How to read a file which is currently used, like Windows does when copying it?

Question

One of my applications is intended to read (and only read) files which may be in use.

But, when reading a file which is already opened in, for example, Microsoft Word, this application throws a System.IO.IOException:

The process cannot access the file '<filename here>' because it is being used by another process.

The code used to read the file is:

using (Stream stream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
{
    // Do stuff here.
}

Of course, since the file is already used, this exception is expected.

Now, if I ask the operating system to copy the file to a new location, then to read it, it works:

string tempFileName = Path.GetTempFileName();
File.Copy(fileName, tempFileName, true);
//                                         ↓ We read the newly created file.
using (Stream stream = new FileStream(tempFileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite | FileShare.Delete))
{
    // Do stuff here.
}

What is the magic of File.Copy which allows to read the file already used by an application, and especially how to use this magic to read the file without making a temporary copy?

Your code works fine for me. Is it possible you're specifying FileAccess.Write in your app, even though it's not in your snippet above? — Ciaran Keating, Dec 07 '10 at 00:20
As Ciaran Keating says above your original code works for me. What are you doing in the "// Do stuff here." — Gern Blanston, Dec 07 '10 at 17:19

score 4 · Accepted Answer · answered Dec 06 '10 at 23:10

4

Nice question there. Have a look at this, it seems to suggest using FileShare.ReadWrite only is the key, it's worth a shot.

http://www.geekzilla.co.uk/viewD21B312F-242A-4038-9E9B-AE6AAB53DAE0.htm

answered Dec 06 '10 at 23:10

Tom

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Reading the comments, it seems that it works in some circumstances, and doesn't in others. In my case, with a file already opened by Microsoft Word, switching to `FileShare.ReadWrite` does not solve the problem. – Arseni Mourzenko Dec 06 '10 at 23:20
That's annoying, this blogpost also suggests the same thing. http://alexpinsker.blogspot.com/2005/03/how-to-read-file-thats-in-use-by.html It does appear to be a problem with it *already* being read, as opposed to subsequent reads (your app). – Tom Dec 07 '10 at 12:35
As a further sidenote, using File.Open returns a FileStream, as suggested here: http://aautar.digital-radiation.com/blog/?p=1292 – Tom Dec 07 '10 at 12:37

skajfes · Answer 2 · 2010-12-06T23:17:32.163

0

try removing FileShare.ReadWrite | FileShare.Delete from the FileStream constructor, or at least FileShare.Delete.

edited Dec 06 '10 at 23:17

answered Dec 06 '10 at 23:09

skajfes

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these flags specify what *other* file streams are allowed to have, not the one that is being opened at the time - as such specifying these flags makes the lock required a *weaker* one, so your statement is wrong. – BrokenGlass Dec 06 '10 at 23:11
Yes, but your sharing mode must be consistent with those of other existing handles to the file. Just what's "consistent" is a little unclear to me. For example, while Word has a file open I can open it with a sharing mode of Read|Write, but not just Read on its own or Write on its own. – Ciaran Keating Dec 07 '10 at 00:07

How to read a file which is currently used, like Windows does when copying it?

2 Answers2