C#对于非托管资源的释放原理探究

前言

我们都知道CLR有一个使用根的可达性算法的垃圾回收机制来回收托管内存，那么对于那些本机资源（非托管内存）他又是怎么清理的呢？

正文

要看他怎么清理资源，首先要知道这个资源是怎么来的，这里我们用FileStream这一经典类来探讨这些问题。首先是它的构造函数

[SecuritySafeCritical]
public FileStream(string path, FileMode mode)
  : this(path, mode, mode == FileMode.Append ? FileAccess.Write : FileAccess.ReadWrite, FileShare.Read, 4096, FileOptions.None, Path.GetFileName(path), false)
{
}

[SecurityCritical]
internal FileStream(
  string path,
  FileMode mode,
  FileAccess access,
  FileShare share,
  int bufferSize,
  FileOptions options,
  string msgPath,
  bool bFromProxy)
{
  //取得内核对象
  Win32Native.SECURITY_ATTRIBUTES secAttrs = FileStream.GetSecAttrs(share);
  //绑定句柄，初始化缓冲区
  this.Init(path, mode, access, 0, false, share, bufferSize, options, secAttrs, msgPath, bFromProxy, false, false);
}

然后我们调用FileStream.Write函数，将一段二进制数组写入文件

//这句代码会把我们的二进制数组写入到缓存区中
Buffer.InternalBlockCopy((Array) array, offset, (Array) this._buffer, this._writePos, count);
//最后调用这个函数把缓存区中的数据写入文件
Win32Native.WriteFile(handle, numPtr + offset, count, IntPtr.Zero, overlapped);

我们知道，释放一个这样的文件流需要调用Dispose函数，找到Dispose函数就能知道它内部怎么处理非托管资源的释放了，我们当前正使用一个本机资源的句柄来连接着这个文件

[SecuritySafeCritical]
protected override void Dispose(bool disposing)
{
  try
  {
    if (this._handle == null  this._handle.IsClosed  this._writePos <= 0)
      return;
    //会把当前缓冲区内容写入磁盘文件
    this.FlushWrite(!disposing);
  }
  finally
  {
    if (this._handle != null && !this._handle.IsClosed)
      //释放句柄及其引用的本机资源
      this._handle.Dispose();
    this._canRead = false;
    this._canWrite = false;
    this._canSeek = false;
    base.Dispose(disposing);
  }
}

对于this._handle.Dispose()，我们只能看到这一步

/// <summary>Releases the unmanaged resources used by the <see cref="T:System.Runtime.InteropServices.SafeHandle" /> class specifying whether to perform a normal dispose operation.</summary>
/// <param name="disposing">
/// <see langword="true" /> for a normal dispose operation; <see langword="false" /> to finalize the handle.</param>
[SecurityCritical]
[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
[__DynamicallyInvokable]
protected virtual void Dispose(bool disposing)
{
  if (disposing)
    this.InternalDispose();
  else
    this.InternalFinalize();
}

再底层就是

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
[MethodImpl(MethodImplOptions.InternalCall)]
private extern void InternalDispose();    

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.Success)]
[MethodImpl(MethodImplOptions.InternalCall)]
private extern void InternalFinalize();