关于IActiveDesktop接口的问题?

[zlcad]

我在程序中要使用IActiveDesktop接口来控制桌面对象，编译如下语句时出错
IActiveDesktop *pActiveDesktop;
//Create an instance of the Active Desktop
hr = CoCreateInstance(CLSID_ActiveDesktop, NULL, CLSCTX_INPROC_SERVER,
IID_IActiveDesktop, (void**)&pActiveDesktop);


error C2027: use of undefined type 'IActiveDesktop'
D:\Program Files\Microsoft Visual Studio\VC98\INCLUDE\Comdef.h(507) : see declaration of 'IActiveDesktop'
其中头文件"Comdef.h"已包含了IActiveDesktop的定义，不知怎么还出现未定义类型错误？