private unsafe ValueTask <int> ReadAsyncInternal(Memory <byte> destination, CancellationToken cancellationToken = default)
{
if (!CanRead)
{
ThrowHelper.ThrowNotSupportedException_UnreadableStream();
}
long positionBefore = _filePosition;
if (CanSeek)
{
long len = Length;
if (positionBefore + destination.Length > len)
{
destination = positionBefore <= len?
destination.Slice(0, (int)(len - positionBefore)) :
default;
}
// When using overlapped IO, the OS is not supposed to
// touch the file pointer location at all. We will adjust it
// ourselves, but only in memory. This isn't threadsafe.
_filePosition += destination.Length;
}
(SafeFileHandle.ValueTaskSource? vts, int errorCode) = RandomAccess.QueueAsyncReadFile(_fileHandle, destination, positionBefore, cancellationToken);
return(vts != null
? new ValueTask <int>(vts, vts.Version)
: (errorCode == 0) ? ValueTask.FromResult(0) : ValueTask.FromException <int>(HandleIOError(positionBefore, errorCode)));
}
public override ValueTask <int> ReadAsync(Memory <byte> destination, CancellationToken cancellationToken = default)
{
if (!CanSeek)
{
return(RandomAccess.ReadAtOffsetAsync(_fileHandle, destination, fileOffset: -1, cancellationToken));
}
if (LengthCachingSupported && _length >= 0 && Volatile.Read(ref _filePosition) >= _length)
{
// We know for sure that the file length can be safely cached and it has already been obtained.
// If we have reached EOF we just return here and avoid a sys-call.
return(ValueTask.FromResult(0));
}
// This implementation updates the file position before the operation starts and updates it after incomplete read.
// This is done to keep backward compatibility for concurrent reads.
// It uses Interlocked as there can be multiple concurrent incomplete reads updating position at the same time.
long readOffset = Interlocked.Add(ref _filePosition, destination.Length) - destination.Length;
(SafeFileHandle.OverlappedValueTaskSource? vts, int errorCode) = RandomAccess.QueueAsyncReadFile(_fileHandle, destination, readOffset, cancellationToken, this);
return(vts != null
? new ValueTask <int>(vts, vts.Version)
: (errorCode == 0) ? ValueTask.FromResult(0) : ValueTask.FromException <int>(HandleIOError(readOffset, errorCode)));
}
