Indexer declaration in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Indexer declaration

Start

↓

Declare class with indexer

↓

Indexer get/set called with index

↓

Access or modify internal data

↓

Return or update value

↓

End

This flow shows how a class with an indexer lets you use array-like access to get or set values inside the class.

Execution Sample

C Sharp (C#)

class Sample {
  private int[] data = {10, 20, 30};
  public int this[int i] {
    get => data[i];
    set => data[i] = value;
  }
}

var s = new Sample();
Console.WriteLine(s[1]);
s[1] = 50;
Console.WriteLine(s[1]);

This code defines a class with an indexer to get and set values in an internal array using square brackets.

Execution Table

Step	Action	Index	Value Before	Value After	Output
1	Create Sample instance	-	-	-	-
2	Read s[1]	1	20	20	20
3	Set s[1] = 50	1	20	50	-
4	Read s[1]	1	50	50	50
5	End	-	-	-	-

💡 Finished accessing and modifying values using the indexer.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
data[0]	10	10	10	10	10
data[1]	20	20	50	50	50
data[2]	30	30	30	30	30

Key Moments - 2 Insights

Why can we use square brackets on the object s like s[1]?

Because the class Sample declares an indexer with 'this[int i]', it allows using square brackets to get or set values inside the object, as shown in steps 2 and 3 of the execution_table.

What happens if we try to access an index outside the array range?

An exception will occur because the indexer accesses the internal array directly without bounds checking. This is not shown in the current execution_table but is important to remember.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when reading s[1] at step 2?

A 20

B 50

C 10

D Error

💡 Hint

Check the Output column at step 2 in the execution_table.

At which step does the value at index 1 change in the data array?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the Value After column for data[1] in variable_tracker after each step.

If we remove the set accessor from the indexer, what would happen when executing step 3?

A It would update the value successfully.

B It would cause a compile-time error.

C It would throw a runtime exception.

D It would ignore the assignment silently.

💡 Hint

Remember that without a set accessor, the indexer is read-only and cannot assign values.

Concept Snapshot

Indexer declaration syntax:
public returnType this[type index] {
  get { ... }
  set { ... }
}

Allows objects to be accessed like arrays.
Get returns value at index.
Set updates value at index.
Useful for internal collections.

Full Transcript

This example shows how to declare and use an indexer in C#. The class Sample has a private array and an indexer that lets you get or set values using square brackets. When you create an instance and use s[1], it calls the get accessor returning the value at index 1. When you assign s[1] = 50, it calls the set accessor updating the value. The execution table traces these steps and shows how the internal array changes. Beginners often wonder why square brackets work on objects; it's because of the indexer declaration. Also, if you remove the set accessor, you cannot assign values, causing a compile error.