Hello World in C#

Every programming journey starts with Hello World. Let’s write our first C# program.

The Code

Create a file named Program.cs:

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

Yes, that’s it! Modern C# (9.0+) supports top-level statements, so you don’t need all the boilerplate that older tutorials show. The compiler automatically wraps this in a Main method for you.

Understanding the Code

• Console.WriteLine() - Writes text to the console followed by a newline
• Top-level statements - C# 9+ allows simple programs without explicit class/method declarations

Traditional Version

If you’re curious, here’s what older C# code (pre-9.0) looks like:

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using System;

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

Both versions produce identical output. The modern version is just cleaner for simple programs.

Running with Docker

The easiest way to run C# without installing the .NET SDK locally:

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# Pull the official .NET SDK image
docker pull mcr.microsoft.com/dotnet/sdk:9.0

# Run the program (creates a temporary project)
docker run --rm -v $(pwd):/app -w /app mcr.microsoft.com/dotnet/sdk:9.0 sh -c "cp Program.cs Program.cs.bak && dotnet new console -o . --force >/dev/null 2>&1 && cp Program.cs.bak Program.cs && rm Program.cs.bak && dotnet run"

The command creates a temporary project file, compiles your Program.cs, and runs it.

Running Locally

If you have the .NET SDK installed (version 6.0+):

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# Create a new project and run
dotnet new console -o HelloWorld
# Replace the generated Program.cs with your code
dotnet run --project HelloWorld

Expected Output

Hello, World!

Key Concepts

1. C# is compiled - Source code is compiled to Intermediate Language (IL)
2. IL runs on the CLR - Common Language Runtime executes the bytecode
3. Top-level statements - Modern C# allows simple programs without ceremony
4. Console is in System - The System namespace is implicitly imported in modern C#

.NET SDK vs Runtime

• SDK: For development - includes compiler, libraries, and CLI tools
• Runtime: For running apps only - smaller, production-focused

We use the SDK image for development because we need to compile the code.