C# Keywords and Identifiers

C# Keywords

Keywords are predefined sets of reserved words that have special meaning in a program. The meaning of keywords can not be changed, neither can they be directly used as identifiers in a program.

For example,

long mobileNum;

Here, long is a keyword and mobileNum is a variable (identifier). long has a special meaning in C# i.e. it is used to declare variables of type long and this function cannot be changed.

Also, keywords like long, int, char, etc can not be used as identifiers. So, we cannot have something like:

long long;

C# has a total of 79 keywords. All these keywords are in lowercase. Here is a complete list of all C# keywords.

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  

Although keywords are reserved words, they can be used as identifiers if @ is added as prefix. For example,

int @void;

The above statement will create a variable @void of type int.

Contextual Keywords

Besides regular keywords, C# has 25 contextual keywords. Contextual keywords have specific meaning in a limited program context and can be used as identifiers outside that context. They are not reserved words in C#.

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)

If you are interested to know the function of every keywords, I suggest you visit C# keywords (official C# docs).

C# Identifiers

Identifiers are the name given to entities such as variables, methods, classes, etc. They are tokens in a program which uniquely identify an element. For example,

int value;

Here, value is the name of variable. Hence it is an identifier. Reserved keywords can not be used as identifiers unless @ is added as prefix. For example,

int break;

This statement will generate an error in compile time.

To learn more about variables, visit C# Variables.

Rules for Naming an Identifier

  • An identifier can not be a C# keyword.
  • An identifier must begin with a letter, an underscore or @ symbol. The remaining part of identifier can contain letters, digits and underscore symbol.
  • Whitespaces are not allowed. Neither it can have symbols other than letter, digits and underscore.
  • Identifiers are case-sensitive. So, getName, GetName and getname represents 3 different identifiers.

Here are some of the valid and invalid identifiers:

Identifiers Remarks
number Valid
calculateMarks Valid
hello$ Invalid (Contains $)
name1 Valid
@if Valid (Keyword with prefix @)
if Invalid (C# Keyword)
My name Invalid (Contains whitespace)
_hello_hi Valid

Example: Find list of keywords and identifiers in a program

Just to clear the concept, let's find the list of keywords and identifiers in the program we wrote in C# Hello World.

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


Keywords Identifiers
using System
namespace HelloWorld (namespace)
class Hello (class)
static Main (method)
void args
string Console

The "Hello World!" inside WriteLine method is a string literal.

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