C#.NET indexers — What kinds of types can declare an indexer and thus be indexed like an array?

Introduction / Context:
Indexers provide array-like access to instances. They are declared with the this keyword and one or more parameters. This question asks which categories in C# are eligible to define indexers.

Given Data / Assumptions:

• C# supports indexers on classes, structs, and interfaces.
• Properties and functions themselves are not entities that “hold” indexers; they are members.

Concept / Approach:
An indexer is a member of a type. The C# language allows an indexer declaration inside a class or a struct, and an indexer signature can also be declared in an interface (with accessors but without bodies). Properties and methods are not containers of indexers; rather, indexers are peers to them.

Step-by-Step Solution:

Verification / Alternative check:
Write minimal declarations in each construct. For interfaces, use signatures like int this[int i] { get; set; } with no bodies.

Why Other Options Are Wrong:

• B/D: Include “function” or “property,” which cannot contain indexers.
• C: Omits class even though it is valid.

Common Pitfalls:
Confusing an indexer with a property that returns an array; the latter is different and does not change indexing rules for the object itself.

Final Answer:
1, 3, 5