When using FileStream.ReadAsync() should I open the file in async mode?

Question

The old .Net way of performing asynchronous I/O for a FileStream is to use FileStream.BeginRead() and FileStream.EndRead().

The MSDN documentation for

Answer 1

In win32, you need to specify FILE_FLAG_OVERLAPPED to make use of asynchronous File IO. In .net world you use isAsync parameter of FileStream to achieve the same. If you fail to do so, operations will not be asynchronous.

It is a shame that FileStream.ReadAsync and its related methods failed to document it.

You can confirm this by peeking into the implementation.

public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
{
   ...
    if (!this._isAsync || !Environment.IsWindowsVistaOrAbove)
    {
        return base.ReadAsync(buffer, offset, count, cancellationToken);
    }
    ...
    return stateObject;
}

base.ReadAsync will eventually call to Stream.Read method synchronously in ThreadPool giving the impression that operation is async, but really not.

Related information from Concurrent Programming On Windows book (Pg:818):

As with CreateFile, you must specify at creation time that you'd like to use a FileStream for asynchronous execution. With FileStream, you do this by passing true as the isAsync argument to the constructor overloads, which accept it. The stream's IsAsync property will subsequently return true. If you fail to pass this value, calls to BeginRead and BeginWrite will succeed. But they will use the base class implementation from Stream, which provides none of the benefits of true asynchronous file I/O.

Above information is about APM methods(as this is old book) but still relevant one.