User753101303 posted
Hi,
All your C# code runs on the web server which doesn't have any access to the client side. The purpose of this server side code is to render an HTML page which is shiown in the browser. The browser is itself a sandbox with very limited access to local resource
for safety reason.
In short the standard way for a web server to access a client side file is to let the user upload this (and for safety reason the input type=file upload control can be populated only by the user).
It's common to be confused when starting as you usually develop on a machine that is both the web server and the browser side machine which can give the impression things that won't ever work with a real web server could work.