Your First Program

A console application is a program with no windows and no buttons. It writes plain text into a terminal and can read plain text back. It is the smallest useful kind of program, so it is where every .NET course starts.

Creating it

Run dotnet new console -o Hello. That creates a folder named Hello holding two files you care about: Hello.csproj, which describes the project, and Program.cs, which holds your code. Then cd Hello and dotnet run. It prints Hello, World!.

Program.cs contains a comment and one statement. A comment starts with // and the compiler ignores it; it is there for people. The statement Console.WriteLine("Hello, World!"); prints one line. The text in double quotes is what gets printed, and the semicolon marks the end of the statement.

The short form and the long form

Those few lines are a complete program because of top-level statements, a shorthand added in C# 9. The long form is what C# always required: an entry point method named Main, inside a type. dotnet new console --use-program-main generates that version instead. Both are valid and both compile to the same program. With top-level statements the compiler writes the type and the entry point for you; the name of the generated method is not Main and your code cannot call it.

What trips people up

Only one file in a project may hold top-level statements, and in that file they must come before any type or namespace declaration.

// Program.cs, as created by: dotnet new console -o Hello
// These statements are the whole program. The compiler wraps them in a
// generated type with a generated entry point method.
Console.WriteLine("Hello, World!");

// Statements run in the order they are written.
Console.WriteLine("The program ends after the last statement.");

// Output:
// Hello, World!
// The program ends after the last statement.

// Generated by: dotnet new console -o Hello --use-program-main
// This file has no top-level statements, so the type may come first.
namespace Hello;

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine("Hello, World!");
        Console.WriteLine("The program ends after the last statement.");
    }
}

// Output:
// Hello, World!
// The program ends after the last statement.

dotnet new console -o Hello
cd Hello
dotnet run
# Hello, World!

# What is in the folder:
#   Hello.csproj   describes the project
#   Program.cs     your code
#   obj/           working files the build needs; never edit these

// In a file with top-level statements, args is already available.
// It holds the words typed after the program name, and is never null.
// Run with: dotnet run -- Ada Grace
Console.WriteLine("argument count: " + args.Length);

// Output, when run with: dotnet run -- Ada Grace
// argument count: 2