How to check if file exists in .Net

I am trying to figure out how to use .NET with matlab... this is probably a really simple question. I want to make the code below work...(and not use the "exist" function)... What did i do wrong with the syntax of FileSystemInfo.Exists?

fstring = [tempname '.txt'];
if System.IO.FileSystemInfo.Exists(fstring)
disp('exists')
else
disp('not exist')
end