Uploading large file using file upload control

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ASP.NET

Hi,

In our website,we are using fileupload control to upload and download files.Before uploading it to the server,we are serialsing the content using XML serialiser and then encrypting the data using Rijndael algorithm and then save the file in server.I have set up the web.config httpRuntime with maxRequestLength="71680" executionTimeout="300"

The problem is that I can able to upload multiple files of size 10 mb.but when i try to upload file of size 40mb,it is throwing " out of memory " exception.

Memorystream is not holding up the filecontent.
Is there any way to do upload??

//Serialise
      public string SerializeAnObject(object anObject, int fileLen)
        {
            XmlDocument xmlDoc = new XmlDocument();
            XmlSerializer xml_Serializer = new XmlSerializer(anObject.GetType());
            MemoryStream memStream = new MemoryStream(fileLen);
            try
            {
                xml_Serializer.Serialize(memStream, anObject);
                memStream.Position = 0;
                xmlDoc.Load(memStream);
                return xmlDoc.InnerXml.ToString();
            }
            finally
            {
                memStream.Close();
            }
        }

// encryption
 public string Encrypt(string textToBeEncrypted, string strConn, int fileLen)
        {
            RijndaelManaged rijndaelCipher = new RijndaelManaged();
            string password = strConn;
            byte[] plainText = new byte[fileLen + 1];
            plainText = System.Text.Encoding.Unicode.GetBytes(textToBeEncrypted);
            byte[] salt = Encoding.ASCII.GetBytes(password.Length.ToString());
            PasswordDeriveBytes secretKey = new PasswordDeriveBytes(password, salt);        
// Creates a symmetric encryptor object.
            ICryptoTransform encryptor = rijndaelCipher.CreateEncryptor(secretKey.GetBytes(32), secretKey.GetBytes(16));
           using (MemoryStream memoryStream = new MemoryStream(fileLen))
            {
                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                {
                    cryptoStream.Write(plainText, 0, plainText.Length);
 // Writes the final state and clears the buffer        
                    cryptoStream.FlushFinalBlock();
                    byte[] cipherBytes = memoryStream.ToArray();
                    memoryStream.Close();
                    cryptoStream.Close();
                    string encryptedData = Convert.ToBase64String(cipherBytes);
                    return encryptedData;
                }
            }
        }