What's the best way to add custom functionality to existing code in C#

interface IFoo { void Bar();}
class Foo : IFoo {public void Bar() { /* imp 1 */ } }
class FooWrapper : IFoo {
    IFoo parent;
    public FooWrapper(IFoo parent) {this. parent = parent;}
    public void Bar() { /* imp 2, perhaps using "parent" }
}

public class BaseUser {

    public BaseUser() {}

    public bool login() {

        return false;

    }

}

public class User : BaseUser {}

User u = new User();
u.login();

public static int WeekNumber(this DateTime dtPassed)
    {
        CultureInfo ciCurr = CultureInfo.CurrentCulture;
        int weekNum = 0;
        try
        {
            weekNum = ciCurr.Calendar.GetWeekOfYear(dtPassed,
           CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Sunday);
        }
        catch (Exception ex)
        {
            //TODO: Add error handling code
        }

        return weekNum;
    }