C# .NET: Will a BinaryWriter flush when the FileStream buffer is filled?

Question

I'm coming across something trivial, but it appears that data is flushed to disk (out of the FileStream's buffer) when the data I'm buffering hits the size of the FileStream's buffer.

//use the FileStream buffer to actually buffer the data to be written, so segments are written as desired.
FileStream writeStream = new FileStream(filename, FileMode.Append, FileAccess.Write, FileShare.None, CommandOperationBufferSize);
BinaryWriter binWriter = new BinaryWriter(writeStream);
byte[] FullSize = new byte[CommandOperationTotalSize];

//the BinaryWriter will flush when the FileStream buffer is hit
binWriter.Write(FullSize); //DATA FLUSHES TO DISK HERE!

//if wait, wait five seconds
if (CommandOperation == "writewait" || CommandOperation == "appendwait")
{
    Thread.Sleep(5000);
    writeStream.Flush();
    Thread.Sleep(5000);
}

writeStream.Close();
writeStream.Dispose();
binWriter.Close();

Can anyone confirm that this is the case? That the FileStream's buffer is actual .Flush() when the FileStream's buffer is filled?

I ask because it appears that if I set CommandOperationTotalSize to 1MB, and set the CommandOperationBufferSize to 64KB, data is flushed to disk when the buffer is filled.

Sounds like I answered my own question, but it seems odd that the FileStream buffer wouldn't just overflow? But maybe the API developers are trying to be nice?

Thanks,

Matt

I just talked to a C++ dev, and yes, this is what I figured. :) — brandeded
– brandeded, Commented Jul 14, 2011 at 19:21

Hans Passant · Accepted Answer · 2011-07-14 18:55:20Z

2

You can readily assume that overflowing the buffer is not possible. The class would be rather hard to use if that was the case, given that FileStream has no properties at all to tell you how much is currently being buffered.

The buffer is only there to reduce the number of calls to the native Windows WriteFile() call. Important when you write small amounts of data, say one byte at a time. If you don't explicitly specify the buffer size then it will use a buffer of 4096 bytes. Which is fine, it is very rare to need something else. Any writes are further buffered by the file system cache. You should only consider a non-standard size when you use FileOptions.WriteThrough

answered Jul 14, 2011 at 18:55

Hans Passant

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2 Comments

brandeded Over a year ago

Thanks Hans. I've tested and it does appear that the FileStream will call .Flush() if the buffer is full, but not before. I am writing a benchmarking program, and benchmarking for very very high I/O over SMB, so adjusting whatever buffers I have is useful (at least for the benchmarking program). Thanks for the reply though. I suppose it is best to allow the buffers to work as they are intended to.

brandeded Over a year ago

I used ILSpy to take a look at the API methods.

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