Expressions in SQL Server Integrations Services 2005 are extremely powerful. You might use them to dynamically concantenate strings, for example, when you want to write, import or export to various files at run time.
But try as you might to use an algorithm like:
@User::FolderPath + "\" + @User::FileName
it just won't work. You'll get a nasty error message that says "Expression cannot be evaluated."
Fix it by adding a second slash, to read:
@User::FolderPath + "\\" + @User::FileName