如何系统.在Windows Phone 7从IsolatedStorageFileStream加载时出现OutOfMem

我有一个应用程序，在隔离的存储中存储一些文件，需要在文件打开时进行解密。我尝试将IsolatedStorageFileStream加载到字节数组中并启动AES256解密。当文件大小很小时，它工作得很好。但是，当文件大小很大时，比如大约120MB，它将抛出System.OutOfMemoryException。以下是我的部分代码:

                        IsolatedStorageFileStream m_rawStream =
                        IsolatedStorageFile.GetUserStoreForApplication().OpenFile("shared''transfers''" +
                        DownloadFileName, FileMode.OpenOrCreate, FileAccess.ReadWrite);

                    byte[] m_rawByte = new byte[_totalByte];
                    m_rawStream.Read(m_rawByte, 0, _totalByte);
                    m_rawStream.Close();
                    //Decrypte downloaded epub
                    byte[] m_decryptedBytes = _decryptStringFromBytesAES(m_rawByte,
                                _hexStringToByteArray(_decryptKey),
                                _hexStringToByteArray(_decryptIV));
                    //Store on the upper layer of isolated storage
                    using (var isfs = new IsolatedStorageFileStream(DownloadFileName,
                                        FileMode.Create,
                                        IsolatedStorageFile.GetUserStoreForApplication()))
                    {
                        isfs.Write(m_decryptedBytes, 0, m_decryptedBytes.Length);
                        isfs.Flush();
                    }
//AES Decrypt helper 
    private byte[] _decryptStringFromBytesAES(byte[] cipherText, byte[] Key, byte[] IV)
    {
        // TDeclare the streams used
        // to decrypt to an in memory
        // array of bytes.
        MemoryStream msDecrypt = null;
        CryptoStream csDecrypt = null;
        // Declare the RijndaelManaged object
        // used to decrypt the data.
        AesManaged aesAlg = null;
        // Declare the byte[] used to hold
        // the decrypted byte.
        byte[] resultBytes = null;
        try
        {
            // Create a RijndaelManaged object
            // with the specified key and IV.
            aesAlg = new AesManaged();
            aesAlg.KeySize = 256;
            aesAlg.Key = Key;
            aesAlg.IV = IV;
            // Create a decrytor to perform the stream transform.
            ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
            // Create the streams used for decryption.
            msDecrypt = new MemoryStream();
            csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Write);
            // Read the decrypted bytes from the decrypting stream
            // and place them in a string.
            csDecrypt.Write(cipherText, 0, cipherText.Length);
            //csDecrypt.FlushFinalBlock();
            resultBytes = msDecrypt.ToArray();
        }
        catch (Exception e)
        {
            Debug.WriteLine(e.Message);
            MessageBoxResult _result = MessageBox.Show("無法開啟檔案，請重試或聯絡技術人員。", "錯誤", MessageBoxButton.OK);
            if (_result == MessageBoxResult.OK)
            {
                new SimpleNavigationService().Navigate("/View/MainPange.xaml");
            }
        }
        return resultBytes;
    }

如何避免获得OutOfMemory异常?谢谢。