C# - Class Extension

Use class extensions to add additional logic to a class without modifying the class itself.

class User {
	public string Firstname;
	public string Lastname;
}

class Extensions {
	public string Fullname(this User user) {
		return $"{user.Firstname} {user.Lastname}";
	}
}

User user;
string name = user.Fullname();

Adds the method Fullname to a instance of User without needing to add the method to the User method.