c# - async/await and opening a FileStream?

Question

I came across the following question when trying to determine if I was using the Stream methods such as ReadAsync and CopyToAsync correctly: C# 4.5 file read performance sync vs async

In this question I read the following in the accepted answer:

Most notably, your "async" test does not use async I/O; with file streams, you have to explicitly open them as asynchronous or else you're just doing synchronous operations on a background thread.

In his asynchronous IO code he was using the following to open the FileStream 'asynchronously':

var file = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, true)

So I was wondering if you intend to use methods such as CopyToAsync whether you should open the underlying FileStream as shown above?, as opposed to doing something simple like the following:

File.Open(filename, FileMode.Open)

Which is how the example in the actual documentation for CopyToAsync demonstrates opening the underlying FileStream: https://msdn.microsoft.com/en-us/library/hh159084(v=vs.110).aspx

If it does not matter which way the underlying FileStream is opened, what does the useAsync parameter of the FileStream constructor do?

Answer 1

So I was wondering if you intend to use methods such as CopyToAsync whether you should open the underlying FileStream as shown above?

Yes. The reason is mostly historical.

First, on Windows, HANDLEs (including file handles) must be opened/created explicitly with an asynchronous flag if you want to do asynchronous (OVERLAPPED) operations on them.

However, the old Windows 95/98/ME line only supported asynchronous operations on serial port and IOCTL (device driver) handles. Asynchronous I/O on disk files wasn't supported on that platform line. And the original .NET did support 98/ME, so the original FileStream just used synchronous I/O. I think (but am not absolutely sure) that APM methods (like FileStream.BeginRead) on Win98/ME were probably just implemented using the so-called "asynchronous delegates" (which just execute a synchronous method like FileStream.Read on a thread pool thread).

So, that's the historical reason why file stream handles were not opened with the asynchronous flag by default.

Which is how the example in the actual documentation for CopyToAsync demonstrates

Unfortunately, a lot of the MSDN examples are rather poor quality. They're OK if you approach them from the perspective of "here's an example of how to call this specific method", but not so great from the perspective of "here's an example of production-quality code that uses this method".