The process cannot access the file because it is being used by another process

Solution 1:

using (FileStream fs = 
    new FileStream(filePath,
        FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
//...

http://msdn.microsoft.com/en-us/library/system.io.fileshare.aspx

Your log may be write locked, so try with FileShare.ReadWrite.

Solution 2:

Try to add the FileShare option, see if that helps:

FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);

EDIT: corrected code, not FileShare.Read but FileShare.ReadWrite does the trick (as Guillaume showed as well). The reason: you want to open your file and allow others to read and write it at the same time.