分段输出到同一个Stream（.NET）

　　某项目需要输出一个数据文件，该文件由2部分组成，即文件头信息和数据。

　　项目是使用C#语言在.NET Framework 4上创建的。

　　拿到这个需求，首先想到的是定义一个Writer类，在写入方法中创建一个文件流，使用BinaryWriter封装，写入所需要的各种数据。看起来就像这样：

public void Write()
{
    using (Stream stream = OpenFileStream())
    using (BinaryWriter writer = new BinaryWriter(stream))
    {
        WriteHeader(writer);
        WriteData(writer);
    }
}
private void WriteHeader(BinaryWriter writer)
{
    // ......
}
private void WriteData(BinaryWriter writer)
{
    // ......
}

　　不过随后需求就发生了变化，因为数据敏感，需要加密。于是想到对文件头部分使用BinaryWriter写入，而后面的数据部分，先使用CryptoStream包装流，再使用BinaryWriter写入。于是改成这样：

public void Write()
{
    using (Stream stream = OpenFileStream())
    {
        using (BinaryWriter headerWriter = new BinaryWriter(stream))
        {
            WriteHeader(headerWriter);
        }
        using (CryptoStream cryptoStream
            = new CryptoStream(stream, GetCryptoTransform(),
                CryptoStreamMode.Write))
        using (BinaryWriter dataWriter = new BinaryWriter(cryptoStream))
        {
            WriteData(dataWriter);
        }
    }
}

　　改过之后，问题产生了——在使用headerWriter的using语句结束时，会自动调用headerWriter.Dispose()，而这个方法会调用BaseStream.Close()，也就是说，文件流被关闭了，那么后面尝试写入数据时就会抛出异常。

　　虽然BinaryWriter有一个构造方法可以申明不关闭流：

public BinaryWriter(
    Stream output,
    Encoding encoding,
    bool leaveOpen
)

　　但这个构造方法是在.NET 4.5才加入的，项目是用的4.0的Framework，所以必须另外想办法。而且后面的CryptoStream也存在同样的问题，而它可没有提供不关闭流的构造。

　　这里可以想到两个办法来处理：

　　1. 在所有内容都写完之后再统一Dispose各种操作对象和流对象。

　　2. 定义一个从Stream继承的StreamWrapper，将Close和Dispose都重载并实现为空方法，再定义一个ReallyClose方法来真正关闭封装的流。

　　使用第1种方法，就像这样：

public void Write()
{
    using (Stream s = OpenFileStream())
    using (BinaryWriter headWriter = new BinaryWriter(s))
    using (CryptoStream cs = new CryptoStream(s, GetCryptoTransform(),
        CryptoStreamMode.Write))
    using (BinaryWriter dataWriter = new BinaryWriter(cs))
    {
        WriterHeader(headWriter);
        WriteData(dataWriter);
    }
}

　　而且如果一个文件分成了很多很多段的话，这个using列表就太长了。但这不是问题，问题是如果WriteHeader中抛出异常，那么cs和dataWriter这两个对象就浪费了。所以，可以考虑使用try {...} finally {...} 来实现using，

public void Write()
{
    Stream stream = null;
    BinaryWriter headWriter = null;
    CryptoStream cs = null;
    BinaryWriter dataWriter = null;
    try
    {
        stream = OpenFileStream();
        headWriter = new BinaryWriter(stream);
        WriterHeader(headWriter);
        cs = new CryptoStream(stream, GetCryptoTransform(), CryptoStreamMode.Write);
        dataWriter = new BinaryWriter(cs);
        WriteData(dataWriter);
    }
    finally
    {
        if (dataWriter != null) { dataWriter.Dispose(); }
        if (cs != null) { cs.Dispose(); }
        if (headWriter != null) { headWriter.Dispose(); }
        if (stream != null) { stream.Dispose(); }
    }
}

　　解决问题，但代码量大，而且容易出错。比如，要记得在WriterHeader里面Flush，这个原本会在Dispose()自动执行的操作（对于CryptoStream，需要执行FlushFinalBlock()）。

　　相对来说，写一个StreamWrapper靠谱多了。不过使用RealClose就失去了IDisposable的意义，所以稍稍改变一下，定义一个变量来允许Dispose时关闭流。

class StreamWrapper : Stream
{
    private readonly Stream stream;
    public Stream Stream { get { return stream; } }
    public StreamWrapper(Stream stream)
    {
        this.stream = stream;
        IsLeavingOpen = true;
    }
    public override void Flush()
    {
        stream.Flush();
    }
    public override long Seek(long offset, SeekOrigin origin)
    {
        return stream.Seek(offset, origin);
    }
    public override void SetLength(long value)
    {
        stream.SetLength(value);
    }
    public override int Read(byte[] buffer, int offset, int count)
    {
        return stream.Read(buffer, offset, count);
    }
    public override void Write(byte[] buffer, int offset, int count)
    {
        stream.Write(buffer, offset, count);
    }
    public override bool CanRead { get { return stream.CanRead; } }
    public override bool CanSeek { get { return stream.CanSeek; } }
    public override bool CanWrite { get { return stream.CanWrite; } }
    public override long Length { get { return stream.Length; } }
    public override long Position
    {
        get { return stream.Position; }
        set { stream.Position = value; }
    }
    public override void Close()
    {
        if (IsLeavingOpen)
        {
            return;
        }
        stream.Close();
        base.Close();
    }
    protected override void Dispose(bool disposing)
    {
        if (IsLeavingOpen)
        {
            return;
        }
        if (disposing)
        {
            stream.Dispose();
        }
        base.Dispose(disposing);
    }
    public bool IsLeavingOpen { get; set; }
}

　　如果稍稍改变一下写入数据的接口，使用起来也非常方便

public void Write()
{
    using (Stream fileStream = OpenFileStream())
    using (StreamWrapper stream = new StreamWrapper(fileStream))
    {
        WriteHeader(stream);
        WriteData(stream);
        stream.IsLeavingOpen = false;
    }
}
private void WriteHeader(Stream stream)
{
    using (BinaryWriter writer = new BinaryWriter(stream))
    {
        // ......
    }
}
private void WriteData(Stream stream)
{
    using (CryptoStream cryptoStream = new CryptoStream(stream,
        GetCryptoTransform(), CryptoStreamMode.Write))
    using (BinaryWriter writer = new BinaryWriter(cryptoStream))
    {
        // ......
    }
}