What are tokens in C#?

A token is the smallest element of a C# program that the compiler can recognize. Tokens are the building blocks of C# code and include keywords, identifiers, literals, operators, and punctuation marks. Understanding tokens is fundamental to writing valid C# programs.

Keywords

Keywords are reserved words predefined by the C# compiler. These keywords have special meanings and cannot be used as identifiers. However, if you want to use these keywords as identifiers, you may prefix the keyword with the @ character.

Example of Using @ Prefix with Keywords

using System;

class Program {
   public static void Main() {
      int @int = 10;      // Using 'int' as variable name with @ prefix
      string @class = "MyClass";  // Using 'class' as variable name
      
      Console.WriteLine("Value of @int: " + @int);
      Console.WriteLine("Value of @class: " + @class);
   }
}

The output of the above code is −

Value of @int: 10
Value of @class: MyClass

The following are some of the reserved keywords in C# −

Identifiers

An identifier is a name used to identify a class, variable, method, or any other user-defined item. Identifiers are tokens that you create to name your program elements.

Rules for Identifiers

• Must begin with a letter (a-z, A-Z) or underscore (_), followed by letters, digits (0-9), or underscores.

• The first character cannot be a digit.

• Cannot contain spaces or special symbols like ? - + ! @ # % ^ & * ( ) [ ] { } . ; : " ' / \

• Cannot be a reserved keyword (unless prefixed with @).

• Are case-sensitive (MyVar and myvar are different).

Example of Valid and Invalid Identifiers

using System;

class Program {
   public static void Main() {
      // Valid identifiers
      int myNumber = 100;
      string firstName = "John";
      double _salary = 50000.50;
      bool isValid123 = true;
      
      // Display values
      Console.WriteLine("myNumber: " + myNumber);
      Console.WriteLine("firstName: " + firstName);
      Console.WriteLine("_salary: " + _salary);
      Console.WriteLine("isValid123: " + isValid123);
   }
}

The output of the above code is −

myNumber: 100
firstName: John
_salary: 50000.5
isValid123: True

Examples of Invalid Identifiers

int 2number;        // Cannot start with digit
string first-name;  // Cannot contain hyphen
double my salary;   // Cannot contain space
bool class;         // Cannot use keyword (unless @class)

Other Types of Tokens

Literals

Literals represent fixed values in the program −

using System;

class Program {
   public static void Main() {
      int intLiteral = 42;           // Integer literal
      double doubleLiteral = 3.14;   // Double literal
      string stringLiteral = "Hello"; // String literal
      char charLiteral = 'A';        // Character literal
      bool boolLiteral = true;       // Boolean literal
      
      Console.WriteLine("Integer: " + intLiteral);
      Console.WriteLine("Double: " + doubleLiteral);
      Console.WriteLine("String: " + stringLiteral);
      Console.WriteLine("Character: " + charLiteral);
      Console.WriteLine("Boolean: " + boolLiteral);
   }
}

The output of the above code is −

Integer: 42
Double: 3.14
String: Hello
Character: A
Boolean: True

Conclusion

Tokens are the fundamental building blocks of C# programs, consisting of keywords, identifiers, literals, operators, and symbols. Understanding token rules, especially for keywords and identifiers, is essential for writing syntactically correct C# code and avoiding compilation errors.