Extension method syntax in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an extension method in C#?

An extension method is a special kind of static method that allows you to add new methods to existing types without modifying their source code or creating a new derived type.

beginner

How do you declare an extension method in C#?

You declare an extension method as a static method inside a static class. The first parameter has the keyword this before the type it extends.

intermediate

Why must the class containing extension methods be static?

Because extension methods are static methods, the class that holds them must also be static to group these methods logically and prevent instantiation.

beginner

Example: What does this extension method do?<br>

public static bool IsEven(this int number) { return number % 2 == 0; }

This method adds IsEven() to all integers. It returns true if the number is even (divisible by 2), otherwise false.

beginner

How do you use an extension method once it is declared?

You call it like a regular instance method on the type it extends. For example, if IsEven() extends int, you can call 5.IsEven().

Which keyword is required before the first parameter in an extension method?

A ref

B static

C public

D this

The this keyword before the first parameter tells the compiler this is an extension method for that type.

Where must extension methods be declared?

A Inside any class

B Inside a static class

C Inside an interface

D Inside a struct

Extension methods must be declared inside a static class.

What type of method is an extension method?

A Static method

B Abstract method

C Virtual method

D Instance method

Extension methods are static methods that appear as if they are instance methods.

How do you call an extension method on a string variable named text?

A text.Method()

B ExtensionClass.Method(text)

C Method(text)

D CallExtension(text)

You call an extension method like an instance method on the variable, e.g., text.Method().

Can extension methods access private members of the type they extend?

A Yes, always

B Only if in the same namespace

C No, they cannot

D Only if marked internal

Extension methods cannot access private members of the type they extend because they are static methods outside the type.

Explain how to write and use an extension method in C#.

Think about how you add a new method to an existing type without changing its code.

What are the benefits of using extension methods?

Consider how extension methods help when you want new features on types you don't own.