SendKeyToBackground.cs

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

namespace SendKeysToAnyApplication
{
    class Program
    {
        delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);

        [DllImport("user32.dll")]
        static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, uint wParam, uint lParam);
        [DllImport("user32.dll")]
        static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
        [DllImport("user32.dll")]
        static extern bool EnumChildWindows(IntPtr hwndParent, EnumWindowsProc lpEnumFunc, IntPtr lParam);
        [DllImport("user32.dll")]
        static extern int GetClassName(IntPtr hWnd, StringBuilder lpClassName, int nMaxCount);

        const uint WM_CHAR = 0x102;
        const uint WM_PASTE = 0x0302;

        static IntPtr m_hWndEdit = IntPtr.Zero;

        [STAThread]
        static void Main(string[] args)
        {
            // for a test we use the notepad window
            // ... so lets have a look if we find an opened notepad window (better you open one first...)
            IntPtr hWnd = FindWindow("Notepad", null);
            // But! we can not send text to the main-window, we have to find the subwindow where the text is entered
            // So lets enumerate all child windows:
            IntPtr hWndEdit = IntPtr.Zero;
            EnumChildWindows(hWnd, EnumChildWindowsCallback, hWndEdit);

            if (m_hWndEdit != null) // edit window found?
            {
                // Now you could use different messages to send text (WM_CHAR, WM_KEYDOWN, ...)
                // I decided to use the clipboard:
                // Copy some text to the clipboard
                Clipboard.SetText("this is magic!");
                // ... and just paste it to the target window
                SendMessage(m_hWndEdit, WM_PASTE, 0, 0);
            }
        }

        static bool EnumChildWindowsCallback(IntPtr hWnd, IntPtr lParam)
        {
            // Search for notepads edit window - if we find it "false" is returned (means stop enumerating windows)

            StringBuilder sb = new StringBuilder();
            GetClassName(hWnd, sb, 256);
            if (!sb.ToString().Contains("Edit"))
            {
                return true;
            }

            m_hWndEdit = hWnd; // Store the handle to notepads edit window (this is the window we want to send the messages to)
            return false;
        }
    }
}