Monday, 27 July 2009

Wait until file is closed/unlocked

Sometimes you want to wait until some specified file is available for reading/writing. For example, you wait until other process finishes packing into a zip file, or wait until some file is fully uploaded to the server so you can compress it at server side, or something like that.
There are some solutions in which checking whether the file can be opened mainly looks like this:

public static bool IsFileAvailable(string path)
{
  try
  {
    if (!File.Exists(path)) return false;
    using (FileStream fs = File.Open(path, FileMode.Open, FileAccess.ReadWrite, FileShare.None)) { };
    return true;
  }
  catch (Exception) { return false; }
}

There are some drawbacks to this approach. Imagine that this code returns true, but before we actually open the file, some other process or thread manages to open and lock the file. Besides, a generic exception should not be caught, because we might “eat” some really important exception like OutOfMemoryException or similar. A better solution would be to return obtained file stream, and catch only file-related exceptions.

After some refactoring, adding extension methods, and adding ‘wait’ methods, the final code looks like this:

C# Download TryOpenFile.cs file
public static class FileExt
{
  public static FileStream TryOpenFile(string path, FileAccess access, FileShare share)
  {
    try
    {
      if (!File.Exists(path)) return null;
      return File.Open(path, FileMode.Open, access, share);
    }
    catch (IOException) { return null; }
    catch (UnauthorizedAccessException) { return null; }
  }

  public static FileStream WaitAndOpenFile(string path, FileAccess access, FileShare share, TimeSpan timeout)
  {
    DateTime dt = DateTime.UtcNow;
    FileStream fs = null;
    while ((fs = TryOpenFile(path, access, share)) == null && (DateTime.UtcNow - dt) < timeout)
    {
      Thread.Sleep(250); // who knows better way and wants a free cookie? ;)
    }
    return fs;
  }

#region " Other Methods"
  public static FileStream TryOpenFileForReading(this FileInfo fileInfo) { return TryOpenFileForReading(fileInfo.FullName); }
  public static FileStream TryOpenFileForReading(string path) { return TryOpenFile(path, FileAccess.Read); }
  public static FileStream TryOpenFileForWriting(this FileInfo fileInfo) { return TryOpenFileForWriting(fileInfo.FullName); }
  public static FileStream TryOpenFileForWriting(string path) { return TryOpenFile(path, FileAccess.ReadWrite); }
  public static FileStream TryOpenFile(this FileInfo fileInfo, FileAccess access) { return TryOpenFile(fileInfo.FullName, access); }
  public static FileStream TryOpenFile(string path, FileAccess access) { return TryOpenFile(path, access, FileShare.None); }
  public static FileStream TryOpenFile(this FileInfo fileInfo, FileAccess access, FileShare share) { return TryOpenFile(fileInfo.FullName, access, share); }
  public static FileStream WaitAndOpenFileForReading(this FileInfo fileInfo, TimeSpan timeout) { return WaitAndOpenFileForReading(fileInfo.FullName, timeout); }
  public static FileStream WaitAndOpenFileForReading(string path, TimeSpan timeout) { return WaitAndOpenFile(path, FileAccess.Read, timeout); }
  public static FileStream WaitAndOpenFileForWriting(this FileInfo fileInfo, TimeSpan timeout) { return WaitAndOpenFileForWriting(fileInfo.FullName, timeout); }
  public static FileStream WaitAndOpenFileForWriting(string path, TimeSpan timeout) { return WaitAndOpenFile(path, FileAccess.ReadWrite, timeout); }
  public static FileStream WaitAndOpenFile(this FileInfo fileInfo, FileAccess access, TimeSpan timeout) { return WaitAndOpenFile(fileInfo.FullName, access, timeout); }
  public static FileStream WaitAndOpenFile(string path, FileAccess access, TimeSpan timeout) { return WaitAndOpenFile(path, access, FileShare.None, timeout); }
  public static FileStream WaitAndOpenFile(this FileInfo fileInfo, FileAccess access, FileShare share, TimeSpan timeout) { return WaitAndOpenFile(fileInfo.FullName, access, share, timeout); }
#endregion
}

This code also is not perfect. I known of no better method of waiting until a file is closed by the previous owner, except waiting in a loop. We can only make this method a little better by using the Thread.Sleep method which prevents CPU from being used all the time.

There is one more thing. Why is the FileShare.None flag used by default? Only because we do not want other processes/threads to mess with our file which we obtained with such “difficulties”.

Well… that is all for today. I hope this code will be helpful for you :)

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1 comments:

jey.carrero said...

Hola, soy de colombia, te felicito por tus colaboraciones, encontré una entrada tuya en CodeProject acerca de controles PopUp en C#, muy bueno este codigo, espero te animes a seguir escribiendo para los que tenemos curiosidad por aprender más cada día

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