Safe stream update of file

The normal way of avoiding the "delete then move fails problem" is:

• Write to file.new
• Move file.current to file.old
• Move file.new to file.current
• Delete file.new

Then when you come to read, use file.new if file.current is missing, deleting file.old if you see it.

Checking for whether or not the file is available: try opening it for write, but appending to the end. Of course, you'll need to close the handle before you then move it, and in-between someone else could open it - but it would at least be a reasonable optimisation.

Not sure about copying summaries etc, I'm afraid.

Why not try checking the FileAttributes first?

Try something like this:

//If File is readonly
if ( (file.Attribute & System.FileAttributes.ReadOnly) == System.FileAttributes.ReadOnly ) 
        //Don't delete. 

Also try using .OpenWrite(). If you can open the file to write to, it is not being accessed and is not currently in use. You can only open a file for writing if its currently in an un-open state. I dont recommend this but it may help you.

  FileStream fs = File.OpenWrite(file);
  fs.Close();
  return false; 

You can also use a FileLock checking method. Something like this:

protected virtual bool IsFileLocked(FileInfo file)
{
    try
    {
        using (file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None))
        {
           return false;
        }
    }

    catch (IOException)
    {
        return true;
    }

}

you may also want to check FileIOPermission.Write. This allows to see if the file is writable (and able for deletion).

fileIOPerm = New FileIOPermission(FileIOPermissionAccess.Write, FileSpec);
fileIOPerm.Demand();

In regards to question #3 in the original post...You can always move the files to a temp folder, using File.Copy(path1,path2,true). You may want to consider using a temp folder and writing better logic for file manipulation.

If you did decide to use a temp folder, or temp files/intermediate files, then you would also fix your question #2. Try moving the files first.