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C# - Add/Send text to notepad process using user32.dll SendMessage from your application

posted Jan 13, 2012, 1:31 AM by Peter Henell   [ updated Jan 25, 2012, 5:13 AM ]
How to add text to another applications text fields from your application?

This is a simple example of starting a Notepad process and then adding text to it.
Will open a new Notepad.exe process and then add the text "Sending a message, a message from me to you" to the text area of notepad.

The constant 0x000c of SendMessage  is documented here  http://msdn.microsoft.com/en-us/library/windows/desktop/ms632644(v=vs.85).aspx .
The constant says SETTEXT which really means that the text in notepad will be replaced if you send more than one message using this constant.
using System;
using System.Text;
using System.Runtime.InteropServices;
using System.Diagnostics;

namespace SendMessageTest
    class Program
        [DllImport("user32.dll", EntryPoint = "FindWindowEx")]
        public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

        public static extern int SendMessage(IntPtr hWnd, int uMsg, int wParam, string lParam);

        static void Main(string[] args)
            DoSendMessage("Sending a message, a message from me to you");

        private static void DoSendMessage(string message)
            Process notepad = Process.Start(new ProcessStartInfo("notepad.exe"));
            if (notepad != null)
                IntPtr child = FindWindowEx(notepad.MainWindowHandle, new IntPtr(0), "Edit", null);
                SendMessage(child, 0x000C, 0, message);