1, if the beginning of the program comprises using System; instruction, can be used directly System classes and methods without fully defined.
For example: Console.WriteLine can be called directly, rather than System.Console.WriteLine
If the System class method called directly, but there is no use using System, it will error --- because this method can not be found which belong to the class
2, C # program may consist of one or more files. Each file can contain zero or more namespaces.
Namespace may comprise a class, structure, interfaces, enumerations, other delegate type, and other namespaces
3, C # identifier
Identifier is assigned to the type (class, interface, structure, delegate or enumeration), a member variable or name of the namespace, valid identifiers must follow these rules:
① identifier must begin with a letter or _
② Unicode identifier may comprise alphabetic characters, decimal numeric characters, Unicode characters are connected, a combination of Unicode character or to Unicode
Naming Conventions - use camelCased PascalCase
① interface names begin with a capital letter I
② Attribute Type Attribute ending with the word
③ enumerated type of non-labeled singular noun, plural noun used to mark
④ identifier should not contain two consecutive characters _, these names are reserved for the compiler-generated identifiers