How to populate a ListBox with SendMessage?

Question

I am trying to populate a ListBox with data I am reading from a text file, but have had no luck. I tried several ways to do SendMessage (please see below) but get the same "invalid cast type" error.

ListBox:

CreateWindow("listbox", NULL, WS_CHILD | WS_VISIBLE | WS_VSCROLL|
                         LBS_NOTIFY, 20, 90, 200, 365,
                         hwnd, (HMENU)LST_LISTBOX, NULL, NULL);

I keep on changing the SendMessage:

SendMessage(LST_LISTBOX, LB_INSERTSTRING, 0, (LPARAM)10); //myline[i]);

SendMessage(LST_LISTBOX, LB_ADDSTRING, 0, (LPARAM) myline[i]);

SendMessage(GetDlgItem(hwnd, LST_LISTBOX), LB_ADDSTRING, 0, (LPARAM) myline[i] );

What would be the best way to populate a listbox from a file?

Answer 1

You need to send the message to the HWND that CreateWindow() creates. You can get that from the return value of CreateWindow(), or from GetDlgItem() after CreateWindow() is finished.

The LPARAM for LB_ADDSTRING and LB_INSERTSTRING needs to be a pointer to the 1st character of a null-terminated C-style string, eg:

char myline[size];
//fill myline as needed...

HWND hwndLB = CreateWindowA(WC_LISTBOXA, ... hwnd, reinterpret_cast<HMENU>(LST_LISTBOX), ...);

// or:
// CreateWindowA(...);
// HWND hwndLB = GetDlgItem(hwnd, LST_LISTBOX);

SendMessage(hwndLB, LB_ADDSTRING, 0, reinterpret_cast<LPARAM>(myline));

Alternatively:

std::string myline;
//fill myline as needed...

HWND hwndLB = ...;

SendMessage(hwndLB, LB_ADDSTRING, 0, reinterpret_cast<LPARAM>(myline.c_str()));

---

UPDATE: based on comments you posted, myline seems to be a std::string[20] array, so you need to index into the array and call c_str() on the desired element, eg:

SendMessage(hwndLB, LB_ADDSTRING, 0, reinterpret_cast<LPARAM>(myline[i].c_str()));