Your First C# Program

Why this matters: Up to now you have been installing things. From this lesson on, you are writing code. The first time your program asks you a question and replies with something based on your answer is a small but real shift. From here you are building, not just configuring.

Step one: Hello, World

The shortest complete C# program in .NET 8:

// This is a comment. The compiler ignores anything after //
Console.WriteLine("Hello, World!");

That one line:

• Uses Console, a built-in class for printing to and reading from the terminal
• Calls WriteLine to print text followed by a newline
• Ends with a semicolon, which every C# statement requires

Run it. You should see Hello, World! in your terminal.

Step two: ask the user a question

A program that talks back is more interesting than one that just announces itself:

Console.WriteLine("=== Welcome ===");

Console.Write("What is your name? ");
string name = Console.ReadLine();

Console.Write("Where are you from? ");
string city = Console.ReadLine();

Console.WriteLine();
Console.WriteLine($"Hello, {name} from {city}. Welcome to C#.");

New things in this program:

• Console.Write is like WriteLine but does not add a newline at the end. Use it for prompts when you want the cursor to stay on the same line.
• Console.ReadLine() waits for the user to type a line and press Enter, then hands you back what they typed as a string.
• string name = Console.ReadLine() declares a variable called name and stores the user's input in it.
• $"..." is string interpolation. The {name} and {city} parts get replaced with the values of those variables when the string is built.

Run it. Answer the prompts. The output should feel like a real conversation.

Step three: a small VAT calculator

Now we add a bit of arithmetic. Enter a price, and the program adds 15% VAT.

Console.WriteLine("=== Price Calculator (with VAT) ===");

Console.Write("Enter price (excluding VAT): R");
string input = Console.ReadLine();
decimal price = decimal.Parse(input);

decimal vat = price * 0.15m;
decimal total = price + vat;

Console.WriteLine();
Console.WriteLine($"Price:       R{price:F2}");
Console.WriteLine($"VAT (15%):   R{vat:F2}");
Console.WriteLine($"Total:       R{total:F2}");

New things in this program:

• decimal is C#'s type for money. It is more accurate than double for currency, which is why banks and accounting systems use it. The m at the end of 0.15m tells the compiler "this is a decimal, not a double".
• decimal.Parse(input) converts the string the user typed (input is always a string) into a decimal number.
• :F2 is a format specifier. It tells the formatter "show this number with exactly two decimal places". Try changing it to :F0 and see what happens.

Run it with a value like 499. You should see:

Price:       R499.00
VAT (15%):   R74.85
Total:       R573.85

Syntax rules you will hit immediately

These are the four rules that catch every beginner.

Semicolons end statements

Console.WriteLine("Hello");  // Correct
Console.WriteLine("Hello")   // Error: missing semicolon

C# is case sensitive

name, Name and NAME are three different things. Built-in types like string are lowercase, classes you create are PascalCase.

string name = "Sipho";   // Correct
String name = "Sipho";   // Works, but the convention is lowercase
STRING name = "Sipho";   // Error: STRING does not exist

Strings use double quotes, characters use single quotes

string greeting = "Hello";  // String, double quotes
char letter = 'A';          // Single character, single quotes
string wrong = 'Hello';     // Error

Braces group code into blocks

You will meet braces properly in the next module, but for now know that { } groups statements that belong together:

if (age >= 18)
{
    Console.WriteLine("You are an adult");
}

Comments

There are two kinds of comment you will write all the time:

// Single-line comment

/* Multi-line comment.
   Useful for longer explanations
   or for temporarily disabling a block of code. */

Use them sparingly. Good code reads almost like English; a comment is most useful for explaining why something is done, not what.

Running your program

Visual Studio

• F5: run with debugging
• Ctrl+F5: run without debugging (faster)

Command line

dotnet run

Four common beginner mistakes

Try it yourself

Write a small program that:

1. Asks the user for their name
2. Asks the user for their age
3. Calculates what year they were born
4. Displays a message like: Hello Lerato, you were born in 1998.

Hint: to do arithmetic with the age, you need to parse it from a string to an int:

int age = int.Parse(Console.ReadLine());

And to get the current year:

int currentYear = DateTime.Now.Year;

What's next

In the next lesson we dive into how your C# code actually becomes a running program: the role of the compiler, the .NET runtime and what "IL" means.