Importing in C#

In java, if I have a file main.java and a file b.java in the folder b, I can import easily the classes from b with an import b; in the main. How can I do the equivalent in C# ? "Using" only seems to provide aliases, it seems like I need to add files to the project in Visual Studio to import files. But it's very annoying as it means I can't just send a few files as in java, I need to send the whole project...