CB_SHOWDROPDOWN An application sends a CB_SHOWDROPDOWN message to show or hide the list box of a combo box that has the CBS_DROPDOWN or CBS_DROPDOWNLIST style. CB_SHOWDROPDOWN wParam = (WPARAM) (BOOL) fShow; // the show/hide flag lParam = 0; // not used; must be zero
Parameters fShow Value of wParam. Specifies whether the drop-down list box is to be shown or hidden. A value of TRUE shows the list box; a value of FALSE hides it. Return Values The return value is always TRUE. Res This message has no effect on a combo box created with the CBS_SIMPLE style. QuickInfo Windows NT: Requires version 3.1 or later. Windows: Requires Windows 95 or later. Windows CE: Requires version 1.0 or later. Header: Declared in winuser.h.
你可以把这个值当作自己的局部变量来使用ByRef是传递地址,源数据可能被修改
你对这个变量的操作将对你传入的那个变量产生影响,就像指针的感觉你这里用的ByVal是一种显式声明,因为默认情况下是ByRef按址传递的。我们所讨论的是在VB6.0的情况下,而在VB.NET下,默认情况为ByVal按值传递。
ByRef是按地址传递
默认情况下是ByRef
所以可以不写.
ByVal是传值,将变量本身压入堆栈
SendMessage的lParam既可以是指针,又可以是值
当它需要0值时,当然写ByVal 0&
An application sends a CB_SHOWDROPDOWN message to show or hide the list box of a combo box that has the CBS_DROPDOWN or CBS_DROPDOWNLIST style. CB_SHOWDROPDOWN
wParam = (WPARAM) (BOOL) fShow; // the show/hide flag
lParam = 0; // not used; must be zero
Parameters
fShow
Value of wParam. Specifies whether the drop-down list box is to be shown or hidden. A value of TRUE shows the list box; a value of FALSE hides it.
Return Values
The return value is always TRUE. Res
This message has no effect on a combo box created with the CBS_SIMPLE style. QuickInfo
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Requires version 1.0 or later.
Header: Declared in winuser.h.