Lesson 1 · ~8 min

Hello World in C#

Write your first line of C#, understand what it does, and learn why you'll meet two different shapes of C# program in the wild.

The smallest useful program

Since C# 9 (.NET 5+), a program can be just statements at the top of the file — no class, no Main. This is what dotnet new console gives you today:

Console.WriteLine("Hello, world!");

Console.WriteLine prints its argument and moves to the next line. Console.Write prints without the newline — run this and watch the two calls share one line:

Console.Write("C# is");
Console.WriteLine("one line");

Comments

Two styles, same as C++ and Java. Comments are for the next reader (often you, three weeks later):

// A single-line comment — everything after // is ignored.

/* A block comment
   can span several lines. */

Console.WriteLine("comments are ignored"); // this part too

The classic template: class Program + Main

Older tutorials, textbooks and a lot of production code use the classic shape. It is not obsolete — it's what you'll see in most jobs. Both shapes mean the same thing: find the entry point, run it.

using System;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello from Main!");
    }
}

Piece by piece:

using System; — imports the standard namespace so you can write Console instead of System.Console. (Our browser engine lets you omit it.)
class Program — a named container for your code. C# is fully object-oriented: all code lives in types.
static void Main(string[] args) — the entry point. static means it runs without creating an object first; void means it returns nothing; args holds command-line arguments.

Your first input

Programs get interesting when they react. Console.ReadLine() reads one line of text — type into the input box below the code before running:

Console.Write("What's your name? ");
string name = Console.ReadLine();
Console.WriteLine($"Hello, {name}!");

Two new ideas in three lines:

string name = Console.ReadLine(); — declares a variable of type string and stores the input in it.
$"Hello, {name}!" — an interpolated string: everything inside {…} is an expression, evaluated and inserted. This is the idiomatic way to build text in modern C#.

Practice right now

Open the playground and change the program to print two lines: your name, then your favorite number. When you're comfortable, the natural next step is the first judged problem: Sum of Two Numbers.