Indexers
Access objects with array-like syntax.
Indexers is a free C# Academy lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.
What Is an Indexer?
An indexer lets an object be accessed with [ ] syntax, like an array. You define it with this[...]. It is perfect for collection-like classes.
using System;
class Week
{
private string[] _days = { "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" };
public string this[int index] => _days[index];
}
class Program
{
static void Main()
{
var w = new Week();
Console.WriteLine(w[0]);
Console.WriteLine(w[6]);
}
}Indexer Syntax
An indexer looks like a property but uses this and takes a parameter list in brackets. It has get and optionally set.
using System;
class Scores
{
private int[] _data = new int[5];
public int this[int i]
{
get { return _data[i]; }
set { _data[i] = value; }
}
}
class Program
{
static void Main()
{
var s = new Scores();
s[0] = 90;
s[1] = 75;
Console.WriteLine(s[0] + ", " + s[1]);
}
}Read and Write Through the Indexer
With both a getter and setter, you can assign and read elements just like an array, while the class controls the underlying storage.
using System;
class Grid
{
private int[] _cells = new int[3];
public int this[int i]
{
get => _cells[i];
set => _cells[i] = value;
}
}
class Program
{
static void Main()
{
var g = new Grid();
for (int i = 0; i < 3; i++) g[i] = i * 10;
Console.WriteLine(g[0] + ", " + g[1] + ", " + g[2]);
}
}Validation in an Indexer
Because an indexer is just code, you can validate the index and throw a clear error for out-of-range access.
using System;
class SafeList
{
private int[] _data = new int[3];
public int this[int i]
{
get
{
if (i < 0 || i >= _data.Length)
throw new IndexOutOfRangeException("Bad index: " + i);
return _data[i];
}
set { _data[i] = value; }
}
}
class Program
{
static void Main()
{
var list = new SafeList();
list[1] = 42;
Console.WriteLine(list[1]);
}
}String-Keyed Indexers
Indexer parameters are not limited to int. A string key turns your class into a dictionary-like lookup.
using System;
using System.Collections.Generic;
class Config
{
private Dictionary<string, string> _map = new Dictionary<string, string>();
public string this[string key]
{
get => _map.ContainsKey(key) ? _map[key] : "(none)";
set => _map[key] = value;
}
}
class Program
{
static void Main()
{
var c = new Config();
c["theme"] = "dark";
Console.WriteLine(c["theme"]);
Console.WriteLine(c["missing"]);
}
}Multiple Parameters
An indexer can take more than one parameter, ideal for matrices or 2D grids accessed as matrix[row, col].
using System;
class Matrix
{
private int[,] _data = new int[2, 2];
public int this[int row, int col]
{
get => _data[row, col];
set => _data[row, col] = value;
}
}
class Program
{
static void Main()
{
var m = new Matrix();
m[0, 0] = 1;
m[1, 1] = 9;
Console.WriteLine(m[0, 0] + ", " + m[1, 1]);
}
}Read-Only Indexers
Omit the setter for a read-only indexer, exposing data without allowing external modification.
using System;
class Fibonacci
{
public int this[int n]
{
get
{
int a = 0, bv = 1;
for (int i = 0; i < n; i++)
{
int t = a + bv;
a = bv;
bv = t;
}
return a;
}
}
}
class Program
{
static void Main()
{
var fib = new Fibonacci();
Console.WriteLine(fib[7]);
}
}Indexers vs Properties
A property is accessed by name (obj.Name); an indexer is accessed by key (obj[key]). Use an indexer when the object behaves like a collection.
using System;
class Inventory
{
private int[] _stock = new int[4];
public int Total => _stock.Length; // property
public int this[int slot] // indexer
{
get => _stock[slot];
set => _stock[slot] = value;
}
}
class Program
{
static void Main()
{
var inv = new Inventory();
inv[2] = 15;
Console.WriteLine("Slot 2: " + inv[2] + ", Total slots: " + inv.Total);
}
}Overloaded Indexers
You can define multiple indexers with different parameter types, and the compiler picks the matching one.
using System;
using System.Collections.Generic;
class People
{
private List<string> _names = new List<string> { "Ann", "Bob", "Cara" };
public string this[int i] => _names[i];
public int this[string name] => _names.IndexOf(name);
}
class Program
{
static void Main()
{
var p = new People();
Console.WriteLine(p[1]);
Console.WriteLine(p["Cara"]);
}
}Iterating an Indexed Object
Combine an indexer with a count property to loop over a custom collection naturally.
using System;
class Ring
{
private int[] _data = { 10, 20, 30 };
public int Count => _data.Length;
public int this[int i] => _data[i % _data.Length];
}
class Program
{
static void Main()
{
var ring = new Ring();
for (int i = 0; i < 5; i++)
Console.Write(ring[i] + " ");
Console.WriteLine();
}
}Putting It Together
Indexers make wrapper classes feel like native collections, hiding storage details behind familiar bracket syntax.
using System;
using System.Collections.Generic;
class Phonebook
{
private Dictionary<string, string> _book = new Dictionary<string, string>();
public string this[string name]
{
get => _book.TryGetValue(name, out var num) ? num : "unknown";
set => _book[name] = value;
}
}
class Program
{
static void Main()
{
var pb = new Phonebook();
pb["Alice"] = "555-1234";
Console.WriteLine("Alice: " + pb["Alice"]);
Console.WriteLine("Zed: " + pb["Zed"]);
}
}Quick Check
Test your understanding of indexers.
Recap
An indexer uses this[...] to give a class array-like access. It supports get/set, any parameter type (int, string, multiple), validation, overloading, and read-only forms. Use indexers for collection-like classes; use named properties for distinct attributes.
using System;
class Demo
{
private int[] _v = new int[3];
public int this[int i]
{
get => _v[i];
set => _v[i] = value;
}
}
class Program
{
static void Main()
{
var d = new Demo();
d[0] = 100;
Console.WriteLine(d[0]);
}
}
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Frequently Asked Questions
Is the “Indexers” lesson free?
Yes — the full text of “Indexers” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C# Academy course, upgrade to CoddyKit PRO. The C# Academy course includes 4 lessons in total.
What will I learn in “Indexers”?
Access objects with array-like syntax. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start C# Academy?
No prior experience is required. C# Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.
How long does the “Indexers” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
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