Unable To Secure/lock Down A File That Is In Use/open

I am writing a program that acceses a file in VB/Visual Studio 2008 on Win7 and running it as a user who has admin credentials. I am opening the file with the following code:

Dim Input As New FileStream("C:FileTest est.txt", FileMode.Append, FileAccess.Write, FileShare.None) To test to make sure I can't access this file, I tried to open it with Notepad and I was able to and got no errors. I wrote a new value using Notepad and even saved it and I didn't get any errors form Notepad or my VB program.

I then went on and treid to open the same file twice from my VB program as:

Dim Input As New FileStream("C:FileTest est.txt", FileMode.Append, FileAccess.Write, FileShare.None)
Dim Input2 As New FileStream("C:FileTest est.txt", FileMode., FileAccess.Read, FileShare.None)

And I receivded an error in my VB program as expected. So the FileShare.None switch kind of works. Although I was still able to open the file with Notepad while my VB program had it open. I was aslo able to open Windows Explorer and reame it and even delte it while my VB program had it "open", in the write mode no less without my VB program knowing anything about it.

Why are other applications able to ignore the FileShare.None switch? Seems pretty useless if that's the case. And a huge secruity problem