Need help with placing text in other program

OK, let's start from scratch:

1.  Create a new Windows Forms application and drag a button onto it (which will be called button1 by default) from the ToolBox.

2. In the form designer, double click on the button and the button1_Click eventhandler will open up. Now paste the appropriate code into that. Next scroll up a bit and add the 3 DllImport declarations (these should be within the Form class but outside of any methods). Then add this line to your using directives:

using System.Runtime.InteropServices;

3. Now build the application - you shouldn't get any errors - and open up a new blank instance of notepad outside Visual Studio.

5. Finally go back to Visual Studio, run the program and click the button. With luck, you should now see some activity in  notepad.

To use this code from a Windows Forms application all the code needs to be in the Form class itself, not the Main() method. You should also be able to use SendKeys.Send() rather than SendKeys.SendWait() in such an application:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;
namespace WindowsApplication4
{
    public partial class Form1 : Form
    {
       [DllImport("User32.dll")]
       static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, string lParam);

       [DllImport("user32.dll")]
       static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
   
       [DllImport("user32.dll")]
       static extern bool SetForegroundWindow(IntPtr hWnd);

       private void button1_Click(object sender, EventArgs e)
       {
           string lpClassName = "Notepad";
           string lpWindowName = "Untitled - Notepad";
           IntPtr hWnd = FindWindow(lpClassName, lpWindowName);
           SetForegroundWindow(hWnd);
           string z = "TEXT";
           const int WM_SETTEXT = 0x0C;
           SendMessage(hWnd, WM_SETTEXT, IntPtr.Zero, z);
           SendKeys.Send("Hello{ENTER}World");
       }
 
        // other code
    }
}

As Ryan said, the Win32 API function SendMessage() takes 4 parameters, though the types of these parameters depends on which Windows message you're using.

You also need to get the window handle and force focus onto it before you can send the WM_SETTEXT message which actually prints text to the window header. To print text within the window body, you can use the managed SendKeys class which sends keys to the application just as if they'd been typed at the keyboard.

If you start a new blank instance of Notepad and run the following console application, you'll see what I mean:

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

class Test
{

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

    [DllImport("user32.dll")]
    static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
   
    [DllImport("user32.dll")]
    static extern bool SetForegroundWindow(IntPtr hWnd);

    static void Main()
    {

        string lpClassName = "Notepad";
        string lpWindowName = "Untitled - Notepad";
        IntPtr hWnd = FindWindow(lpClassName, lpWindowName);
        SetForegroundWindow(hWnd);
        string z = "TEXT";
        const int WM_SETTEXT = 0x0C;
        SendMessage(hWnd, WM_SETTEXT, IntPtr.Zero, z);
        SendKeys.SendWait("Hello{ENTER}World");
    }

}