Variables

Variables are like labeled boxes where you store data. You put something in, give it a name, and pull it out whenever you need it. Every C# program uses variables constantly, so this is one of the first things to get comfortable with.

Declaring Variables

In C#, you declare a variable by specifying its type followed by its name:

int age;           // Declares an integer variable named age
string name;       // Declares a string variable named name
double price;      // Declares a decimal number variable
    

After declaring, you can assign a value:

age = 25;
name = "Alice";
price = 19.99;
    

Or you can combine declaration and assignment in one step:

int age = 25;
string name = "Alice";
double price = 19.99;
    

Type Inference with var

C# can figure out the type for you using the var keyword. When you write var, the compiler looks at what you're assigning and determines the type automatically:

var age = 25;        // Compiler figures out this is int
var name = "Alice";  // Compiler figures out this is string
var price = 19.99;   // Compiler figures out this is double
    

This isn't magic — the variable still has a fixed type. var just saves you typing. Once the compiler decides it's an int, you can't store text in it later.

Naming Rules

C# has some rules for variable names, plus common conventions the community follows:

• Names must start with a letter or underscore.
• They can contain letters, digits, and underscores.
• They're case-sensitive — myVar and MyVar are different.
• They can't be reserved keywords like int, class, or static.
• Convention: use camelCase for local variables (firstName, totalPrice).

int myVar = 10;      // Valid
int _count = 5;      // Valid, underscore prefix is allowed
int 2fast = 3;       // Invalid — starts with a digit
int class = 7;       // Invalid — class is a keyword
    

Assigning Values

The assignment operator (=) puts a value into your variable. You can also reassign variables — change what they store:

int score = 10;
score = 20;       // Now score is 20, the old value is gone
score = score + 5;  // Now score is 25
    

Variables must be assigned a value before you use them. The compiler will yell at you if you try to read from an unassigned variable. This is actually a good thing — it catches bugs before they happen.