Identifier and Keywords

Identifier

Identifiers are character strings used to name things such as class, variables, methods, parameters and so on. You can create self-documenting identifiers by concatenating meaningful words into a single descriptive name, using uppercase and lowercase letters (e.g., FirstName, LastName, SocialSecurityNum, etc.). These are the identifiers in previous example program:
System Program1 Program Main args Console WriteLine Name ReadLine Read
There's some rule for defining identifier:

• Identifiers are case-sensitive. For instance, the variable names myVariable and myvariable are different identifiers.
• The alphabetic and underscore characters (a through z, A through Z, and _) are allowed at any position.
• Digits are not allowed in the first position but are allowed everywhere else. Therefore: person1 is legal identifier, but 1Person is illegal
• The @ character is allowed in the first position of an identifier but not at any other position. Although allowed, its use is generally discouraged. @FirstName is legal but First@name will be resulting compile error.

Commonly Used Identifier Naming Styles

Style Name	Description	Recommended Use	Examples
Pascal casing	Each word in the identifier is capitalized.	Use for type names and member names visible outside the class. These include the names of the following: classes, methods, namespaces, properties, and public fields.	FirstName,LastName
Camel casing	Each word in the identifier, except the first, is capitalized.	Use for local variable names and the names of formal parameters in method declarations.	firstName,lastName
Camel case with leading underscore	This is a camel-cased identifier that starts with an underscore.	Use for the names of private and protected fields.	_firstName,_lastName

Keywords

Keywords are the character string tokens used to define the C# language. The important thing about keyword is that they can not be used as identifier.

abstract	const	extern	int	out	short	typeof
as	continue	false	interface	override	sizeof	uint
base	decimal	finally	internal	params	stackalloc	ulong
bool	default	fixed	is	private	static	unchecked
break	delegate	float	lock	protected	string	unsafe
byte	do	for	long	public	struc	ushort
case	double	foreach	namespace	readonly	switch	using
catch	else	goto	new	ref	this	virtual
char	enum	if	null	return	throw	void
checked	event	implicit	object	sbyte	true	volatile
class	explicit	in	operator	sealed	try	while

Keywords cannot be used as variable names or any other form of identifier, unless prefaced with the @ character.

Contextual keywords are identifiers that act as keywords only in certain language constructs, this mean we can still use this as identifier.

add	ascending	async	awai	by	descending	dynamic
equals	from	get	global	group	in	into
join	let	on	orderby	partial	remove	select
set	value	var	where	yield

Main Method

Every C# program must have one class with a method called Main. In program shown previously, it was declared in a class called Program.

• The starting point of execution of every C# program is at the first instruction in Main.
• The name Main must be capitalized.

The simplest form of Main is the following:

static void Main( )
{
    Statements
}

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