Reading a file used by another process

If notepad can read the file then so can you, clearly the program didn't put a read lock on the file. The problem you're running into is that StreamReader will open the file with FileShare.Read. Which denies write access. That can't work, the other program already gained write access.

You'll need to create the StreamReader like this:

using (var fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var sr = new StreamReader(fs, Encoding.Default)) {
    // read the stream
    //...
}

Guessing at the Encoding here. You have to be careful with this kind of code, the other program is actively writing to the file. You won't get a very reliable end-of-file indication, getting a partial last line is quite possible. In particular troublesome when you keep reading the file to try to get whatever the program appended.

Use

File.Open(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);

This should work as long as the other application has not locked the file exclusively.