Indexer declaration in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this indexer usage?

Consider the following C# class with an indexer. What will be printed when the program runs?

C Sharp (C#)

using System;

class Sample {
    private string[] data = {"zero", "one", "two"};
    public string this[int index] {
        get { return data[index]; }
        set { data[index] = value; }
    }
}

class Program {
    static void Main() {
        Sample s = new Sample();
        s[1] = "ONE";
        Console.WriteLine(s[1]);
    }
}

A two

B one

C zero

D ONE

💡 Hint

Think about what happens when you assign a new value to s[1].

The indexer allows accessing the data array by index. Assigning s[1] = "ONE" changes the second element. So printing s[1] outputs "ONE".

❓ Predict Output

intermediate

What error does this indexer code produce?

What error will this C# code produce when compiled?

C Sharp (C#)

class Test {
    private int[] values = new int[3];
    public int this[int index] {
        get { return values[index]; }
        // Missing set accessor
    }
}

class Program {
    static void Main() {
        Test t = new Test();
        t[0] = 10;
    }
}

A Compile-time error: Property or indexer cannot be assigned to -- it is read only

B Runtime error: Index out of range

C No error, outputs 10

D Compile-time error: Missing get accessor

💡 Hint

Check if the indexer allows setting a value.

The indexer only has a get accessor, so it is read-only. Trying to assign t[0] = 10 causes a compile-time error.

🔧 Debug

advanced

Why does this indexer code cause a runtime error?

Examine the code below. Why does it throw an exception at runtime?

C Sharp (C#)

class Container {
    private int[] arr = new int[2];
    public int this[int index] {
        get {
            if (index < 0 || index >= arr.Length) throw new System.IndexOutOfRangeException();
            return arr[index];
        }
        set {
            arr[index] = value;
        }
    }
}

class Program {
    static void Main() {
        Container c = new Container();
        c[2] = 5;
    }
}

A Because the get accessor throws an exception for index 2

B Because the set accessor does not check index bounds before assignment

C Because the array size is zero

D Because the indexer is missing a set accessor

💡 Hint

Look at how the set accessor handles the index.

The set accessor assigns arr[index] = value without checking if index is valid. Since arr has length 2, index 2 is out of range, causing a runtime exception.

📝 Syntax

advanced

Which option correctly declares a string indexer?

Which of the following C# indexer declarations is syntactically correct for indexing by a string key?

A public string this(key string) { get { return dict[key]; } set { dict[key] = value; } }

B public string this[string key] get { return dict[key]; } set { dict[key] = value; }

C public string this[string key] { get { return dict[key]; } set { dict[key] = value; } }

D public string this[string key] => dict[key];

💡 Hint

Remember the syntax for indexers uses square brackets and braces.

Option C correctly uses square brackets for the parameter and braces for get/set accessors. Option C uses wrong parentheses syntax. Option C misses braces around get/set. Option C uses expression body but lacks set accessor.

🚀 Application

expert

What is the output of this complex indexer example?

Given the following C# code, what will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class MultiIndexer {
    private Dictionary<(int, int), string> data = new();

    public string this[int x, int y] {
        get => data.TryGetValue((x, y), out var val) ? val : "none";
        set => data[(x, y)] = value;
    }
}

class Program {
    static void Main() {
        MultiIndexer mi = new MultiIndexer();
        mi[1, 2] = "A";
        mi[3, 4] = "B";
        Console.WriteLine(mi[1, 2]);
        Console.WriteLine(mi[2, 3]);
    }
}

A

A
none

B

none
none

C

A
B

D

B
none

💡 Hint

Check which keys are set and which are not.

The indexer uses a dictionary keyed by a tuple (x,y). Only (1,2) and (3,4) are set. Accessing (1,2) returns "A". Accessing (2,3) returns "none" because it is not set.