C#

using System;

class Program
{
    static void Main(string[] args)
    {
        A a = new A();
        a.PerformFoo();
        a.UseDefault = false;
        a.PerformFoo();
        Console.ReadKey();
    }
}

public class A
{
    private delegate void FooDelegate();
    private FooDelegate fooToUse;
    private bool _UseDefault;

    public A()
    {
        fooToUse = DefaultFoo;
    }

    public bool UseDefault
    {
        get { return _UseDefault; }
        set
        {
            _UseDefault = value;
            if (_UseDefault)
                fooToUse = DefaultFoo;
            else
                fooToUse = OtherFoo;
        }
    }

    private void DefaultFoo()
    {
        Console.WriteLine("Default");
    }
    private void OtherFoo()
    {
        Console.WriteLine("Other");
    }
    public void PerformFoo()
    {
        fooToUse.Invoke();
    }
}