Your First Program: Main and Top-Level Statements

The starting point of a program

Every program has a starting point: the place where the computer begins to run it. In C#, for many years this point was a method called Main. In modern C# (since version 9), thanks to top-level statements, you do not have to write this skeleton; the compiler generates it for you.

The new style: top-level statements

The single line the template gives you is a complete program:

Console.WriteLine("Hello, Byte Akademi!");

The classic style: the Main method

In older books and in some projects you will see the following form. Both do the same job:

using System;

namespace HelloWorld
{
    internal class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, Byte Akademi!");
        }
    }
}

Here using System; means "I will use the ready-made classes in the System namespace"; the Console class lives there. In new .NET projects, commonly used namespaces are added behind the scenes (this is called implicit usings), which is why you could use Console in the new style without writing using.

Printing: Write and WriteLine

Console.WriteLine moves to the next line after printing; Console.Write does not.

Console.Write("Voltage: ");
Console.Write(12);
Console.WriteLine(" V");
Console.WriteLine("This line starts below.");

Voltage: 12 V
This line starts below.

Reading user input: ReadLine

Console.ReadLine() returns the line the user typed as text. If the user closes the input stream without typing anything, it may return null; that is why the return type is written as string? (the question mark means "can be empty"; we will see this in later lessons).

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

What is your name? Fatih
Hello, Fatih!

The $ sign before the quotation mark means string interpolation: the variable or expression you write inside the curly braces is placed into the text. It is much more readable than joining pieces of text with +.

Comments

You can write notes inside your code that do not affect how the program runs. Your future self (or someone else) will thank you for them.

// A single-line comment

/* A comment that
   spans several lines */

Console.WriteLine("Comments do not run"); // an end-of-line comment is fine too

Escape sequences

A backslash is used for special characters inside text: \n is a new line, \t is a tab and \" is a quotation mark.

Console.WriteLine("Channel\tValue");
Console.WriteLine("CH1\t3.3 V");
Console.WriteLine("A \"quoted\" word");

Reading compiler error messages

When you make a mistake, the compiler prints the file name, the line number and an error code (such as CS1002). Read the whole message to the end: most of the time it tells you directly what the problem is and where it is. If there are several errors, fix the topmost one first; the ones below are often side effects of it. For the time it will save you in programming, this habit is one of the most valuable skills.

Try it yourself

Write a program that asks the user for their name and their department, then prints a single sentence such as "Hello Alice, you study Electrical-Electronics Engineering." Use two separate ReadLine calls and one interpolated string. Then try turning the output into two columns with \t.