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.

Practice

1.

What is the main purpose of an indexer in a C# class?

Choose the best description.

easy

A. To create a constructor for initializing objects.

B. To define a method that returns the size of a collection.

C. To allow objects to be accessed using array-like bracket syntax with custom keys.

D. To overload the + operator for custom types.

Solution

1. Step 1: Understand what an indexer does

   An indexer lets you use square brackets [] to access elements inside an object, similar to arrays.

2. Step 2: Identify the correct description

   Only To allow objects to be accessed using array-like bracket syntax with custom keys. describes this behavior correctly by mentioning bracket syntax and custom keys.

3. Final Answer:

   To allow objects to be accessed using array-like bracket syntax with custom keys. -> Option C

4. Quick Check:

   Indexer = bracket access [OK]

Hint: Indexers let you use [] on objects like arrays [OK]

Common Mistakes:

• Confusing indexers with constructors
• Thinking indexers define collection size
• Mixing indexers with operator overloading

2.

Which of the following is the correct syntax to declare an indexer in a C# class using a custom type Key as the index?

public class MyCollection {
    // Indexer declaration here
}

easy

A. public string this[Key key] { get; set; }

B. public string this(Key key) { get; set; }

C. public string this[Key] { get; set; }

D. public string this[Key key] => get; set;

Solution

1. Step 1: Recall indexer syntax

   Indexers use the keyword this followed by square brackets with the parameter type and name inside.

2. Step 2: Check each option

   public string this[Key key] { get; set; } correctly uses this[Key key] with get and set accessors. Others have syntax errors or wrong brackets.

3. Final Answer:

   public string this[Key key] { get; set; } -> Option A

4. Quick Check:

   Indexer syntax = this[Type name] { get; set; } [OK]

Hint: Indexer uses this[Type name] with get/set inside braces [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Omitting parameter name in brackets
• Using expression-bodied syntax incorrectly

3.

Given the following code, what will be the output?

public record Key(int Id);

public class Store {
    private Dictionary<Key, string> data = new();
    public string this[Key key] {
        get => data.TryGetValue(key, out var val) ? val : "Not found";
        set => data[key] = value;
    }
}

var store = new Store();
store[new Key(1)] = "Apple";
store[new Key(2)] = "Banana";
Console.WriteLine(store[new Key(1)]);
Console.WriteLine(store[new Key(3)]);

medium

A. Not found\nNot found

B. Apple\nBanana

C. Compilation error

D. Apple\nNot found

Solution

1. Step 1: Understand how the indexer accesses data

   The indexer uses a Dictionary with Key as key type. It returns the value if found, else "Not found".

2. Step 2: Check the keys used

   Keys with Id 1 and 2 are added. Accessing new Key(1) finds "Apple". Accessing new Key(3) is missing, so returns "Not found".

3. Final Answer:

   Apple Not found -> Option D

4. Quick Check:

   Key(1) found, Key(3) missing = Apple, Not found [OK]

Hint: Indexer uses dictionary lookup; missing keys return default [OK]

Common Mistakes:

• Assuming new Key(1) is different from stored Key(1)
• Expecting Banana for Key(3)
• Ignoring TryGetValue fallback

4.

Identify the error in the following code snippet that implements an indexer with a custom type Key:

public class DataStore {
    private Dictionary<Key, int> dict = new();
    public int this[Key key] {
        get { return dict[key]; }
        set { dict.Add(key, value); }
    }
}

medium

A. Using dict.Add in set causes exception if key exists; should use dict[key] = value.

B. Indexer must return string, not int.

C. Key type cannot be used as indexer parameter.

D. Missing constructor for DataStore class.

Solution

1. Step 1: Analyze the set accessor

   The set accessor uses dict.Add(key, value), which throws an exception if the key already exists.

2. Step 2: Suggest correct usage

   To update or add, use dict[key] = value; which overwrites existing or adds new key.

3. Final Answer:

   Using dict.Add in set causes exception if key exists; should use dict[key] = value. -> Option A

4. Quick Check:

   dict.Add throws if key exists; use dict[key] = value [OK]

Hint: Use dict[key] = value to avoid exceptions on set [OK]

Common Mistakes:

• Using Add instead of index assignment in set
• Thinking Key cannot be indexer parameter
• Confusing return types in indexer

5.

You want to create a class Matrix that uses a custom struct Position as the indexer key to store integer values. The Position struct has two int fields: Row and Col. Which approach correctly implements the indexer to allow getting and setting values by Position?

public struct Position {
    public int Row, Col;
    public Position(int r, int c) { Row = r; Col = c; }
}

public class Matrix {
    private Dictionary<Position, int> values = new();
    // Implement indexer here
}

hard

A. public int this[Position pos] { get => values[pos]; set => values[pos] = value; }

B. public int this[Position pos] { get => values.TryGetValue(pos, out var v) ? v : 0; set => values[pos] = value; }

C. public int this[Position pos] { get { return values[pos]; } set { values.Add(pos, value); } }

D. public int this[int row, int col] { get => values[new Position(row, col)]; set => values[new Position(row, col)] = value; }

Solution

1. Step 1: Consider safe get behavior

   Using TryGetValue with fallback 0 avoids exceptions if the position key is missing.

2. Step 2: Check set accessor correctness

   Using values[pos] = value; safely adds or updates the dictionary entry.

3. Step 3: Evaluate other options

   public int this[int row, int col] { get => values[new Position(row, col)]; set => values[new Position(row, col)] = value; } uses two parameters, not matching the question's single Position key. public int this[Position pos] { get { return values[pos]; } set { values.Add(pos, value); } } uses Add in set, which throws if key exists. public int this[Position pos] { get => values[pos]; set => values[pos] = value; }'s get accessor throws if key missing.

4. Final Answer:

   public int this[Position pos] { get => values.TryGetValue(pos, out var v) ? v : 0; set => values[pos] = value; } -> Option B

5. Quick Check:

   Safe get with TryGetValue + index assignment set [OK]

Hint: Use TryGetValue in get to avoid exceptions on missing keys [OK]

Common Mistakes:

• Using Add in set causing exceptions
• Not handling missing keys in get
• Using multiple parameters instead of custom type