那么如果创建FileStream的时候指定了异步会是什么结果呢(这里的实现在BeginReadCore方法里)?
[SecuritySafeCritical]
private unsafe FileStreamAsyncResult BeginReadCore(byte[] bytes, int offset, int numBytes, AsyncCallback userCallback, object stateObject, int numBufferedBytesRead)
{
NativeOverlapped* overlappedPtr;
FileStreamAsyncResult ar = new FileStreamAsyncResult {
_handle = this._handle,
_userCallback = userCallback,
_userStateObject = stateObject,
_isWrite = false,
_numBufferedBytes = numBufferedBytesRead
};
ManualResetEvent event2 = new ManualResetEvent(false);
ar._waitHandle = event2;
Overlapped overlapped = new Overlapped(0, 0, IntPtr.Zero, ar);
//...
overlappedPtr = overlapped.Pack(IOCallback, bytes);
//...
ar._overlapped = overlappedPtr;
//...
ReadFileNative(this._handle, bytes, offset, numBytes, overlappedPtr, out hr)
private unsafe FileStreamAsyncResult BeginReadCore(byte[] bytes, int offset, int numBytes, AsyncCallback userCallback, object stateObject, int numBufferedBytesRead)
{
NativeOverlapped* overlappedPtr;
FileStreamAsyncResult ar = new FileStreamAsyncResult {
_handle = this._handle,
_userCallback = userCallback,
_userStateObject = stateObject,
_isWrite = false,
_numBufferedBytes = numBufferedBytesRead
};
ManualResetEvent event2 = new ManualResetEvent(false);
ar._waitHandle = event2;
Overlapped overlapped = new Overlapped(0, 0, IntPtr.Zero, ar);
//...
overlappedPtr = overlapped.Pack(IOCallback, bytes);
//...
ar._overlapped = overlappedPtr;
//...
ReadFileNative(this._handle, bytes, offset, numBytes, overlappedPtr, out hr)
上面代码中的NativeOverlapped就是在上一节我们提到的保存有回调等信息的OVERLAPPED结构，在这里也是一样，它保存有我们的userCallback回调。然后通过调用ReadNative发起IO请求，并将这个数据结构传递进去，这里的ReadNative就是对Win32 的ReadFile的封装。发起异步IO请求完毕，BeginRead返回，过了一会儿磁盘驱动程序将数据读回来了，对应的IO完成端口收到通知，IO完成端口把刚才传递进去的NativeOverlapped结构传递给IO线程，IO线程从中取出IOCallback回调，IOCallback回调里有对我们的userCallback回调的调用：
IOCallback = new IOCompletionCallback(FileStream.AsyncFSCallback);
private static unsafe void AsyncFSCallback(uint errorCode, uint numBytes, NativeOverlapped* pOverlapped)
{
FileStreamAsyncResult asyncResult = (FileStreamAsyncResult) Overlapped.Unpack(pOverlapped).AsyncResult;
//...
AsyncCallback callback = asyncResult._userCallback;
if (callback != null)
{
callback(asyncResult);
}
}
private static unsafe void AsyncFSCallback(uint errorCode, uint numBytes, NativeOverlapped* pOverlapped)
{
FileStreamAsyncResult asyncResult = (FileStreamAsyncResult) Overlapped.Unpack(pOverlapped).AsyncResult;
//...
AsyncCallback callback = asyncResult._userCallback;
if (callback != null)
{
callback(asyncResult);
}
}
在这个回调里我们会对EndRead进行调用，我们看看EndRead的代码会发现其他一些东西：
public override unsafe int EndRead(IAsyncResult asyncResult)
{
//...
WaitHandle handle = result._waitHandle;
if (handle != null)
{
try
{
handle.WaitOne();
}
finally
{
handle.Close();
}
}
NativeOverlapped* nativeOverlappedPtr = result._overlapped;
if (nativeOverlappedPtr != null)
{
Overlapped.Free(nativeOverlappedPtr);
}
//...
return (result._numBytes + result._numBufferedBytes);
}
{
//...
WaitHandle handle = result._waitHandle;
if (handle != null)
{
try
{
handle.WaitOne();
}
finally
{
handle.Close();
}
}
NativeOverlapped* nativeOverlappedPtr = result._overlapped;
if (nativeOverlappedPtr != null)
{
Overlapped.Free(nativeOverlappedPtr);
}
//...
return (result._numBytes + result._numBufferedBytes);
}
首先是销毁我们在BeginRead里初始化的WaitHandle内核对象，然后将NativeOverlapped结构也销毁。所以EndRead除了取回读了多少个字节的作用外，还起了销毁资源的作用。所以有的时候我们想进行这么一个操作：异步的发起请求，但是我们并不关心该请求是否成功。如是我们就假想能不能只调用BeginXXX方法就可以了?从这里看我们不能简单的调用一下BeginXXX就了事了，因为在BeginXXX里分配的一些句柄和内核资源需要在EndXXX里销毁，不然会造成资源泄露。
