C# | Keywords

Keywords or Reserved words are the words in a language that are used for some internal process or represent some predefined actions. These words are therefore not allowed to use as variable names or objects. Doing this will result in a compile-time error.

Example:

// C# Program to illustrate the keywords
using System;

class GFG {

    // Here static, public, void
    // are keywords
    static public void Main () {

        // here int is keyword
        // a is identifier
        int a = 10;

        Console.WriteLine("The value of a is: {0}",a);

        // this is not a valid identifier

        // removing comment will give compile time error
        // double int = 10;

    }
}

Output:

The value of a is: 10

There are total 78 keywords in C# as follows:

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

int

interface

internal

is

lock

long

namespace

new

null

object

operator

out

override

params

private

protected

public

readonly

ref

return

sbyte

sealed

short

sizeof

stackalloc

static

string

struct

switch

this

throw

true

try

typeof

unit

ulong

unchecked

unsafe

ushort

using

using static

virtual

void

volatile

while

Keywords in C# is mainly divided into 10 categories as follows:

1. Value Type Keywords: There are 15 keywords in value types which are used to define various data types.
   

   bool	byte	char	decimal
   double	enum	float	int
   long	sbyte	short	struct
   unit	ulong	ushort

   Example:

   // C# Program to illustrate the
   // value type keywords
   using System;
   
   class GFG {
   
       // Here static, public, void
       // are keywords
       static public void Main () {
   
           // here byte is keyword
           // a is identifier
           byte a = 47;
           Console.WriteLine("The value of a is: {0}",a);
   
   
           // here bool is keyword
           // b is identifier
           // true is a keyword
           bool b = true;
   
           Console.WriteLine("The value of b is: {0}",b);
   
   
   
       }
   }

   Output:

   The value of a is: 47
   The value of b is: True
   

2. Reference Type Keywords: There are 6 keywords in reference types which are used to store references of the data or objects. The keywords in this category are: class, delegate, interface, object, string, void.
3. Modifiers Keywords: There are 17 keywords in modifiers which are used to modify the declarations of type member.
   

   public	private	internal	protected	abstract
   const	event	extern	new	override
   partial	readonly	sealed	static	unsafe
   virtual	volatile

   Example:

   // C# Program to illustrate the
   // modifiers keywords
   using System;
   
   class Geeks {
   
       class Mod
       {
   
           // using public modifier
           // keyword
           public int n1;
   
       }
   
       // Main Method
       static void Main(string[] args) {
   
           Mod obj1 = new Mod();
   
           // access to public members
           obj1.n1 = 77;
   
           Console.WriteLine("Value of n1: {0}", obj1.n1);
   
       }
   
   }

   Output:

   Value of n1: 77

4. Statements Keywords: There are total 18 keywords which are used in program instructions.
   

   if	else	switch	do	for
   foreach	in	while	break	continue
   goto	return	throw	try	catch
   finally	checked	unchecked

   Example:

   // C# program to illustrate the statement keywords
   using System;
   
   class demoContinue
   {
       public static void Main()
       {
   
           // using for as statement keyword
           // GeeksforGeeks is printed only 2 times
           // because of continue statement
           for(int i = 1; i < 3; i++)
           {
   
               // here if and continue are keywords
               if(i == 2)
               continue;
   
               Console.WriteLine("GeeksforGeeks");
           }
       }
   }

   Output:

   GeeksforGeeks

5. Method Parameters Keywords: There are total 4 keywords which are used to change the behavior of the parameters that passed to a method. The keyword includes in this category are: params, in, ref, out.
6. Namespace Keywords: There are total 3 keywords in this category which are used in namespaces. The keywords are: namespace, using, extern.
7. Operator Keywords: There are total 8 keywords which are used for different purposes like creating objects, getting a size of object etc. The keywords are: as, is, new, sizeof, typeof, true, false, stackalloc.
8. Conversion Keywords: There are 3 keywords which are used in type conversions. The keywords are: explicit, implicit, operator.
9. Access Keywords: There are 2 keywords which are used in accessing and referencing the class or instance of the class. The keywords are base, this.
10. Literal Keywords: There are 2 keywords which are used as literal or constant. The keywords are null, default.

Important Points:

• Keywords are not used as an identifier or name of a class, variable, etc.
• If you want to use a keyword as an identifier then you must use @ as a prefix. For example, @abstract is valid identifier but not abstract because it is a keyword.

Example:

int a = 10;              // Here int is a valid keyword
 
double int = 10.67;     // invalid because int is a keyword

double @int = 10.67;   // valid identifier, prefixed with @

int @null = 0;       // valid

// C# Program to illustrate the use of
// prefixing @ in keywords
using System;

class GFG {

    // Here static, public, void
    // are keywords
    static public void Main () {

        // here int is keyword
        // a is identifier
        int a = 10;

        Console.WriteLine("The value of a is: {0}",a);

        // prefix @ in keyword int which
        // makes it a valid identifier
        int @int = 11;

        Console.WriteLine("The value of a is: {0}",@int);

    }
}

Output:

The value of a is: 10
The value of a is: 11

Contextual Keywords

These are used to give a specific meaning in the program. Whenever a new keyword comes in C#, it is added to the contextual keywords, not in the keyword category. This helps to avoid the crashing of programs which are written in earlier versions.

Important Points:

• These are not reserved words.
• It can be used as identifiers outside the context that’s why it named contextual keywords.
• These can have different meanings in two or more contexts.
• There are total 30 contextual keywords in C#.

add

alias

ascending

async

await

by

descending

dynamic

equals

from

get

global

group

into

join

let

nameof

on

orderby

partial(type)

partial(method)

remove

select

set

value

var

when

where

where

yield

Example:

// C# program to illustrate contextual keywords
using System;

public class Student {

    // Declare name field
    private string name = "GeeksforGeeks";

    // Declare name property
    public string Name
    {

    // get is contextual keyword
    get
        {
            return name;
        }

        // set is a contextual
        // keyword
        set
        {
            name = value;
        }
    }
}

class TestStudent {

    // Main Method
    public static void Main(string[] args)
    {
        Student s = new Student();

        // calls set accessor of the property Name,
        // and pass "GFG" as value of the
        // standard field 'value'.
        s.Name = "GFG";

        // displays GFG, Calls the get accessor
        // of the property Name.
        Console.WriteLine("Name: " + s.Name);

        // using get and set as identifier
        int get = 50;
        int set = 70;

        Console.WriteLine("Value of get is: {0}",get);
        Console.WriteLine("Value of set is: {0}",set);
    }
}

Output:

Name: GFG
Value of get is: 50
Value of set is: 70

Reference: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/