windows-phone-7 - オブジェクトを複数回破棄しないことを示すコード分析

Question

私の暗号コードは

{
    AesManaged aes = null;
    MemoryStream memoryStream = null;
    CryptoStream cryptoStream = null;

    try
    {
        Rfc2898DeriveBytes rfc2898 = new Rfc2898DeriveBytes(password, Encoding.UTF8.GetBytes(salt), 10000);

        aes = new AesManaged();
        aes.Key = rfc2898.GetBytes(32);
        aes.IV = rfc2898.GetBytes(16);

        memoryStream = new MemoryStream();
        cryptoStream = new CryptoStream(memoryStream, aes.CreateEncryptor(), CryptoStreamMode.Write);

        byte[] data = Encoding.UTF8.GetBytes(dataToEncrypt);
        cryptoStream.Write(data, 0, data.Length);
        cryptoStream.FlushFinalBlock();

        return Convert.ToBase64String(memoryStream.ToArray());
    }
    finally
    {
        if (cryptoStream != null)
            cryptoStream.Close();

        if (memoryStream != null)
            memoryStream.Close();

        if (aes != null)
            aes.Clear();
    }
}

コード分​​析を試してみたところ、

CA2202
Do not dispose objects multiple times 
Object 'memoryStream' can be disposed more than once in method 'EncryptDecrypt.Encrypt(string, string, string)'. 
To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object.

しかし、私のコードは正常に動作しています。memoryStream と cryptoStream を作成し、後でそれらを閉じました...しかし、なぜ複数のオブジェクトが複数回表示されるのか理解できません