将序列化的XML字符串写入XML文件

我有一个解密的XML字符串,它通过电线发送到我的代码所在的接收框。现在,我想把这个XML字符串写入一个XML文件。

[更新]: 我的问题是,我看不到一种从XML字符串编写/创建XML文件的方法。。。我可以看到使用流、URL的示例,但这对我没有帮助。

protected string DecryptForm(byte[] encryptedString, byte[] key, byte[] vector) 
{
    rijndael = new RijndaelManaged();
    rijndael.Mode = CipherMode.CBC;

    // Create a decryptor to perform the stream transform
    ICryptoTransform decryptor = rijndael.CreateDecryptor(key, vector);

    string plainText = null;
    try
    {
        //Create the streams used for decryption
        using (MemoryStream msStream = new MemoryStream(encryptedString))
        {
            using (CryptoStream csStream = new CryptoStream(msStream,
                decryptor, CryptoStreamMode.Read))
            {
                using (StreamReader readerStream = new StreamReader(csStream))
                {
                    // Read the decrypted bytes from the decrypting stream
                    plainText = readerStream.ReadToEnd();
                }
            }   
        }
        finally
        {
            // Clear the RijndaelManaged object
            if(rijndael != null)
            rijndael.Clear();
        }   
        // Return the decrypted string
        return plainText;
    }

}