Private Type NMTOOLBAR
    hdr As NMHDR
    iItem As Long
    tbBtn As TBBUTTON
    cchText As Long
    lpszString As Long
End Type
Private Const TBN_FIRST = -700&
Private Const TBN_DROPDOWN = (TBN_FIRST - 10)

Private Const WM_USER = &H400
Private Const TB_GETBUTTON As Long = (WM_USER + 23)
Private Const WM_NOTIFY As Long = &H4E&
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long

Private Sub Toolbar1_ButtonClick(ByVal Button As MSComctlLib.Button)
    If Button.Index = 4 Then '假如toolbar上的第4个按钮有buttonmenu，同时其样式为5
       showbuttonmenu Toolbar1, Button.Index - 1
    End If
End Sub
Private Sub showbuttonmenu(Toolbar1 As MSComctlLib.Toolbar, ByVal Buttonindex As Long) 'buttonindex从0开始
    Dim tButton As TBBUTTON
    Dim tNotify As NMTOOLBAR
    Dim lResult As Long
    Dim mhwnd As Long
    Dim lCommandId As Long
    mhwnd = FindWindowEx(Toolbar1.hwnd, 0, "msvb_lib_toolbar", vbNullString)
    lResult = SendMessage(mhwnd, TB_GETBUTTON, Buttonindex, tButton)
    lCommandId = tButton.idCommand
    With tNotify
        .hdr.code = TBN_DROPDOWN
        .hdr.hwndFrom = mhwnd
        .iItem = lCommandId
    End With
    lResult = SendMessage(Toolbar1.hwnd, WM_NOTIFY, 0, tNotify)
End Sub