What is an indexer in C#?

An indexer allows objects to be indexed like arrays using the 'this' keyword, enabling cleaner access to internal data.

How is an indexer defined in C#?

You define an indexer using 'this' with a parameter list and get/set accessors inside a class or struct.

What are Indexers in C#? | FullStackPrep.Dev

💡 Concept: Indexers

Indexers in C# allow objects of a class or struct to be indexed just like arrays. They are defined using the this keyword and enable intuitive access to internal collections or logic.

📘 Quick Intro

Instead of calling methods like GetValue(i), indexers allow you to use obj[i]. This makes class instances feel like collections and improves readability in data structures or APIs.

🧠 Analogy

Think of an indexer like a vending machine. You insert a slot number (index) and get a snack (value). Internally, logic determines what snack to return—but you only interact using a simple number.

🔧 Technical Explanation

🔹 Declared using the this keyword with a parameter list.
🔹 Can have both get and set accessors.
🔹 Can be overloaded based on parameter signatures.
🔹 Provide cleaner syntax for class consumers.
🔹 Common in wrapper classes around collections or custom containers.

🎯 Use Cases

✅ Wrapping collections with business rules.
✅ Exposing custom internal structures as if they were arrays.
✅ Implementing matrix-like structures with two-dimensional indexers.
✅ Providing multiple views into internal data (e.g., string indexer, int indexer).

💻 Code Example

public class Week
{
    private string[] days = { ""Sun"", ""Mon"", ""Tue"", ""Wed"", ""Thu"", ""Fri"", ""Sat"" };

    public string this[int index]
    {
        get => days[index];
        set => days[index] = value;
    }
}

// Usage
var week = new Week();
Console.WriteLine(week[1]); // Output: Mon

❓ Interview Q&A

Q1: What is an indexer?
A: A member that allows access to a class or struct instance using array-like syntax.

Q2: How do you define an indexer in C#?
A: Using the this keyword followed by a parameter inside square brackets.

Q3: Can indexers be overloaded?
A: Yes, with different parameter types or counts.

Q4: Can indexers be static?
A: No, indexers must be instance members.

Q5: Are indexers required to have both get and set?
A: No, you can have read-only or write-only indexers.

Q6: Can you use multiple parameters in indexers?
A: Yes, multi-dimensional indexers are allowed.

Q7: Is it possible to throw exceptions in indexers?
A: Yes, for example, when an index is out of range.

Q8: Are indexers inherited?
A: Yes, like methods and properties.

Q9: What is the return type of an indexer?
A: Any valid type—it’s defined like a property.

Q10: How are indexers different from arrays?
A: Arrays are data structures; indexers are access mechanisms within a class.

📝 MCQs

Q1. What keyword is used to define an indexer?

base
var
this
index

Q2. Can you use multiple parameters in an indexer?

No
Only with arrays
Yes
Only with structs

Q3. Are indexers required to be public?

Yes
No
Only static ones
Only virtual ones

Q4. Can you throw exceptions in an indexer?

No
Yes
Only get
Only set

Q5. What does an indexer return?

Only string
Only int
Nothing
Any defined type

Q6. Can indexers be abstract?

No
Yes
Only in structs
Only in arrays

Q7. Can you define a write-only indexer?

No
Yes
Only in sealed class
Only in base class

Q8. Which access modifier is valid for indexers?

Only public
Only private
Any valid modifier
None

Q9. Are indexers similar to which other member?

Methods
Fields
Properties
Events

Q10. Can indexers be virtual?

No
Yes
Only in interfaces
Only in records

💡 Bonus Insight

Indexers enhance usability by making your custom objects feel like arrays. They help encapsulate logic and improve API design for collection wrappers, caches, or matrix-like classes.

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What are Indexers in C#?