Checking for Open Text File in Notepad, and Closing it

Question

I wonder if anyone might be able to resolve the following issue: I have a text file, which I write to using a StreamWriter. I want to be able to append to the file, but beforehand I must ensure that the file is closed. I have tried using System.Threading.Mutex(), System.Diagnostics.Process.GetCurrentProcess(), and System.Diagnostics.Process.GetProcessesByName(). But with no success. I think my problem mainly exists with either detecting whether a file is open (i.e. file locked), or the specific file is open, as opposed to the application (Notepad) which it runs in. Any assistance would be greatly appreciated. Thanks