Problem in reading a log file

[C# Programming]

Hi every one
I have a log file that continually is filling by a application.
How can I read the log file?
I tried
StreamReader tr=new StreamReader("d:\\theLogFile.txt");
MessageBox.Show(tr.ReadLine());


but got "The process cannot access the file because it is being used by another process." error.
help please?
thanks.