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