Adding an extension method to a class in C#

Extension methods can be used to add static methods to an existing class or type without modifying the original copy of it.

Although they are static, they are called as part of the original instance by importing the namespace where the extension method exists.

You can find out more about extension methods on the Microsoft website.

We have this CategoryService class:

public class CategoryService {
	public List<string> GetAllCategoryNames() {	
		return new List<string>();	

And we have this CategoryHelper static class that has a GetAllCategoryIds method added.

public static class CategoryHelper {
	public static List<int> GetAllCategoryIds() {	
		return new List<int>();	

We want to add the GetAllCategoryIds method to the CategoryService as an extension method. How do we do that?