("Attempting to access index 5...");
         int value = numbers[5]; // This will throw an exception
      }
      catch (ArgumentOutOfRangeException ex) {
         Console.WriteLine("Error: " + ex.Message.Split('.')[0]);
      }
   }
}

The output of the above code is −

List count: 3
Valid index range: 0 to 2
Element at index 2: 30
Attempting to access index 5...
Error: Index was out of range

Conclusion

The indexer property in List<T> provides a convenient way to get and set elements using square bracket notation. Always ensure the index is within the valid range (0 to Count-1) to avoid runtime exceptions, and consider using bounds checking for safer code.