Keywords in C#

C# keywords are reserved words with predefined meanings used by the compiler to perform specific operations. They are part of the C# language syntax and generally cannot be used as identifiers unless the @ prefix is used.

Keywords cannot normally be used as identifiers such as variable, method, class, or object names.
C# also has contextual keywords, which have special meaning only in specific contexts.
Examples include int, class, if, return, public, etc.

using System;

class Geeks {
    static void Main()
    {
        const int limit = 20;
        int marks = 25;

        if (marks >= limit) {
            Console.WriteLine("Passed");
        }
        else {
            Console.WriteLine("Failed");
        }
    }
}

Output

Passed

Explanation:

using imports a namespace.
class defines a class named Program.
static declares a method that belongs to the class itself.
void indicates that the method does not return a value.
const declares a constant.
int specifies an integer data type.
if and else control which block of code executes.

Types of Keywords

C# keywords are classified into two main categories based on how they are interpreted by the compiler.

1. Reserved Keywords

Reserved keywords have predefined meanings in C# and cannot normally be used as names for variables, classes, or methods.

Examples include int, class, if, while, return, and public.

int age = 25;

if (age >= 18)
{
    Console.WriteLine("Eligible");
}

Here, int declares an integer variable, while if checks a condition.

2. Contextual Keywords

Contextual keywords have special meanings only in particular programming contexts. Outside those contexts, they can often be used as identifiers.

Examples include var, async, await, record, partial, and where.

var name = "GeeksforGeeks";
Console.WriteLine(name);

Here, var tells the compiler to infer the local variable's type from its initializer. In this case, the inferred type is string.

List of C# Keywords

C# keywords can be grouped according to their common purposes, such as declaring data types, controlling execution, handling exceptions, and defining classes.

The following sections organize the main reserved keywords into functional categories.

Data Type Keywords

Used to define variables and specify the kind of data they can hold.

Keyword	Usage
bool	Represents a Boolean value, true or false.
byte	Represents an 8-bit unsigned integer.
sbyte	Represents an 8-bit signed integer.
char	Represents a 16-bit Unicode character.
short	Represents a 16-bit signed integer.
ushort	Represents a 16-bit unsigned integer.
int	Represents a 32-bit signed integer.
uint	Represents a 32-bit unsigned integer.
long	Represents a 64-bit signed integer.
ulong	Represents a 64-bit unsigned integer.
float	Represents a single-precision floating-point number.
double	Represents a double-precision floating-point number.
decimal	Represents a high-precision decimal number.
string	Represents a sequence of characters.
object	Represents the base type from which all C# types ultimately derive.
void	Specifies that a method does not return a value.

Control Flow Keywords

Used to control the execution flow of a program.

Keyword	Usage
if	Executes code when a condition is true.
else	Defines an alternative block when an if condition is false.
switch	Selects a block of code based on a value.
case	Defines an individual branch in a switch statement.
default	Defines the block executed when no case matches.
for	Starts a for loop.
foreach	Iterates through elements in a collection.
while	Starts a while loop.
do	Starts a do-while loop.
break	Terminates the nearest loop or switch.
continue	Skips to the next iteration of a loop.
goto	Transfers execution to a labeled statement.
return	Exits from a method and optionally returns a value.

Exception Handling Keywords

Used for handling runtime errors and exceptions.

Keyword	Usage
try	Defines a block of code in which exceptions may occur.
catch	Handles an exception thrown by a try block.
finally	Defines code that executes after try/catch.
throw	Throws an exception.

Object-Oriented Keywords

Used to define classes, objects, inheritance, interfaces, and related object-oriented features.

Keyword	Usage
class	Declares a class.
interface	Declares an interface.
struct	Declares a value-type structure.
enum	Declares a set of named constants.
delegate	Defines a delegate type for referencing methods.
new	Creates an object or invokes a constructor.
this	Refers to the current object.
base	Refers to members of the base class.
abstract	Declares a class or member that requires implementation by derived types.
sealed	Prevents inheritance or further overriding.
virtual	Allows a derived class to override a member.
override	Provides a new implementation of an inherited virtual or abstract member.

Access Control Keywords

Define the accessibility of classes and their members.

Keyword	Usage
public	Accessible from anywhere that can access the containing type.
private	Accessible only within the containing type.
protected	Accessible within the containing type and derived types.
internal	Accessible within the current assembly.

