[c#] Show a message box from a class in c#?

How do you get a class to interact with the form to show a message box?

System.Windows.MessageBox.Show("Hello world"); //WPF
System.Windows.Forms.MessageBox.Show("Hello world"); //WinForms

using System.Windows.Forms;

public class message
{
    static void Main()
    {  
        MessageBox.Show("Hello World!"); 
    }
}

Try this:

System.Windows.Forms.MessageBox.Show("Here's a message!");