The MSDN constructor for a FileStream says that it may throw either an UnauthorizedAccessException or a SecurityException. Here's what MSDN says about these exceptions.
UnauthorizedAccessException: The exception that is thrown when the operating system denies access because of an I/O error or a specific type of security error.
SecurityException: The exception that is thrown when a security error is detected.
How are these two similar exceptions different? What situations will trigger either of them?
A UnauthorizedAccessException is thrown when there is a permissions error accessing the file on disk. That is an error at the operating system level such as a normal user trying to overwrite an operating system file (like kernel32.dll).
A SecurityException is thrown if there is a security violation at the CLR level. For example if you are running as a low access ClickOnce application and attempt to read / write to a place in the file system forbidden by the CLR security settings in the process.
来源：https://stackoverflow.com/questions/2339251/unauthorizedaccessexception-vs-securityexception