Indexer with custom types in C Sharp (C#)

Introduction

Indexers let you access objects like arrays using custom keys. Using custom types as keys makes your code clearer and more organized.

You want to access elements in a collection using a special key type, not just numbers.

You have a class that stores data identified by a custom object, like a coordinate or a name.

You want to make your class easier to use by allowing bracket [] access with your own key type.

Syntax

C Sharp (C#)

public ReturnType this[CustomKeyType key] {
    get { /* return value for key */ }
    set { /* set value for key */ }
}

The keyword this defines the indexer.

You can define get and set to read and write values.

Examples

Indexer using a custom Coordinate type as key.

C Sharp (C#)

public string this[Coordinate coord] {
    get { return data[coord]; }
    set { data[coord] = value; }
}

Indexer using a Person object as key to store ages.

C Sharp (C#)

public int this[Person person] {
    get { return ages[person]; }
    set { ages[person] = value; }
}

Sample Program

This program creates a Grid class with an indexer that uses a Coordinate record as the key. It stores and retrieves strings at specific coordinates.

C Sharp (C#)

using System;
using System.Collections.Generic;

public record Coordinate(int X, int Y);

public class Grid {
    private Dictionary<Coordinate, string> data = new();

    public string this[Coordinate coord] {
        get => data.ContainsKey(coord) ? data[coord] : "Empty";
        set => data[coord] = value;
    }
}

class Program {
    static void Main() {
        var grid = new Grid();
        grid[new Coordinate(0, 0)] = "Start";
        grid[new Coordinate(1, 2)] = "Tree";

        Console.WriteLine(grid[new Coordinate(0, 0)]);
        Console.WriteLine(grid[new Coordinate(1, 2)]);
        Console.WriteLine(grid[new Coordinate(2, 2)]); // no value set
    }
}

Important Notes

Make sure your custom key type implements equality correctly (like a record) so dictionary lookups work.

Indexers make your class feel like a collection with easy access syntax.

Summary

Indexers let you use bracket [] access on your classes.

You can use any custom type as the key for an indexer.

Implement get and set to control how values are accessed and stored.

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