Control is visible but not loaded (winAPI Programming)

Congratulations on solving the problem and successfuly sending a click event to solve it instead of sending keys to solve it. I am sorry I was not able to help with this.

You are sure getting good with Spy++. It can sure help with things like this. It is good that you have learned how to monitor messages too. I enjoy helping people that are good at figuring things out such as you are.

Ok, I finally solved this issue.

The issue contained 2 problems:
1. Each tab was created only when a click event occurred on it.
2. There was no way to switch between the tabs using the regular methods of TCM_SETCURSEL and such.

The answer to both questions is to generate a click event for each tab you want to use during the program. The click must happen from the tab container (The empty window which has the sys32tabcontrol class) and you must supply the right control coordinates (this can be retrieved from spy++ WM_LBUTTONUP/WM_LBUTTONDOWN events). Finally, use the MakeLParam function to combine the x and y coordinates of the click and insert them as parameters to SendMessage function.

Here's the code to generate these clicks:

//Force creation of Additions Tab
                SendMessage(hwnd_tabContainer, WM_LBUTTONDOWN, 0, (IntPtr)MakeParam(120, 10));
                SendMessage(hwnd_tabContainer, WM_LBUTTONUP, 0, (IntPtr)MakeParam(120, 10));

                //Force creation of Settings Tab
                SendMessage(hwnd_tabContainer, WM_LBUTTONDOWN, 0, (IntPtr)MakeParam(210, 10));
                SendMessage(hwnd_tabContainer, WM_LBUTTONUP, 0, (IntPtr)MakeParam(210, 10));

                //Force creation of Queue Tab
                SendMessage(hwnd_tabContainer, WM_LBUTTONDOWN, 0, (IntPtr)MakeParam(270, 10));
                SendMessage(hwnd_tabContainer, WM_LBUTTONUP, 0, (IntPtr)MakeParam(270, 10));

If the application supports keyboard shortcuts - even if it's only 'tabbing' from one control to the next - you might be able to move to the 'drawn' tabs (if that's in fact what they are) by sending keystrokes to the application whilst it has the input focus.

You can do this either using the managed SendKeys.Send  or SendWait methods (http://msdn.microsoft.com/en-us/library/system.windows.forms.sendkeys.send.aspx) or its unmanaged equivalents keybd_event (http://msdn.microsoft.com/en-us/library/windows/desktop/ms646304(v=vs.85).aspx) or SendInput (http://msdn.microsoft.com/en-us/library/windows/desktop/ms646310(v=vs.85).aspx).

So what am I supposed to do if that's the problem?

The problem is not only that, I simply can't switch between the tabs no matter what I try. Even when I manage to get the handle correctly after I click on it manually and then run the program it still doesn't switch between the tabs.

The program is AVI ReComp, if you want to help you could try to download this program and just try to switch between the tabs.

Thanks for the help!