The interfaces themselves do not contain data. Although you may see IEnumerable as a type when using a collection, the underlying object is always of a specific type implementing this interface.
The object actually containing your data may be of any type implementing that interface. Please refer to:
And if you would like to implement your own version of IEnumerable, inherit from the interface . Try searching for "A Beginners Tutorial on Implementing IEnumerable" on codeproject ;-)