[2.0/3.0] SendMessage or PostMessage to send keystrokes.

I'm wondering how to use SendMessage() or PostMessage() to send keystrokes to an inactive window. Could someone provide me with a simple code that sends a few characters and the enter button to notepad, while it's in the background?

This code is VB but it should be fairly obvious what it's doing:

vb.net Code:

Dim loginDialogueHandle As IntPtr = IntPtr.Zero
 
'Get the handle of the POS login dialogue.
Do
    Threading.Thread.Sleep(50)
    loginDialogueHandle = NativeMethods.FindWindow(My.Resources.RmsPosFormClassNameString, "Login")
Loop Until loginDialogueHandle <> IntPtr.Zero
 
'TODO: Hide login window in release.
'NativeMethods.ShowWindow(loginDialogueHandle, NativeConstants.SW_HIDE)
 
'Get the handles of the user ID and password fields and the OK button.
Dim userIDFieldHandle As IntPtr = NativeMethods.FindWindowEx(loginDialogueHandle, _
                                                             IntPtr.Zero, _
                                                             My.Resources.RmsPosTextBoxClassNameString, _
                                                             String.Empty)
Dim passwordFieldHandle As IntPtr = NativeMethods.FindWindowEx(loginDialogueHandle, _
                                                               userIDFieldHandle, _
                                                               My.Resources.RmsPosTextBoxClassNameString, _
                                                               String.Empty)
Dim frameHandle As IntPtr = NativeMethods.FindWindowEx(loginDialogueHandle, _
                                                       IntPtr.Zero, _
                                                       My.Resources.RmsPosFrameClassNameString, _
                                                       String.Empty)
Dim okButtonHandle As IntPtr = NativeMethods.FindWindowEx(frameHandle, _
                                                          IntPtr.Zero, _
                                                          My.Resources.RmsPosButtonClassNameString, _
                                                          "OK")
 
If userIDFieldHandle <> IntPtr.Zero Then
    'Enter the user ID.
    NativeMethods.SendMessage(userIDFieldHandle, _
                              NativeConstants.WM_SETTEXT, _
                              0, _
                              "1")
End If
 
If passwordFieldHandle <> IntPtr.Zero Then
    'Enter the password.
    NativeMethods.SendMessage(passwordFieldHandle, _
                              NativeConstants.WM_SETTEXT, _
                              0, _
                              "password")
End If
 
If okButtonHandle <> IntPtr.Zero Then
    'Click the OK button.
    NativeMethods.SendMessage(okButtonHandle, _
                              NativeConstants.BM_CLICK, _
                              0, _
                              0)
End If

Here are the API declarations, which are in C#:

Code:

---

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern IntPtr FindWindowEx(IntPtr hwndParent,
                                        IntPtr hwndChildAfter,
                                        string lpszClass,
                                        string lpszWindow);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int SendMessage(IntPtr hwnd,
                                        int wMsg,
                                        int wParam,
                                        int lParam);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
public static extern int SendMessage(IntPtr hwnd,
                                        int wMsg,
                                        int wParam,
                                        string lParam);

---

You should download and install the API Viewer, which can help you with unmanaged function and constant declarations.