Indexer with custom types in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an indexer in the class.

C Sharp (C#)

public string this[[1] index] { get { return "Value" + index; } }

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

Aint

Bstring

Cdouble

Dbool

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of int for the index parameter.

Forgetting to specify the parameter type.

The indexer uses an int parameter to access elements by integer index.

2fill in blank

medium

Complete the code to define a custom type used as an indexer parameter.

C Sharp (C#)

public struct [1] { public int Id; }

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

AIndex

BKey

CItem

DValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class instead of a struct.

Using a generic or unrelated name.

The struct Key is used as a custom type for the indexer parameter.

3fill in blank

hard

Fix the error in the indexer declaration using the custom type.

C Sharp (C#)

public string this[[1] key] { get { return "Value" + key.Id; } }

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

AKey

Bint

Cstring

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using int or string instead of the custom type.

Using object which does not have the Id property.

The indexer parameter must be of the custom type Key to access its Id property.

4fill in blank

hard

Fill both blanks to complete the indexer with get and set accessors using the custom type.

C Sharp (C#)

private Dictionary<[1], string> data = new Dictionary<[2], string>();
public string this[Key key] {
    get => data[key];
    set => data[key] = value;
}

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

AKey

Bint

Cstring

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using different types for the dictionary keys.

Using primitive types instead of the custom type.

The dictionary uses the custom type Key as its key type, so both blanks must be Key.

5fill in blank

hard

Fill all three blanks to complete the indexer that checks if the key exists before returning a value or default.

C Sharp (C#)

public string this[[1] key] {
    get {
        if (data.[2](key))
            return data[key];
        else
            return [3];
    }
}

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

AKey

BContainsKey

Cnull

DTryGetValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using TryGetValue incorrectly.

Returning an empty string instead of null.

Using wrong parameter type.

The indexer uses Key as parameter type, checks existence with ContainsKey, and returns null if not found.

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