SendMessage function

[SteveOshnall]

Have to cast SendMessage function as (HWND)for the program to compile. But the software freezes.
Also when debugging VS2013 complains

+ hActiveWnd 0x00500454 {unused=??? } HWND__ *

void pass(WPARAM wParam,LPARAM lParam)
{
    HWND hActiveWnd;
    hActiveWnd=(HWND)SendMessage(hMDIWnd,WM_MDIGETACTIVE,0,0L);
    if(IsWindow(hActiveWnd)) SendMessage(hActiveWnd,WM_COMMAND,wParam,lParam);
}

[skaa]

What is hMDIWnd?

[SteveOshnall]

Main Windows handle.