{{works with|Mono|1.2}}
{{works with|Visual C sharp|Visual C#|2003}}
namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            System.Console.WriteLine("Hello world!");
        }
    }
}

{{works with|Visual C sharp|Visual C#|9.0+}}
C# 9.0 allows statements at the top level of a source file (a ''compilation_unit'' in the specification) between any using statements and namespace declarations. These statements become the program entry point and are placed in a method in a compiler-generated type.
System.Console.WriteLine("Hello world!");
or
using System;
Console.WriteLine("Hello world!");