Indexer declaration in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an indexer that returns an int.

C Sharp (C#)

public int this[[1] index] { get { return index; } }

Drag options to blanks, or click blank then click option'

Aint

Bstring

Cbool

Ddouble

Attempts:

3 left

💡 Hint

Common Mistakes

Using a return type instead of parameter type inside the brackets.

Using a type that is not valid for an indexer parameter.

The indexer parameter must have a type, here it is int because the index is an integer.

2fill in blank

medium

Complete the code to declare a read-only indexer that returns a string.

C Sharp (C#)

public string this[int index] { [1] { return "value"; } }

Drag options to blanks, or click blank then click option'

Aset

Bprivate

Cinit

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using set instead of get for reading values.

Using invalid accessor keywords.

The get accessor is used to return a value from the indexer.

3fill in blank

hard

Fix the error in the indexer declaration by completing the code.

C Sharp (C#)

public int this[int index] { get { return [1]; } }

Drag options to blanks, or click blank then click option'

Athis

Bindex

Cvalue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Returning value inside a get accessor.

Returning this which is invalid here.

The indexer should return the parameter index to work correctly.

4fill in blank

hard

Fill both blanks to declare an indexer with get and set accessors.

C Sharp (C#)

public string this[int index] { [1] { return data[index]; } [2] { data[index] = value; } }

Drag options to blanks, or click blank then click option'

Aget

Bset

Cinit

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping get and set keywords.

Using invalid accessor names.

The get accessor returns the value, and the set accessor assigns a value.

5fill in blank

hard

Fill all three blanks to declare an indexer that returns the uppercase key if value is not empty.

C Sharp (C#)

public Dictionary<string, string> data = new Dictionary<string, string>();
public string this[[1] key] {
    get {
        return data.ContainsKey(key) && data[key] [2] "" ? key.[3]() : null;
    }
}

Drag options to blanks, or click blank then click option'

Astring

B!=

CToUpper

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter type like int.

Using == instead of != in the condition.

Using incorrect method name for uppercase conversion.

The indexer parameter type is string. The condition checks if the value is not empty using !=. The key is converted to uppercase with ToUpper().

Practice

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

easy

A. To declare a constant value inside the class

B. To allow objects to be accessed like arrays using square brackets

C. To create a new class instance

D. To define a method that returns the class name

Solution

1. Step 1: Understand what an indexer does

   An indexer lets you access elements in an object using square brackets, similar to arrays.

2. Step 2: Compare options with this definition

   Only To allow objects to be accessed like arrays using square brackets describes this behavior correctly.

3. Final Answer:

   To allow objects to be accessed like arrays using square brackets -> Option B

4. Quick Check:

   Indexer = access with [] [OK]

Hint: Indexers let objects act like arrays with [] access [OK]

Common Mistakes:

• Confusing indexers with methods
• Thinking indexers create new objects
• Mixing indexers with constants

2. Which of the following is the correct syntax to declare an indexer in C#?

easy

A. public int this[int index] get set;

B. public int indexer(int index) { get; set; }

C. public int this[index] { get; set; }

D. public int this[int index] { get; set; }

Solution

1. Step 1: Recall indexer syntax

   An indexer uses the keyword this with a parameter list in square brackets and defines get and/or set accessors inside braces.

2. Step 2: Check each option

   public int this[int index] { get; set; } matches the correct syntax exactly. public int indexer(int index) { get; set; } uses a method-like name, which is incorrect. public int this[index] { get; set; } misses the type in the parameter. public int this[int index] get set; misses braces and semicolons.

3. Final Answer:

   public int this[int index] { get; set; } -> Option D

4. Quick Check:

   Indexer syntax = this[type param] { get; set; } [OK]

Hint: Indexer syntax always uses 'this' with parameter in brackets [OK]

Common Mistakes:

• Using method name instead of 'this'
• Omitting parameter type
• Missing braces around get/set

3. What will be the output of the following code?

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

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

medium

A. 50

B. 20

C. 30

D. Compilation error

Solution

1. Step 1: Understand the indexer behavior

   The indexer accesses the private array data. Setting s[1] = 50; changes data[1] from 20 to 50.

2. Step 2: Check the output of Console.WriteLine(s[1])

   This prints the updated value at index 1, which is 50.

3. Final Answer:

   50 -> Option A

4. Quick Check:

   Indexer set changes value, output = 50 [OK]

Hint: Indexer set changes array value, get returns updated value [OK]

Common Mistakes:

• Assuming original value prints
• Confusing indexer with method call
• Expecting compilation error

4. Identify the error in this indexer declaration:

public string this[int index]
{
  get { return data[index]; }
  set data[index] = value;
}

medium

A. Parameter name cannot be 'index'

B. Indexer must return int, not string

C. Missing braces around set accessor body

D. Missing semicolon after get accessor

Solution

1. Step 1: Review accessor syntax

   Both get and set accessors must have braces enclosing their bodies.

2. Step 2: Locate the error

   The set accessor lacks braces around data[index] = value;, causing a syntax error.

3. Final Answer:

   Missing braces around set accessor body -> Option C

4. Quick Check:

   Accessor bodies need braces { } [OK]

Hint: Always use braces { } for get and set bodies [OK]

Common Mistakes:

• Omitting braces in set accessor
• Wrong return type assumption
• Thinking parameter name is restricted

5. You want to create a class that stores names and allows access by index using an indexer. Which of the following indexer declarations correctly supports both reading and writing names by index?

hard

A. public string this[int i] { get { return names[i]; } set { names[i] = value; } }

B. public string this[int i] { set { names[i] = value; } }

C. public string this[int i] { get { return names[i]; } }

D. public string this[string name] { get { return name; } set { name = value; } }

Solution

1. Step 1: Understand requirements for read/write access

   The indexer must have both get and set accessors to allow reading and writing by index.

2. Step 2: Evaluate each option

   public string this[int i] { get { return names[i]; } } only has get, so no writing. public string this[int i] { set { names[i] = value; } } only has set, so no reading. public string this[int i] { get { return names[i]; } set { names[i] = value; } } has both get and set with int index parameter, which is correct. public string this[string name] { get { return name; } set { name = value; } } uses a string parameter, which does not match the requirement to access by index.

3. Final Answer:

   public string this[int i] { get { return names[i]; } set { names[i] = value; } } -> Option A

4. Quick Check:

   Read/write indexer needs both get and set [OK]

Hint: Read/write indexers need both get and set accessors [OK]

Common Mistakes:

• Defining only get or only set
• Using wrong parameter type
• Confusing indexer parameter with property