IDispose和最佳实践

Question

我使用了代码分析工具,它提供了以下警告.

严重级代码描述项目文件行抑制状态警告CA2202对象'stream'可以在方法'Cipher.Encryptor(string)'中多次处理.为避免生成System.ObjectDisposedException,不应在对象上多次调用Dispose:Lines:43 Service Layer ...\Cipher.cs 43 Active

它源于"权力之塔":

    public static string Encryptor(string input)
    {
        var content = String.Empty;

        var cipher = new RijndaelManaged();
        var plain = Encoding.Unicode.GetBytes(input);
        var key = new PasswordDeriveBytes(password, salt);

        using (var encrypt = cipher.CreateEncryptor(key.GetBytes(32), key.GetBytes(16)))
        using (var stream = new MemoryStream())
        using (var cryptographic = new CryptoStream(stream, encrypt, CryptoStreamMode.Write))
        {
            cryptographic.Write(plain, 0, plain.Length);
            cryptographic.FlushFinalBlock();
            content = Convert.ToBase64String(stream.ToArray());
        }

        return content;
    }

在这种情况下,我利用MemoryStream,CryptoStream和ICryptoTransform.为什么Visual Studio的代码分析会将此标记为警告？这来自Visual Studio 2015中内置的代码分析.

=================

这是一个较短的repro:

using System.IO;
using System.Security.Cryptography;

namespace ClassLibrary1
{
   public class Class1
   {
      void foo()
       {
          using (var memStream = new MemoryStream())
          using (var xForm = new FromBase64Transform())
          using (var cStream = new CryptoStream(memStream, xForm, CryptoStreamMode.Read))
             ;
       }
   }
}

......这可以在VS2013 Up5中看到