Lesson 1: Introduction to Variables in C#

Variables are the building blocks of every C# program. Before you can work with data — numbers, text, dates, or anything else — you need to know how to store it. In this lesson, we'll cover exactly what variables are, how to declare and initialize them, and the rules you must follow when naming them.

What is a Variable?

A variable is a named storage location in memory that holds a value. Think of it like a labeled box — the label is the variable name, and whatever you put inside is the value.

In C#, every variable has a type (what kind of data it stores) and a name (how you refer to it in code).

Declaring a Variable

To declare a variable in C#, you specify the type followed by the name:

// Syntax: type variableName;
int age;
string name;
double salary;
bool isActive;

At this point the variables are declared but not yet assigned a value. Trying to use them before assigning a value will cause a compile-time error in C#.

Initializing a Variable

You can declare and assign a value at the same time — this is called initialization:

// Declare and initialize in one line
int age = 25;
string name = "Alice";
double salary = 72500.50;
bool isActive = true;

// Or declare first, then assign later
int score;
score = 100;

Console.WriteLine(age);      // Output: 25
Console.WriteLine(name);     // Output: Alice
Console.WriteLine(isActive); // Output: True

The var Keyword

C# supports type inference using the var keyword. The compiler automatically determines the type based on the assigned value. The type is still fixed at compile time — var is not the same as JavaScript's dynamic typing.

var age = 25;           // inferred as int
var name = "Alice";   // inferred as string
var pi = 3.14;         // inferred as double
var flag = true;       // inferred as bool

// This would cause a compile error — type is locked:
// age = "hello";  ❌ Cannot convert string to int

Variable Naming Rules

C# enforces strict naming rules. Your variable names must follow these:

Naming Conventions

Following conventions makes your code easier for others (and future you) to read:

Constants: Variables That Don't Change

If a value should never change after it's set, use the const keyword. Constants are evaluated at compile time and cannot be modified at runtime.

const double Pi = 3.14159265358979;
const int MaxRetries = 3;
const string AppName = "FormatBase";

// This would cause a compile error:
// Pi = 3.14;  ❌ Cannot assign to a constant

A Complete Example

Let's put it all together in a simple console program:

// A simple student profile
using System;

const string SchoolName = "FormatBase Academy";

string studentName = "Ravi Patel";
int studentAge = 21;
double gpa = 3.85;
bool isEnrolled = true;

Console.WriteLine($"School: {SchoolName}");
Console.WriteLine($"Student: {studentName}, Age: {studentAge}");
Console.WriteLine($"GPA: {gpa}, Enrolled: {isEnrolled}");

/* Output:
   School: FormatBase Academy
   Student: Ravi Patel, Age: 21
   GPA: 3.85, Enrolled: True
*/

Lesson Summary