Compilation units

A compilation-unit defines the overall structure of a source file. A compilation unit consists of zero or more using-directives followed by zero or more global-attributes followed by zero or more namespace-member-declarations. compilation-unit : using-directives global-attributes namespace-member-declarations
A C# program consists of one or more compilation units, each contained in a separate source file. When a C# program is compiled, all of the compilation units are processed together. Thus, compilation units can depend on each other, possibly in a circular fashion.
The using-directives of a compilation unit affect the global-attributes and namespace-member-declarations of that compilation unit, but have no effect on other compilation units.
The global-attributes (24) of a compilation unit permit the specification of attributes for the target assembly. Assemblies act as physical containers for types.
The namespace-member-declarations of each compilation unit of a program contribute members to a single declaration space called the global namespace. [Example: For example: File A.cs: class A {}
File B.cs: class B {}
The two compilation units contribute to the single global namespace, in this case declaring two classes with the fully qualified names A and B. Because the two compilation units contribute to the same declaration space, it would have been an error if each contained a declaration of a member with the same name. end example]