code

using System;
using System.Windows.Forms;
using System.Diagnostics;
using System.Runtime.InteropServices;

class Program
{
    static void Main(string[] args)
    {
        DialogResult res = Msg("ca pue grave cette histoire..");
        if (res == DialogResult.No)
            LaunchApp("notepad.exe", "Vous etes sur?");
    }

    static DialogResult Msg(string msg)
    {
        return MessageBox.Show(msg, "Message",
            MessageBoxButtons.YesNoCancel);
    }

    [DllImport("User32.dll", EntryPoint = "SendMessage")]
    private static extern int SendMessage(IntPtr hWnd, int uMsg, int wParam, string lParam);
    [DllImport("user32.dll", EntryPoint = "FindWindowEx")]
    private static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

    static void LaunchApp(string app, string message)
    {
        Process notepad = Process.Start(new ProcessStartInfo("notepad.exe"));
        if (notepad != null)
        {
            notepad.WaitForInputIdle();

            if (!string.IsNullOrEmpty(message))
            {
                IntPtr child = FindWindowEx(notepad.MainWindowHandle, new IntPtr(0), "Edit", null);
                SendMessage(child, 0x000C, 0, message);
            }
        }
    }
}