gistfile1.cs

public class FooRepository
{
	public void Find(int fooId, Action<Foo> foundFoo, Action noFooFound)
	{
		var foo = LookUpFooFromDatabase();
		if(foo == null)
		{
			noFooFound();
		}	
		foundFoo(foo);
	}
}

public class FooController 
{
	public void TheAction(int fooId)
	{
		fooRepository.Find(fooId, 
				   foo => //do something with foo, 
				   () => throw new Exception("couldn't find a foo"))
	}	

}