分段輸出到同一個Stream（.NET）

　　某項目需要輸出一個數據文件，該文件由2部分組成，即文件頭信息和數據。

　　項目是使用C#語言在.NET Framework 4上創建的。

　　拿到這個需求，首先想到的是定義一個Writer類，在寫入方法中創建一個文件流，使用BinaryWriter封裝，寫入所需要的各種數據。看起來就像這樣：

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)
{
    // ......
}

　　不過隨后需求就發生了變化，因為數據敏感，需要加密。于是想到對文件頭部分使用BinaryWriter寫入，而后面的數據部分，先使用CryptoStream包裝流，再使用BinaryWriter寫入。于是改成這樣：

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);
        }
    }
}

　　改過之后，問題產生了——在使用headerWriter的using語句結束時，會自動調用headerWriter.Dispose()，而這個方法會調用BaseStream.Close()，也就是說，文件流被關閉了，那么后面嘗試寫入數據時就會拋出異常。

　　雖然BinaryWriter有一個構造方法可以申明不關閉流：

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

　　但這個構造方法是在.NET 4.5才加入的，項目是用的4.0的Framework，所以必須另外想辦法。而且后面的CryptoStream也存在同樣的問題，而它可沒有提供不關閉流的構造。

　　這里可以想到兩個辦法來處理：

　　1. 在所有內容都寫完之后再統一Dispose各種操作對象和流對象。

　　2. 定義一個從Stream繼承的StreamWrapper，將Close和Dispose都重載并實現為空方法，再定義一個ReallyClose方法來真正關閉封裝的流。

　　使用第1種方法，就像這樣：

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);
    }
}

　　而且如果一個文件分成了很多很多段的話，這個using列表就太長了。但這不是問題，問題是如果WriteHeader中拋出異常，那么cs和dataWriter這兩個對象就浪費了。所以，可以考慮使用try {...} finally {...} 來實現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(); }
    }
}

　　解決問題，但代碼量大，而且容易出錯。比如，要記得在WriterHeader里面Flush，這個原本會在Dispose()自動執行的操作（對于CryptoStream，需要執行FlushFinalBlock()）。

　　相對來說，寫一個StreamWrapper靠譜多了。不過使用RealClose就失去了IDisposable的意義，所以稍稍改變一下，定義一個變量來允許Dispose時關閉流。

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; }
}

　　如果稍稍改變一下寫入數據的接口，使用起來也非常方便

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))
    {
        // ......
    }
}