If you get the following error when trying to write an expression there is an easy solution

Attempt to parse the expression "@[User::FilePath] + "\" + @[User::FileName] + ".raw"" failed.  The token "." at line number "1", character number "<some position>" was not recognized. The expression cannot be parsed because it contains invalid elements at the location specified.

The SSIS expression language is a C based language and the \ is a token, this means you have to escape it with another one. i.e "\" becomes "\\", unlike C# you can't prefix the string with a @, you have to use the escaping route.

In summary when ever you want to use \ you need to use two \\