Indexer with custom types in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Indexer with custom types

Create class with custom type

↓

Define private storage (e.g., array)

↓

Define indexer method: get/set

↓

Use indexer to access elements

↓

Get or set custom type elements

↓

End

The flow shows creating a class with a private storage of custom types, defining an indexer to get or set elements by index, and then using that indexer to access or modify elements.

Execution Sample

C Sharp (C#)

class Box {
  public string Content;
}

class Container {
  private Box[] boxes = new Box[2];
  public Box this[int i] {
    get => boxes[i];
    set => boxes[i] = value;
  }
}

var c = new Container();
c[0] = new Box { Content = "Apple" };
var fruit = c[0].Content;

This code defines a Container class with an indexer to get/set Box objects by index, then stores a Box with content "Apple" at index 0 and retrieves its content.

Execution Table

Step	Action	Indexer Call	Index	Value Set/Get	Result/Output
1	Create Container instance	N/A	N/A	N/A	Container object created with boxes array of size 2 (all null)
2	Set c[0]	set	0	Box { Content = "Apple" }	boxes[0] now holds Box with Content "Apple"
3	Get c[0]	get	0	N/A	Returns Box with Content "Apple"
4	Access Content property	N/A	N/A	N/A	"Apple" string retrieved from Box
5	End	N/A	N/A	N/A	Execution complete

💡 Indexer calls complete; program ends after retrieving content.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
c	null	Container instance with boxes[null, null]	Same instance	Same instance
boxes	null	[Box{Content="Apple"}, null]	[Box{Content="Apple"}, null]	[Box{Content="Apple"}, null]
fruit	undefined	undefined	"Apple"	"Apple"

Key Moments - 2 Insights

Why do we use an indexer instead of a normal method to access boxes?

The indexer lets us use array-like syntax (c[0]) to get or set elements, making code cleaner and more intuitive, as shown in execution_table steps 2 and 3.

What happens if we try to get an index that was never set?

The indexer returns the default value in the array, which is null for reference types like Box. This is implied in step 1 where boxes start as null.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of boxes[0] after step 2?

A null

B Box with Content "Apple"

C Empty Box

D String "Apple"

💡 Hint

Check the 'Value Set/Get' and 'Result/Output' columns in step 2.

At which step is the indexer 'get' accessor called?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Indexer Call' column to find where 'get' is used.

If we set c[1] to a new Box with Content "Orange", what changes in variable_tracker after step 2?

A boxes remains [Box{Content="Apple"}, null]

B fruit changes to "Orange"

C boxes becomes [Box{Content="Apple"}, Box{Content="Orange"}]

D c becomes null

💡 Hint

Think about how setting an indexer updates the boxes array.

Concept Snapshot

Indexer with custom types:
- Define private storage (e.g., array of custom objects)
- Use 'public Type this[int index]' with get and set
- Access elements like array: obj[index]
- Enables clean syntax to get/set custom objects
- Useful for container-like classes

Full Transcript

This example shows how to create a class with an indexer that works with custom types. We start by defining a class Container that holds an array of Box objects. The indexer allows us to get or set Box objects by index using array-like syntax. When we create a Container instance, its boxes array starts with nulls. Setting c[0] to a new Box with Content "Apple" stores that Box at index 0. Getting c[0] returns that Box, and accessing its Content property gives us the string "Apple". The execution table traces these steps, showing how the indexer is called for get and set, and how variables change. Key points include understanding why indexers simplify access and what happens if an index is not set. The quiz questions reinforce these ideas by asking about variable states and indexer calls.