How to send a CBN_SELCHANGE message when using CB_SETCURSEL?

Question

When using the CB_SETCURSEL message, the CBN_SELCHANGE message is not sent.

How to notify a control that the selection was changed

Answer 1

This might help the next person out:

[DllImport("User32.dll", EntryPoint = "SendMessage")]
private static extern int SendMessage(IntPtr hwnd, int msg, int wParam, int lParam);

[DllImport("user32.dll", EntryPoint = "GetWindowLong")]
private static extern IntPtr GetWindowLongPtr32(IntPtr hWnd, int nIndex);

[DllImport("user32.dll", EntryPoint = "GetWindowLongPtr")]
private static extern IntPtr GetWindowLongPtr64(IntPtr hWnd, int nIndex);

public static IntPtr GetWindowLongPtr(IntPtr hWnd, int nIndex)
{
   if (IntPtr.Size == 8)
        return GetWindowLongPtr64(hWnd, nIndex);
   else
        return GetWindowLongPtr32(hWnd, nIndex);
}

static int MakeWParam(int loWord, int hiWord)
{
     return (loWord & 0xFFFF) + ((hiWord & 0xFFFF) << 16);
}

public const int CB_SETCURSEL = 0x014E;
public const int CBN_SELCHANGE = 0x0001;

public enum GWL
{
            GWL_WNDPROC = (-4),
            GWL_HINSTANCE = (-6),
            GWL_HWNDPARENT = (-8),
            GWL_STYLE = (-16),
            GWL_EXSTYLE = (-20),
            GWL_USERDATA = (-21),
            GWL_ID = (-12)
}

public static IntPtr Hwnd_select_control_parent = IntPtr.Zero;
public static IntPtr Hwnd_select_control = IntPtr.Zero;

static void changeit()

{

// Google WinSpy for tips on how to figure out how to get window handles from known ctrl_id

   Hwnd_select_control = 14298; // or whatever the handle of the combo box is

// Get the parent of the selectbox control

   Hwnd_select_control_parent = GetWindowLongPtr(service_window_control, (int)GWL.GWL_HWNDPARENT);

// Get the control id of the selectbox if you don't already have it

   IntPtr nID = GetWindowLongPtr(Hwnd_select_control, (int)GWL.GWL_ID);
   int ctrl_id = nID.ToInt32();

// Change the combo box to the value "My Value"

   SendMessage(Hwnd_select_control, CB_SETCURSEL, "My Value", null);

// low ID is the ctrl_id of the combo box, high id is CBN_SELCHANGE

   int send_cbn_selchange = MakeWParam(ctrl_id, CBN_SELCHANGE); 

// Send the WM_COMMAND to the parent, not the control itself

   SendMessage(Hwnd_serviceselect_control_parent, 0x111 /* WM_COMMAND */, send_cbn_selchange, Hwnd_serviceselect_control.ToInt32()); 


}

Answer 2

I just discovered calling these SendMessages to the Combobox twice works... I know it's not perfect but it worked for me. (Written in VB6)

    For looper = 1 To 2

        bVal = SendMessage(inHandle, COMBO.CB_SHOWDROPDOWN, True, 0)
        count = SendMessage(inHandle, COMBO.CB_SETCURSEL, 1, 0)
        count = SendMessage(inHandle, WIND.WM_LBUTTONDOWN, 0, -1)

    Next

Answer 3

You're not supposed to use CBN_SELCHANGE unless the change in selection was made by the user.

You don't indicate what language you're using; it would make it easier to provide you with a workaround if you did so.

In Delphi, where an OnChange() would be associated with the combobox, you just call the event method directly:

// Send the CB_SETCURSEL message to the combobox
PostMessage(ComboBox1.Handle, CB_SETCURSEL, Whatever, WhateverElse);

// Directly call the OnChange() handler, which is the equivalent to CBN_SELCHANGE
ComboBox1Change(nil);