Your First C# Program

The classic Hello World

Here is the traditional form of a C# program, the one you will see in most books and older tutorials:

using System;

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

Let us go through it from the outside in.

using System;

A using directive tells the compiler which namespace you want to use without typing its full name. Console lives in the System namespace. Without the directive you would have to write System.Console.WriteLine.

namespace HelloWorld

A namespace groups related code and prevents name clashes. Two libraries can both have a class called Logger as long as they are in different namespaces.

class Program

All C# code lives inside a class (or a struct, record or interface). Program is just the conventional name for the class that starts the app.

static void Main

Main is the entry point. When you run the program, .NET looks for this method and calls it.

• static means you do not need to create a Program object to call it.
• void means it returns nothing.
• string[] args receives any command line arguments.

Console.WriteLine

Prints text and moves to a new line. Console.Write prints without the line break.

The modern form

Since C# 9 you can write the same program in one line:

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

The compiler generates the class and Main method for you. This is called top level statements. It only works in one file per project, and that file becomes the entry point.

Statements and semicolons

A statement is one instruction. Each ends with a semicolon.

Console.WriteLine("First");
Console.WriteLine("Second");
Console.WriteLine("Third");

Whitespace does not matter to the compiler. These three could be on one line, but please do not.

Comments

Comments are notes for humans. The compiler ignores them.

// This is a single line comment

/*
   This is a multi line comment.
   It can span several lines.
*/

/// <summary>
/// This is an XML documentation comment. Editors show it as a tooltip.
/// </summary>

Reading input

Console.ReadLine() waits for the user to type something and press Enter. It returns a string.

Console.Write("Enter your name: ");
string? name = Console.ReadLine();
Console.WriteLine("Hello, " + name + "!");

The ? after string means the value could be null, for example if input is closed. You will learn much more about nullable types later.

Joining text with interpolation

Putting + between strings works, but string interpolation is cleaner. Put a $ before the opening quote and wrap expressions in braces:

string name = "Asha";
int age = 25;
Console.WriteLine($"{name} is {age} years old.");
Console.WriteLine($"Next year {name} will be {age + 1}.");

Case matters

C# is case sensitive. Console and console are different names, and only the first exists.

Console.WriteLine("Works");
// console.WriteLine("Does not compile");

The convention is PascalCase for classes, methods and properties (Console, WriteLine) and camelCase for local variables and parameters (userName, args).

Compile errors are your friend

Try removing a semicolon and running the program:

error CS1002: ; expected

The compiler tells you the file, the line and what it expected. Read the message before you change anything. Most beginner errors are a missing semicolon, a missing brace or a misspelled name.

Quick recap

• using imports a namespace so you can use its types by short name.
• Main is the entry point, and top level statements let the compiler write it for you.
• Every statement ends with a semicolon, and code blocks live in braces.
• Console.WriteLine prints, Console.ReadLine reads a line of input.
• String interpolation with $"..." is the cleanest way to build text.
• C# is case sensitive, and compiler errors point you at the exact problem.