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C# · Basics

Variables and Data Types

A variable is a labelled box that holds a value. In C#, every box has a type that says what can go inside — and the compiler enforces it.

Beginner 5 min Lesson 3 of 26Examples target C# 13 on .NET 9 unless stated otherwise.
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On this page · 5 min · Beginner

On this page

  1. In simple words
  2. Learning objectives
  3. Explanation
  4. Syntax
  5. Code example
  6. Step by step
  7. Real-world use
  8. Common mistakes
  9. Best practices
  10. Remember this
  11. Practice exercise
  12. Mini quiz
  13. Related topics
  14. My notes
Reading time
5 min
Difficulty
Beginner
Updated
Oct 28, 2026

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In simple words#

A variable is a labelled box on a shelf. The data type is the shape of the box: an int box holds whole numbers, a string box holds text, a bool box holds only yes/no. You cannot put a sentence into a number box.

Variable
A named place to store a value.
Data type
What kind of value it may hold (int, string, bool, decimal…).
var
Let the compiler work out the type from the value.

Learning objectives#

  • Declare variables with a type
  • Choose between int, long, double and decimal
  • Use bool, char and string
  • Use var and const correctly

Explanation#

Declare a variable as type name = value; for example int age = 21;. Once declared, the type cannot change: you cannot later put text into age.

Whole numbers: int (about ±2.1 billion) and long (much larger). Decimal numbers: double for science and measurements, decimal for money because it stores 0.1 exactly.

bool holds true or false. char holds one character in single quotes ('A'). string holds text in double quotes ("Pune").

var lets the compiler infer the type from the value: var count = 3; is still an int. It is not “any type” — it is just shorter.

const declares a value that can never change, such as a tax rate. Number literals have suffixes: 499.99m is a decimal, 3.5f a float, 10L a long.

Syntax#

C# Illustrative
int count = 10;
double weight = 72.5;
decimal price = 199.99m;   // m = decimal
bool isActive = true;
char grade = 'A';
string city = "Pune";
var total = 5;             // inferred as int
const double Pi = 3.14159;

Code example: Prices, flags and text#

C#Program.cs Illustrative
int age = 21;
double rating = 4.5;
decimal price = 499.99m;
bool isMember = true;
char grade = 'A';
string city = "Pune";
var quantity = 3;                 // the compiler infers int
const decimal GstRate = 0.18m;


decimal total = price * quantity * (1 + GstRate);


Console.WriteLine(
quot;Age {age}, rating {rating}, grade {grade}, member {isMember}");
Console.WriteLine(
quot;{quantity} items in {city}: {total:F2}");
Console.WriteLine(int.MaxValue);
Console.WriteLine(0.1 + 0.2);     // double: tiny rounding error
Console.WriteLine(0.1m + 0.2m);   // decimal: exact
Expected output
Age 21, rating 4.5, grade A, member True
3 items in Pune: 1769.96
2147483647
0.30000000000000004
0.3

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

  1. Each variable is declared with its type, so the compiler knows what it can hold.
  2. price uses the m suffix, which makes the literal a decimal — right for money.
  3. quantity uses var, but its type is still int because 3 is an int.
  4. const GstRate can never be reassigned.
  5. {total:F2} formats the result with exactly 2 decimal places.
  6. The last two lines show why money uses decimal: double cannot store 0.1 exactly.

Real-world use case#

An invoice service stores amounts as decimal, quantities as int, the paid flag as bool and the customer name as string. Using double for money would create tiny rounding errors that add up across thousands of invoices.

Common mistakes#

Using double for money

0.1 + 0.2 is 0.30000000000000004 with double. Use decimal for currency.

Forgetting the m suffix

decimal price = 9.99; does not compile: 9.99 is a double literal. Write 9.99m.

Using a variable before assigning it

int x; Console.WriteLine(x); gives “Use of unassigned local variable”. Give local variables a value first.

Best practices#

  • Use decimal for money, double for measurements, int for counts
  • Use var when the type is obvious from the right side
  • Name variables in camelCase and constants in PascalCase
  • Declare variables close to where they are used

Remember this#

Money → decimal (with m). Measurements → double. Counting → int.

var is still strongly typed — the compiler picks the type once.

'A' is a char, "A" is a string.

Practice exercise#

Your task: Store a product name, its unit price (decimal) and a quantity (int). Print the total price with 2 decimal places.

Need a hint?
Multiply price by quantity and use the :F2 format inside the interpolated string.
Show solution
C# Illustrative
string product = "Notebook";
decimal unitPrice = 45.50m;
int quantity = 4;
decimal total = unitPrice * quantity;
Console.WriteLine(
quot;{quantity} x {product} = {total:F2}");   // 4 x Notebook = 182.00

Mini quiz#

Variables and Data Types quizMultiple choice Question 1 of 3

Which type should store a product price?

Score so far: 0/0

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My notes#

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