reading while open/writing to file in .net?

I notice many applications like firefox allow me to watch part of a video (using VLC) when the file is still do开发者_运维问答wnloading. I would like to do that with my application. When i tried opening the video with VLC i get an error.

How do i allow reading when i write to a file? my open line is

File.Open(fn, FileMode.Append)

i do append so i can resume partial files.

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You should use the Open overload, that takes a FileShare parameter:

File.Open(fn, FileMode.Append, FileAccess.Write, FileShare.Read)

This way you explicitly state that other processes are allowed to open the file for reading while you are still writing to it.

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You are leaving it up to the Open() method to pick the FileShare value passed to the FileStream constructor. Which is FileShare.None. Specify your own.