Sends `Hello world!` to one instance of notepad that is currently running.

TextToNotepad.cs

using System;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;

namespace Virtlink
{
    public sealed class Program
    {
        #region PInvoke
        [DllImport("user32.dll")]
        public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

        [DllImport("User32.dll")]
        public static extern int SendMessage(IntPtr hWnd, int uMsg, int wParam, string lParam);
        #endregion

        private const int WM_SETTEXT = 0x000C;

        static void Main(string[] args)
        {
            // Get the 'notepad' process.
            var notepad = Process.GetProcessesByName("notepad").FirstOrDefault();
            if (notepad == null)
                throw new Exception("Notepad is not running.");

            // Find its window.
            IntPtr window = FindWindowEx(notepad.MainWindowHandle, IntPtr.Zero, "Edit", null);

            // Send some string.
            SendMessage(window, WM_SETTEXT, 0, "Hello world!");
        }
    }
}