There are two reasons for don't write the whole list once.
1. The original value type in the list is `IListBlobItem`, we only need to output the `ICloudBlob` object. So we need to filter some elements in the list.

2. We use `IEnumerable`. Suppose there are 10000 elements, we may take a long time to wait for that all the elements are ready to output if use list. Use IEnumerable, we can get a better user experience and the 1000 elements will output one by one.