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How do you overload the '+' operator to support adding two custom objects together? Which operators can be overloaded in C#, and which ones (like && and ||) generally CANNOT be overloaded directly? How do you overload comparison operators like < and >, and what consistency rule must they maintain relative to Equals()? How do you define a custom implicit or explicit conversion operator between your type and another type? How would you implement a multi-dimensional indexer, like a Matrix class supporting matrix[row, col] syntax? How do you overload the true/false operators to make a custom type usable directly in an 'if' statement or as a boolean condition? How would you implement a fluent, chainable custom '+' operator overload for building an immutable Vector3 math library, and what performance consideration applies for structs? How do you implement the 'checked' operator overload variant (introduced in C# 11) to provide overflow-checking behavior for a custom numeric type? How would you use an indexer together with a custom enumerator (implementing IEnumerable) to build a fully iterable, indexable custom collection class?
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C# › Indexers & Operator Overloading

What is an indexer, and how does it let a custom class support square-bracket syntax like an array?

Beginner

Answer

An indexer is a special property-like member defined with 'this[...]' syntax, letting instances of your class be accessed using square brackets (obj[key]) just like an array or dictionary — you define get/set accessors just as with a normal property.
public class WeekSchedule {
  private readonly string[] _days = new string[7];
  public string this[int dayIndex] {
    get => _days[dayIndex];
    set => _days[dayIndex] = value;
  }
}
var schedule = new WeekSchedule();
schedule[0] = "Meeting";
Console.WriteLine(schedule[0]); // "Meeting"
Real-world example Building a custom collection-like class (e.g., a Matrix or Grid) that should support natural array-style access syntax.

Common follow-ups: Can an indexer accept a non-integer parameter type, like a string, the way Dictionary does?

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All 10 Indexers & Operator Overloading questions