UnauthorizedAccessException opening UNC path file with FileStream

Message

UnauthorizedAccessException: Access to the path '...' is denied.

Context
Executing the following line of code (C# ASP.NET under Visual Studio, localFile or UNC path)

FileStream fs = new FileStream(fileName, FileMode.Open);

Explanation
ND

Solution
Add the fileaccess mode option "FileAccess.Read":
FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);