Extension methods for built-in types 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 for a built-in type like string?

You declare a static method inside a static class, and the first parameter has the <code>this</code> keyword followed by the type you want to extend, for example: <br><pre>public static class StringExtensions {
  public static bool IsCapitalized(this string s) { ... }
}</pre>

intermediate

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

No, extension methods cannot access private or protected members of the type they extend. They only work with the public interface of the type.

intermediate

Why use extension methods instead of inheritance for built-in types?

Built-in types like <code>string</code> are sealed and cannot be inherited. Extension methods let you add functionality without inheritance or modifying the original type.

beginner

How do you call an extension method in your code?

You call an extension method as if it were an instance method on the object, for example: "hello".IsCapitalized(). The compiler translates this to a static method call behind the scenes.

What keyword must the first parameter of an extension method have?

A static

B ref

C public

D this

The first parameter of an extension method must be preceded by the keyword this to specify the type being extended.

Where must extension methods be declared?

A Inside a static class

B Inside any class

C Inside an interface

D Inside a sealed class

Extension methods must be declared inside a static class.

Can extension methods override existing methods of a type?

A No, they cannot override existing methods

B Yes, always

C Only if the method is virtual

D Only for built-in types

Extension methods cannot override existing methods; they only add new methods.

Which of these is a valid extension method signature for int?

A static bool IsEven(int this number)

B public static bool IsEven(this int number)

C public bool IsEven(int this number)

D public static bool IsEven(int number)

The correct syntax uses this before the type in the first parameter inside a static method.

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

A WordCount.text()

B WordCount(text)

C text.WordCount()

D text->WordCount()

Extension methods are called like instance methods on the variable.

Explain how to create and use an extension method for a built-in type in C#.

Think about how you add new methods without changing the original type.

Why are extension methods useful for built-in types like string or int?

Consider the limits of inheritance with built-in types.