wat.

using System;
using Gtk;


public partial class MainWindow: Gtk.Window
{
    public MainWindow () : base (Gtk.WindowType.Toplevel)
    {
        Build ();
        textview2.Buffer.InsertAtCursor("text\n");  //Works!
    }

    protected void OnDeleteEvent (object sender, DeleteEventArgs a)
    {
        Application.Quit ();
        a.RetVal = true;
    }
    public void displayText(string input)
    {
        Console.WriteLine ("Before: "+textview2.Buffer.Text);
        textview2.Buffer.Text = input + "\n";
        Console.WriteLine ("After: "+textview2.Buffer.Text);
    }
}

################################## other file ######################################

namespace Launcher
{
    public class MainClass
    {
        public static void Main (string[] args)
        {
            Application.Init ();
            MainWindow win = new MainWindow ();
            win.Show ();
            Application.Run ();
            MainClass run = new MainClass();
            run.start ();
        }

        public void start()
        {
            win.displayText ("text"); //Here I call the method!
        }

    }
}