public Aligned_PageBlobStream(CloudPageBlob blob)
        {
            _blob = blob;

            if (!_blob.Exists())
                throw new ArgumentException();
            //_stream = _blob.OpenRead();
               // _reader = new BinaryReader(_blob.OpenRead());
        }
        public PageBlobReadStream(CloudPageBlob blob)
        {
            _blob = blob;

            if (!_blob.Exists())
                throw new ArgumentException();

            _reader = new BinaryReader(_blob.OpenRead());
        }
        public PageBlobAppendStream(CloudPageBlob blob)
        {
            _blob = blob;
            if (blob == null)
                throw new ArgumentNullException("blob");

            _reader = new BinaryReader(_blob.OpenRead());
            if (!blob.Exists())
            {
                _blob.Create(0);
                _lastPageIndex = -1;
                return;
            }

            _blobLength = _blob.Properties.Length;
            _lastPageIndex = (_blobLength / PageSize) - 1;
        }