Depending on what you're really looking for, it might already be done for you. If you're trying to use an indexer on the Bars collection, it's already done for you::
Foo myFoo = new Foo();
Bar myBar = myFoo.Bars[1];
Or if you're trying to get the following functionality:
Foo myFoo = new Foo();
Bar myBar = myFoo[1];
Then:
public Bar this[int index]
{
get { return Bars[index]; }
}