Your first program: output, input and the Main method

Statements and semicolons

A statement is one instruction, like "print this". In C#, every statement ends with a semicolon (;). Blocks of statements are wrapped in curly braces { }. C# is case-sensitive: Console and console are different names, and only Console exists.

Console.WriteLine("Hello, World!");
Console.WriteLine("Welcome to C#");

Top-level statements

Since C# 9, you can write statements directly in Program.cs. This is called top-level statements. The compiler quietly wraps them in a Main method for you. Only one file in a project may have top-level statements, because a program can have only one starting point.

The classic Main method

Older projects, many tutorials and many company codebases write the full form. Main is the entry point, the first method that runs. It must be static (it belongs to the class, not to an object, explained in module 4).

namespace Hello;

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

Comments

// A single-line comment: the compiler ignores it
/* A comment
   over many lines */
Console.WriteLine("Comments are for humans"); // after code too

Write and WriteLine

Console.Write("Rs ");
Console.Write(250);
Console.WriteLine();
Console.WriteLine("Line one\nLine two");
Console.WriteLine("Path: C:\\Users\\Asha");

Rs 250
Line one
Line two
Path: C:\Users\Asha

Reading input: Console.ReadLine

Console.ReadLine() waits for the user to type a line and press Enter, then gives it back as a string. Its type is string? with a question mark, meaning it may be null (no value) if there is no more input, for example when input is redirected from an empty file. Module 3 explains null in depth.

Console.Write("Your name: ");
string? name = Console.ReadLine();
Console.WriteLine($"Namaste, {name}!");

Typing Asha shows Namaste, Asha!. ReadLine always gives text. To get a number you must convert it, which the next topics cover.

using directives

Console lives in the System namespace. Its full name is System.Console. A using directive at the top of a file (using System;) lets you write the short name. New projects have ImplicitUsings turned on, which adds the common ones (System, System.IO, System.Linq, System.Collections.Generic and a few more) for you.

Common mistakes and fixes