Understanding C# Keywords and Identifiers: Rules, Lists, and Best Practices

C# Keywords and Identifiers

Master the fundamentals of C# keywords and identifiers, the building blocks that define your code’s structure and behavior.

C# Keywords

Keywords are predefined words that the C# compiler reserves for its own use. Their meaning is fixed; they cannot be altered, nor can they be used directly as identifiers.

For example:

long mobileNum;

Here, long is a keyword that declares a long‑typed variable named mobileNum. Keywords such as int, char, and long cannot be used as variable names without modification.

Attempts to use a keyword as an identifier, like long long;, will cause a compilation error. C# contains 79 reserved keywords, all in lowercase. Below is the complete list:

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

Keywords can be used as identifiers when prefixed with @:

int @void;

This declares an int variable named @void.

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Contextual Keywords

In addition to reserved keywords, C# defines 25 contextual keywords. These words acquire special meaning only in specific contexts and can otherwise serve as regular identifiers.

add	alias	ascending
async	await	descending
dynamic	from	get
global	group	into
join	let	orderby
partial (type)	partial (method)	remove
select	set	value
var	when (filter condition)	where (generic type constraint)
yield

For a deeper dive into each keyword, consult the official documentation: C# Keywords.

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C# Identifiers

Identifiers give names to variables, methods, classes, and other program elements. They uniquely distinguish each element within a scope.

Example:

int value;

Here, value is an identifier. Attempting to use a reserved keyword without the @ prefix, such as int break;, will trigger a compile‑time error.

Learn more about variables in C# Variables.

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Rules for Naming an Identifier

• It cannot be a C# keyword.
• It must begin with a letter, underscore (_), or the @ symbol; subsequent characters may be letters, digits, or underscores.
• Whitespace and other symbols (e.g., $) are disallowed.
• Identifiers are case‑sensitive: getName, GetName, and getname are distinct.

Valid vs. invalid identifiers:

Identifier	Remark
number	Valid
calculateMarks	Valid
hello$	Invalid (contains $)
name1	Valid
@if	Valid (keyword with @)
if	Invalid (C# keyword)
My name	Invalid (whitespace)
_hello_hi	Valid

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Example: Extracting Keywords and Identifiers from a Program

Let’s examine a simple Hello World program to identify its keywords and identifiers.

using System;
namespace HelloWorld
{
    class Hello
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

Keyword–Identifier pairs in this snippet:

Keyword	Identifier
using	System
namespace	HelloWorld
class	Hello
static	Main
void	args
string	Console
	WriteLine

The string literal "Hello World!" is passed to WriteLine.