namespace issues

Question

i'm working on a project and i've stumbled on an annoying issue. say i have a file that belongs to a namspace Foo. and then another file that belongs to a namespace Bar. Foo is my main file and it has a line at the beginning which attempts to include the Bar namespace. namely, "using Bar;" however i get the following on compiling: error CS0246: The type or namespace name 'Bar' could not be found (are you missing a using directive or an assembly reference?) whats going on here? i know this is possible in theory. havent seen any practical code that used it though....would i have to have some sort of compiletime flags added, or put everything in the same file or something?