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FileInfo.LastWriteTime discrepancies [Lakshan Fernando]

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I recently investigated a customer issue of incorrect time reporting with FileInfo.LastWriteTime and thought of sharing this information in the blog. In the customer case, the last write time was off by about one hour for some files.

 

We depend on two Win32 APIs to get the file time information, GetFileAttributesEx in the normal case and FindFirstFile if there is a problem with the call to GetFileAttributesEx. One case where we will call FindFirstFile is if there is already an open handle to the specific file. As File Times section on MSDN describes, when writing to a file, the last write time is not fully updated until all handles that are used for writing are closed. This is the most likely reason that the customer saw the last write time reported incorrectly..

Comments

  • Anonymous
    July 26, 2006
    I'm not sure I agree with your diagnosis.

    From my own investigations looking at various different files on my hard disc, plus my searching around on the Internet (e.g. http://www.dotnet247.com/247reference/msgs/54/273622.aspx) it would seem to have more to do with daylight saving time than with a failure to update files' times.

    I could be wrong... but I would urge you to do further investigation.
  • Anonymous
    August 07, 2006
    Hi Andrew,
    Thanks for your comments and apologies for the delay in responding to you - I was on vacation till today.

    The time lag can be seen with a simple repro when LastWriteTime is called on a file that has a handle already opened. In the specific customer case, the affected file was a commonly used one in ASP.NET. But I'm interested in your investigation information and it will be great if you can share a small repro of the daylight issue.
    Thanks
    Lakshan Fernando
  • Anonymous
    August 09, 2006
    Hi Lakshan.  Just spotted your reply.  Certainly if the difference between FileInfo.LastWriteTime and what's displayed in Windows Explorer is off by exactly one hour, you should strongly consider daylight saving time as a cause.  Will try and come up with a repro when I'm back on my home machine.
  • Anonymous
    August 09, 2006
    The comment has been removed