How can I determine the index of the top item displayed in the drop-down list of a TComboBox?

Question

How can I find the index of the top item in the drop-down list of a TComboBox?

I know that a TListBox has a TopIndex property, but I can't find anything similar to this for a TComboBox.

I'm using C++Builder in RAD Studio 10.4 Update 2.

Answer 1

Since FMX's TListBox does not have a TopIndex property, I'm going to assume you are referring to VCL instead.

In the VCL, you can access the HWND of the TComboBox's drop-down ListBox by calling the Win32 GetComboBoxInfo() function on (or sending a CB_GETCOMBOBOXINFO message to) the HWND returned by the TComboBox::Handle property. And then you can send an LB_GETTOPINDEX message to the ListBox HWND.

COMBOBOXINFO info = { sizeof(COMBOBOXINFO) };
GetComboBoxInfo(ComboBox1->Handle, &info);
// or: SendMessage(ComboBox1->Handle, CB_GETCOMBOBOXINFO, 0, (LPARAM)&info);

int index = SendMessage(info.hwndList, LB_GETTOPINDEX, 0, 0);