Namespace and Using Keywords

Used to organize code and access types from namespaces.

Keyword	Usage
namespace	Defines a scope for organizing related types and namespaces.
using	Imports a namespace or defines other using directives.

C# namespaces organize related types and can also be declared using file-scoped syntax.

Modifier Keywords

Used to define additional characteristics of types and members.

Keyword	Usage
static	Indicates that a member belongs to the type rather than an instance.
const	Declares a compile-time constant.
readonly	Prevents a field from being assigned except in permitted initialization contexts.
volatile	Indicates that a field can be modified by multiple threads or external mechanisms.
extern	Indicates that a method is implemented externally.
unsafe	Enables unsafe code operations.

Parameter and Reference Keywords

Used with method parameters and references.

Keyword	Usage
ref	Passes a parameter by reference.
out	Passes a parameter by reference for output.
in	Passes a parameter by readonly reference.
params	Allows a method to accept a variable number of arguments.

Type and Conversion Keywords

Keyword	Usage
as	Performs a compatible reference, nullable, or boxing conversion.
is	Checks whether an object is compatible with a specified type or pattern.
typeof	Obtains the System.Type object for a type.
sizeof	Obtains the size of a value type in bytes in permitted contexts.
default	Produces the default value of a type or specifies a default case.

Operator Keywords

Keyword	Usage
operator	Declares an overloaded operator.
implicit	Defines an implicit user-defined conversion.
explicit	Defines an explicit user-defined conversion.

Memory Management and Synchronization Keywords

Keyword	Usage
lock	Acquires an exclusive lock for a critical section.
fixed	Prevents the garbage collector from relocating a variable during an unsafe operation.
stackalloc	Allocates memory on the stack.

Checked and Unchecked Keywords

Keyword	Usage
checked	Enables overflow checking for integral arithmetic operations.
unchecked	Disables overflow checking for integral arithmetic operations.

Contextual Keywords

Contextual keywords have special meaning only in particular contexts and may be used as ordinary identifiers outside those contexts.

Keyword	Usage
var	Enables implicitly typed local variables.
dynamic	Defers type checking for certain operations until runtime.
async	Marks a method, lambda, or local function as asynchronous.
await	Suspends an asynchronous operation until the awaited task completes.
nameof	Produces the name of a symbol as a string.
record	Declares a record type.
required	Indicates that a member must be initialized when an object is created.
partial	Allows a type or member declaration to be split across multiple declarations.
get	Defines the getter accessor of a property or indexer.
set	Defines the setter accessor of a property or indexer.
init	Defines an accessor that can assign a property during object initialization.
value	Represents the value being assigned in a property or indexer setter.
yield	Produces values from an iterator.
with	Creates a copy of an object with specified changes.
when	Specifies a filter condition in certain contexts.
where	Specifies generic constraints or filters query results.

LINQ Query Keywords

C# query expressions use contextual keywords such as from, where, select, group, orderby, join, let, into, on, equals, by, ascending, and descending.

Keyword	Usage
from	Specifies the data source of a query.
where	Filters elements based on a condition.
select	Specifies the result of a query.
group	Groups query results.
orderby	Sorts query results.
join	Combines elements from two data sources.
let	Introduces a variable inside a query.
into	Stores an intermediate query result.
on	Specifies the join condition.
equals	Specifies equality between join keys.
by	Specifies a grouping key.
ascending	Sorts query results in ascending order.
descending	Sorts query results in descending order.

Special Literals

C# provides predefined literal values that represent special values.

Literal	Usage
true	Represents the Boolean value true.
false	Represents the Boolean value false.
null	Represents a null reference or null value where applicable.

Keywords vs Identifiers

Feature	Keywords	Identifiers
Definition	Reserved or context-specific words with predefined meanings in C#	User-defined names for variables, methods, classes, etc.
Purpose	Define the syntax and structure of C# programs	Identify program elements
Can be Modified	No	Yes
Usage as Variable Name	Not normally allowed	Allowed
Predefined by C#	Yes	No
Examples	int, class, if, return	studentName, calculateSum, Employee
Case Sensitive	Yes	Yes
Compiler Meaning	Defined by the C# language	Defined by the programmer

Important: Unlike Java's final, C# uses const, readonly, and other mechanisms for different forms of immutability; and unlike Java, C# has the @ escape mechanism for using keywords as identifiers.