C# Variable

A variable is a name of memory location. It is used to store data. Its value can be changed and it can be reused many times. It is a way to represent memory location through symbol so that it can be easily identified. The basic variable type available in C# can be categorized as:

• int - stores integers (whole numbers), without decimals, such as 123 or -123
• double - stores floating point numbers, with decimals, such as 19.99 or -19.99
• char - stores single characters, such as 'a' or 'B'. Char values are surrounded by single quotes
• string - stores text, such as "Hello World". String values are surrounded by double quotes
• bool - stores values with two states: true or false

Declaring (Creating) Variables

Syntax for variable definition in C# is -

<data_type> <variable_list>;

Here, data_type must be a valid C# data type including char, int, float, double, or any user-defined data type, and variable_list may consist of one or more identifier names separated by commas.

Some valid variable definitions are shown here -

int i, j, k;
char c, ch;
float f, marks;
double d;

You can initialize a variable at the time of definition as -

int i = 100;

Above, int is a data type, i is a variable name (identifier). The = operator is used to assign a value to a variable. The right side of the = operator is a value that will be assigned to left side variable. Above, 100 is assigned to a variable i.

Initializing Variables

Variables are initialized (assigned a value) with an equal sign followed by a constant expression. The general form of initialization is -

variable_name = value;

Variables can be initialized in their declaration. The initializer consists of an equal sign followed by a constant expression as −

<data_type> <variable_name> = value;

A demonstration of how to declare variables of other types:

int myNum = 5;
double 
myDoubleNum = 5.99D;

char myLetter = 'D';

bool myBool = true;

string myText = "Hello";

Rules for defining variables

• A variable can have alphabets, digits and underscore.
• A variable name can start with alphabet and underscore only. It can't start with digit.
• No white space is allowed within variable name.
• A variable name must not be any reserved word or keyword e.g. char, float etc.

Example

using System;

public class MyApplication
{
	public static void Main()
	{
		int i = 200;
		int j = i + 20; 
		Console.WriteLine("j = {0}", j);
		
		i = 300;
		j = i + 20; 
		Console.WriteLine("j = {0}", j);
		
		i = 400;
		Console.WriteLine("j = {0}", j);  

	}
}

OUTPUT

j = 220
j = 320
j = 320

In the above example, value of j depends on the value of i. You must re-execute expression each time you change the value of i; otherwise, value of j would not change based on the value of i.

Valid variable names:

int y;      
int _y;      
int x20;      

Invalid variable names:

int 6;      
int a b;      
int float;      

Multiple Variables in a Single Line

using System;
					
public class MyApplication
{
	public static void Main()
	{
		int i = 0, j = 10, k = 200;
		
		Console.WriteLine(j);
	}
}

OUTPUT

10

C# Identifiers

All C# variables must be identified with unique names. These unique names are called identifiers. Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume). Note: It is recommended to use descriptive names in order to create understandable and maintainable code:

using System;

namespace MyApplication
{
  class Program
  {
    static void Main(string[] args)
    {
      // Good
      int age = 30;

      // OK, but not so easy to understand what a actually is
      int a = 30;
      
      Console.WriteLine(age);
      Console.WriteLine(a);
    }
  }
}

OUTPUT
30
30

Note: C# is the strongly typed language. It means you can assign a value of the specified data type. You cannot assign an integer value to string type or vice-versa.

There are few example which create error by this declaration of variable

int num = "Hello";  // Cannot assign string to int type variable

int num;
num = 200;            //Variables can be declared first and initialized later.

int i;
int j = i;  //compile-time error: Use of unassigned local variable 'i'