I am reading from a CryptoStream which in turn reads from a BlobReadStream. On Dispose, CryptoStream will (depending on internal state, such as not completely reading all data) call Flush on the underlying stream. This will cause a NotSupportedException to be thrown by BlobReadStreamBase.Flush. Since BCL Streams may call Flush on Streams when reading, the correct behavior is for BlobReadStreamBase to do nothing for calls to Flush rather than throw an exception.