There is no one specifier for all objects within an application.
For classes and structs immediately within a namespace (i.e. at the top level) the default access specifier is
internal
For classes and structs, properties, fields, enums and methods within a class it is
private
However, it is a very, very good idea to absolutely state the access specifier at all times - that way you cannot make a mistake, and it forces you to think about what kind of access is appropriate for it.
See
MDSN[
^]