Indexers in C#

Short, interview-ready summary.

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What is an Indexer?

An indexer lets an object be indexed like an array using [ ] syntax. Instead of calling GetItem(0), you write obj[0].

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Memory hook

"Indexer = this[key] — make your type indexable like array or Dictionary"

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Basic syntax

public class MyCollection
{
    private readonly string[] _items = new string[10];

    // Indexer: object[key]
    public string this[int index]
    {
        get => _items[index];
        set => _items[index] = value;
    }
}

var col = new MyCollection();
col[0] = "hello";
string s = col[0];

• this[...] — indexer declaration
• get / set — accessors (can be read-only with get only)
• Parameter — usually int (array-like) or string (dictionary-like); can have multiple params

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Multiple parameters

public class Matrix
{
    private readonly int[,] _data = new int[10, 10];

    public int this[int row, int col]
    {
        get => _data[row, col];
        set => _data[row, col] = value;
    }
}

var m = new Matrix();
m[2, 3] = 42;

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String (or other type) as key

public class DataStore
{
    private readonly Dictionary<string, string> _data = new();

    public string this[string key]
    {
        get => _data.TryGetValue(key, out var v) ? v : null;
        set => _data[key] = value;
    }
}

var store = new DataStore();
store["name"] = "Alice";

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Indexer vs property vs method

Feature	Indexer	Property	Method
Name	this[...]	Named (e.g. Count)	Named (e.g. GetItem)
Parameters	Required (key/index)	None	Optional
Access	obj[key]	obj.Count	obj.GetItem(0)

• Use indexer when the type is logically a collection or map and indexing is natural.
• Use property for a single, named value.
• Use method when you need logic beyond simple get/set or multiple overloads with different semantics.

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Read-only indexer

public string this[int index] => _items[index];

• No set — assignment like obj[0] = "x" is not allowed.

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Common use cases

• Custom collection — wrap array or list; expose [i]
• Dictionary-like access — key-value by [key]
• Matrix / grid — [row, col]
• Configuration — config["SettingName"]

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Interview one-liner

"An indexer lets an object be indexed with [ ] like an array or dictionary. It's declared with this[params] and has get/set accessors. Use it when your type is naturally indexable (collection, map, matrix)."

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Cheat sheet

this[key] = indexer; get/set accessors
Parameter = int, string, or multiple (e.g. row, col)
Read-only = get only, no set
Use when type is logically indexable