In Mono, you can write “Hello World!” as follows:

using System;

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

Let me explain what’s going on:

• using System; imports the System namespace, which provides basic functionality for .NET programs.
• class HelloWorld { ... } defines a new class called HelloWorld.
• static void Main(string[] args) { ... } defines the entry point of the program. The Main method is called when the program starts. It takes an array of strings as arguments, which are ignored in this case.
• Console.WriteLine("Hello World!"); uses the Console.WriteLine method to print the string “Hello World!” to the console, followed by a newline character.

To compile and run this code, you can use the Mono compiler (mcs) and runtime (mono):

$ mcs HelloWorld.cs
$ mono HelloWorld.exe
Hello World!