Lesson 02 · Basics

Your first C# program

Make a console project with dotnet new console -o Hello, then grow Program.cs into five lines: a comment and four statements. Watch dotnet run build the code into Hello.dll and run it line by line: WriteLine prints a line and moves the cursor down, Write stays on the line, ReadLine waits for your typing and keeps it in a variable, and an interpolated string puts that value into a greeting.

12 min11 steps5 questionsBeginner

Fig 2 · Interactive

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5 quick questions

Pick an answer to see why. Your score saves with your progress.

01The program runs Console.Write("A"); then Console.WriteLine("B"); then Console.Write("C");. What does the terminal show?

Write prints its text and no line break, so B follows A on the same line. WriteLine prints B and then a line break, so C starts a new line.

02You forget the semicolon: Console.WriteLine("Hello, World!") . What happens when you type dotnet run?

dotnet run builds first. The compiler checks every line before anything runs, and a statement without its semicolon is error CS1002, so no program is made.

03What does the line // My first C# program do when the program runs?

Everything from // to the end of the line is a comment: a note for people. The compiler ignores it, so it never changes what the program does.

04After var name = Console.ReadLine(); you type Ada and press Enter. What does Console.WriteLine($"Hi, {name}!"); print?

The $ makes it an interpolated string: each {name} is replaced by the current value of the variable name, which ReadLine set to the line you typed.

05What does dotnet new console -o Hello create?

It makes a project: the folder Hello holding the project file Hello.csproj and Program.cs. Hello.dll is only made later, when the project is built.

Key terms

Glossary

Statement: One instruction in a program, such as Console.WriteLine("Hi");. Most statements end with a semicolon, and they run in order.
Comment: Text after // (to the end of the line) that the compiler ignores: a note for people reading the code.
String literal: Fixed text written in the code inside double quotes, such as "Hello, World!".
Method: A named action you can call, such as Console.WriteLine. You call it with brackets and pass it values inside them.
Console.Write / WriteLine: Write prints text and leaves the cursor on the same line; WriteLine prints text and then a line break.
Variable: A named box in memory that holds a value, such as name. var lets the compiler work out its type.
Interpolated string: A string starting with $, in which each {name} is replaced by that variable's value.
Project file (.csproj): The file dotnet new makes next to your code; it holds the settings the build uses, such as the .NET version.

Fine print

Notes and sources

In .NET 6 and later, dotnet new console -o Hello writes a Program.cs with two lines: a comment linking to aka.ms/new-console-template and the statement Console.WriteLine("Hello, World!");.
The letters you type are shown by the terminal, not by the program (the terminal echoes them). If the input came from a file instead, the program's own output would be Hello, World!, then Your name? Hi, Ada! on one line.
WriteLine ends the line with Environment.NewLine: a line feed (\n) on macOS and Linux, a carriage return and line feed (\r\n) on Windows.
ReadLine returns null when there is no more input (Ctrl+Z then Enter on Windows, Ctrl+D on macOS and Linux). That is why its type is string?, a string that may be null. With var, name gets that type, and {name} prints nothing for null.
Top-level statements with no return statement end with exit code 0. The build writes Hello.dll into a folder such as bin/Debug/net8.0 (the last part names the .NET version).
Console works without a "using System;" line because new console projects turn on implicit usings (.NET 6 and later).
The program is real C#. Put the five lines in Program.cs, type dotnet run, enter Ada, and it prints Hi, Ada!.

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