Indexer with custom types in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of indexer accessing custom type property

What is the output of this C# code?

C Sharp (C#)

public class Person {
    public string Name { get; set; }
    public Person(string name) { Name = name; }
}

public class Group {
    private Person[] people = { new Person("Alice"), new Person("Bob"), new Person("Charlie") };
    public Person this[int index] {
        get { return people[index]; }
        set { people[index] = value; }
    }
}

var group = new Group();
Console.WriteLine(group[1].Name);

A IndexOutOfRangeException

B Charlie

C Bob

D Alice

💡 Hint

Remember that arrays are zero-based indexed.

The indexer returns the Person object at the given index. Index 1 corresponds to the second element, which is "Bob".

❓ Predict Output

intermediate

Indexer set accessor effect on custom type array

What will be printed after running this code?

C Sharp (C#)

public class Item {
    public int Value { get; set; }
    public Item(int value) { Value = value; }
}

public class Container {
    private Item[] items = { new Item(10), new Item(20), new Item(30) };
    public Item this[int index] {
        get => items[index];
        set => items[index] = value;
    }
}

var container = new Container();
container[1] = new Item(99);
Console.WriteLine(container[1].Value);

A 99

B 20

C 10

D NullReferenceException

💡 Hint

Check what happens when you assign a new Item to the indexer.

The set accessor replaces the Item at index 1 with a new Item whose Value is 99. So printing container[1].Value outputs 99.

🔧 Debug

advanced

Identify the error in this indexer implementation

What error will this code produce when compiled?

C Sharp (C#)

public class Box {
    private int[] data = new int[3];
    public int this[int index] {
        get { return data[index]; }
        set { data[index] = value; }
    }
}

A SyntaxError: Cannot use parentheses to index an array

B IndexOutOfRangeException at runtime

C No error, compiles and runs fine

D NullReferenceException at runtime

💡 Hint

Look carefully at how the array is accessed in the set accessor.

In C#, arrays are indexed with square brackets [], not parentheses (). Using parentheses causes a syntax error.

❓ Predict Output

advanced

Output of indexer with custom struct type

What is the output of this code?

C Sharp (C#)

public struct Point {
    public int X, Y;
    public Point(int x, int y) { X = x; Y = y; }
    public override string ToString() => $"({X},{Y})";
}

public class Polygon {
    private Point[] points = { new Point(1,2), new Point(3,4), new Point(5,6) };
    public Point this[int index] {
        get => points[index];
        set => points[index] = value;
    }
}

var poly = new Polygon();
poly[2] = new Point(7,8);
Console.WriteLine(poly[2]);

A Compilation error

B (3,4)

C (5,6)

D (7,8)

💡 Hint

Remember structs are value types and can be replaced via indexer.

The indexer set replaces the point at index 2 with (7,8). Printing poly[2] calls ToString() and outputs "(7,8)".

🧠 Conceptual

expert

Why use indexers with custom types?

Which of the following is the best reason to implement an indexer with a custom type in C#?

A To force all elements to be immutable and prevent changes

B To allow accessing elements of a collection-like class using array syntax for better readability and encapsulation

C To automatically serialize the class to JSON without extra code

D To replace the need for constructors in the class

💡 Hint

Think about what indexers provide in terms of syntax and usage.

Indexers let you access objects like arrays, improving code readability and hiding internal data structures. They do not enforce immutability, serialization, or replace constructors.