The lparam and wparam parameters in this specific sendmessage function? - vb.net

Question

I was going through this code here and:
Private Sub Button3_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button3.Click
SendMessage(Me.Handle, WM_APPCOMMAND, &H200EB0,
APPCOMMAND_VOLUME_MUTE * &H10000)
End Sub
I have no idea what the last two parameters of the sendmessage functions are and what is going on in these two parameters? the "&H200EB0" and "APPCOMMAND_VOLUME_MUTE * &H10000" Parameters?
Here's the full code:
Imports System.Runtime.InteropServices
Public Class Form1
<DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)>
Private Shared Function SendMessage(ByVal hWnd As IntPtr,
ByVal Msg As UInteger, ByVal wParam As IntPtr,
ByVal lParam As IntPtr) As IntPtr
End Function
Const WM_APPCOMMAND As UInteger = &H319
Const APPCOMMAND_VOLUME_UP As UInteger = &HA
Const APPCOMMAND_VOLUME_DOWN As UInteger = &H9
Const APPCOMMAND_VOLUME_MUTE As UInteger = &H8
Private Sub Button1_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button1.Click
SendMessage(Me.Handle, WM_APPCOMMAND, &H30292,
APPCOMMAND_VOLUME_UP * &H10000)
End Sub
Private Sub Button2_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button2.Click
SendMessage(Me.Handle, WM_APPCOMMAND, &H30292,
APPCOMMAND_VOLUME_DOWN * &H10000)
End Sub
Private Sub Button3_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles Button3.Click
SendMessage(Me.Handle, WM_APPCOMMAND, &H200EB0,
APPCOMMAND_VOLUME_MUTE * &H10000)
End Sub
End Class

Answer 1

SendMessage is a method which can be used to send a specified message to a window or windows.
Documentation is here: http://msdn.microsoft.com/en-us/library/windows/desktop/ms644950%28v=vs.85%29.aspx
The first parameter is the "handle" (HWND) to which the message is send. The second parameter is a message id (see http://msdn.microsoft.com/en-us/library/windows/desktop/ms644927(v=vs.85).aspx#system_defined for System-Defined Messages).
The last two parameters can be used to pass more data to the receiver. - Normally both parameters have message dependent meanings.
In your case the WM_APPCOMMAND message (http://msdn.microsoft.com/en-us/library/windows/desktop/ms646275%28v=vs.85%29.aspx) is passed (here it is a keyboard command). I think the first parameter could also be NULL (according to the link above it should be a handle to the window where the user clicked the button or pressed the key), but the second one has to specify the command which should be passed (as an APPCOMMAND). The second parameter is 8 (8=APPCOMMAND_VOLUME_MUTE according to the list on the linked page), however we have to do a bitshift, because the information has to be encoded in the high-order bits of the parameter (i.e., 0x80000 - that's APPCOMMAND_VOLUME_MUTE*&H10000; see lParam section on the page I linked).