C# Variables

Variables in C# are the same as variables in mathematics. Before we understand what is a variable, let's understand the expressions. The followings are examples of expressions.

Example: Expressions

10 + 20
5 * 2
10/2

The result of the above expressions are fixed e.g. 10 + 20 = 30, 5 * 2 = 10 and 10/2 = 5. Now, consider the following expressions.

Example: Expressions with Variable

x + 20
y * 2
z/2

The result of the above expression depends on the value of x, y and z. For example, if x = 5 then the result of x + 20 would be 25 and if x = 20 then the result of x + 20 would be 40. In the same way, the result of y * 2 depends on the value of y and the result of z/2 depends on the value of z. Here, x, y, z are called variables because their values can be changed.

The same concept is used in C#. In C#, a variable stores a value of the specific data type. It can store a numeric, char, string, or other types of value. You can declare and assign a value to a variable like int x = 5; where int is the data type, x is the name of a variable, = is an operator that assigns the value to a variable, and 5 is the integer value assigned to a variable x.

Syntax

<data type> <variable name> = <value>;

The following declares and initializes a variable of an int type.

Example: C# Variable

int num = 100;

Above, int is a data type, num 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 num.

The following declares and initializes variables of different data types.

Example: C# Variables

int num = 100;
float rate = 10.2f;
decimal amount = 100.50M;
char code = 'C';
bool isValid = true;
string name = "Steve";

The followings are naming conventions for declaring variables in C#:

• Variable names must be unique.
• Variable names can contain letters, digits, and the underscore _ only.
• Variable names must start with a letter.
• Variable names are case-sensitive, num and Num are considered different names.
• Variable names cannot contain reserved keywords. Must prefix @ before keyword if want reserve keywords as identifiers.

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.

Example: Cannot assign string to int type variable

int num = "Steve";

Variables can be declared first and initialized later.

Example: Late Initialization

int num;
num = 100;

A variable must be assigned a value before using it, otherwise, C# will give a compile-time error.

Error: Invalid Assignment

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

The value of a variable can be changed anytime after initializing it.

Example: C# Variable

int num = 100;
num = 200;
Console.WriteLine(num); //output: 200

Multiple variables of the same data type can be declared and initialized in a single line separated by commas.

Example: Multiple Variables in a Single Line

int i, j = 10, k = 100;

Multiple variables of the same type can also be declared in multiple lines separated by a comma. The compiler will consider it to be one statement until it encounters a semicolon ;.

Example: Multi-Line Declarations

int i = 0, 
    j = 10, 
    k = 100;

The value of a variable can be assigned to another variable of the same data type. However, a value must be assigned to a variable before using it.

Example: Variable Assignment

int i = 100;

int j = i; // value of j will be 100

Variables can be used in an expression and the result of the expression can be assigned to the same or different variable.

Example: Variable & Expression

int i = 100;

int j = i + 20; // j = 120

i = 200;
j = i + 20;// j = 220

i = 300;
Console.WriteLine("j = {0}", j);// j = 220

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.

In C#, variables are categorized based on how they store their value in memory. Variables are categorized into value type or reference type or pointer type variables.

It is not necessary to specify the specific type when declaring variables. Use the var keyword instead of a data type.

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C# - var

In C#, variables must be declared with the data type. These are called explicitly typed variables.

Example: Explicitly Typed Variable

int i = 100;// explicitly typed variable

C# 3.0 introduced var keyword to declare method level variables without specifying a data type explicitly.

Example: Implicitly Typed Local Variable

var j = 100; // implicitly typed local variable

The compiler will infer the type of a variable from the expression on the right side of the = operator. Above, var will be compiled as int.

The following infers the type from an expression.

Example: var from expression

int i = 10;
var j = i + 1; // compiles as int

var can be used to declare any built-in data type or a user-defined type or an anonymous type variable. The following example shows C# compiler infers type based on the value:

Example: Implicitly-Typed Variable

static void Main(string[] args)
{
    var i = 10;
    Console.WriteLine("Type of i is {0}", i.GetType());

    var str = "Hello World!!";
    Console.WriteLine("Type of str is {0}", str.GetType());

    var dbl = 100.50d;
    Console.WriteLine("Type of dbl is {0}", dbl.GetType());

    var isValid = true;
    Console.WriteLine("Type of isValid is {0}", isValid.GetType());

    var ano = new { name = "Steve" };
    Console.WriteLine("Type of ano is {0}", ano.GetType());

    var arr = new[] { 1, 10, 20, 30 };
    Console.WriteLine("Type of arr is {0}", arr.GetType());

    var file = new FileInfo("MyFile");
    Console.WriteLine("Type of file is {0}", file.GetType());

}

Implicitly-typed variables must be initialized at the time of declaration; otherwise C# compiler would give an error: Implicitly-typed variables must be initialized.

var i; // Compile-time error: Implicitly-typed variables must be initialized
i = 100;

Multiple declarations of var variables in a single statement are not allowed.

var i = 100, j = 200, k = 300; // Error: cannot declare var variables in a single statement

//The followings are also valid
var i = 100; 
var j = 200; 
var k = 300;

var cannot be used for function parameters.

void Display(var param) //Compile-time error
{
    Console.Write(param);
}

var can be used in for, and foreach loops.

for(var i = 0; i < 10; i++)
{
    Console.WriteLine(i);
}

var can also be used with LINQ queries.

Example: LINQ Query Syntax in C#

// string collection
IList<string> stringList = new List<string>() { 
    "C# Tutorials",
    "VB.NET Tutorials",
    "Learn C++",
    "MVC Tutorials" ,
    "Java" 
};

// LINQ Query Syntax
var result = from s in stringList
            where s.Contains("Tutorials") 
            select s;