WM_CONTROL (in Combination Boxes) - Parameters

usid (USHORT)

Control window identity.

usnotifycode (USHORT)

Notify code.

CBN_EFCHANGE

The content of the entry field control has changed, and the change has been displayed on the screen.
CBN_MEMERROR
The entry field control cannot allocate the storage necessary to accommodate window text of the length implied by the EM_SETTEXTLIMIT message.
CBN_EFSCROLL
The entry field control is about to scroll horizontally. This can happen in these circumstances:

• The application has issued a WinScrollWindow call. 
  The content of the entry field control has changed.
  
  The caret has moved. The entry field control must scroll to show the caret position.
  CBN_LBSELECT
  An item in the list box control has been selected.
  CBN_LBSCROLL
  The list box is about to scroll.
  CBN_SHOWLIST
  The list box is about to be displayed.
  CBN_ENTER
  The user has depressed the ENTER key or double clicked (single clicked in the case of a drop-down list) on an item in the list box control.

  hwndcontrolspec (HWND)

  Combination (combo) window handle.

  ulReserved (ULONG)

  Reserved value, should be 0.

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