首先，需要指出的是toolbar的ButtonMenu不是标准的menu，所以用TrackPopupMenu之类的api函数无法达到我们的目的，事实上当用户点击dropdown button时，toolbar会发送TBN_DROPDOWN这条通知消息，所以我们只要用代码模拟发送这条通知消息即可。代码如下：
Option Explicit
Private Type TBBUTTON
iBitmap As Long
idCommand As Long
fsState As Byte
fsStyle As Byte
bReserved1 As Byte
bReserved2 As Byte
dwData As Long
iString As Long
End Type
Private Type NMHDR
hwndFrom As Long
idfrom As Long
code As Long
End Type
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
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