Does a IEnumerable store objects after use

Question

If I have a very large IEnumerable collection, in the order of millions of objects, that would be too large to load all at once. The collections is returned via a yield method:

 private static IEnumerable<MyData> ExtractModelsFromDb()
    {
        using (DbDataReader reader = cmd.ExecuteReader())
        {
            while (reader.Read())
            {
                Load MyData
                yield return MyData;
            }
            reader.Close();
        }
    }

This Data is consumed in a single Foreach loop:

public void RunStuff()
    {
        var myCollection = ExtractModelsFromDb();

        foreach (var data in myCollection)
        {
           //DO STUFF 
        }
    }

I know that the collection under the IEnumerable is getting loaded one object at a time, but are the objects getting removed once they have been used in the ForEach or do they remain in the collection until the ForEach is complete.

Answer 1

It depends on the implementation. It is not against the specification for an IEnumerable to store the values in the sequence that it represents. In fact, a List<T> is an IEnumerable<T> that does exactly that!

Now, for the IEnumerable that you have, it will not store the objects after it returns them. DbDataReaders merely stream the results to you, and yield returning them will not store them. So, your foreach is just streaming through the result set from DbDataReader, and no one is storing them as you go gently down the stream.