Overload indexer to have foreach'able class

I tried to do something like this but this doesn't work:

    class Garage
    {
        private List<Car> cars = new List<Car>();

        public Car this[int i]
        {
            get { return cars[i]; }
        }
         //...
    }

       Garage g =  new Garage();
       //get CS1579 - no GetEnumerator definition
       foreach (Car c in g)
       {
           //...
       }

As MSDN says indexers can be overloaded, so I decided to ask experts here. How to overload indexers to cooperate with foreach loop?