2004.11.29,Mon - IDBPromptInitialize::PromptDataSource的最后一个参数

为了调用“数据连接属性”对话框，需要类似如下的代码：

IDBPromptInitialize *pDBPrompt;
IUnknown *pDataSource = NULL;
CoCreateInstance(CLSID_DataLinks, NULL, CLSCTX_INPROC_SERVER, IID_IDBPromptInitialize, (LPVOID *)&pDBPrompt);
pDBPrompt->PromptDataSource(NULL, hParent, DBPROMPTOPTIONS_PROPERTYSHEET, 0, NULL, NULL, IID_IUnknown, (IUnknown **)&pDataSource);

但是，如果将
IUnknown *pDataSource = NULL;
一句的指针初始化去掉，程序则会崩溃。

MSDN的解释为：
If *ppDataSource is null on entry, then Prompt generates a new DSO（Data Source Object，数据源对象） based on the information specified by the user, and returns a pointer to that DSO in *ppDataSource.
If *ppDataSource is non-null on entry, Prompt uses the properties returned by IProperties::GetProperties() as initial values. If the user selects a different provider, Prompt will release the original *ppDataSource and create a new data source. On exit, *ppDataSource will be set to a pointer to the interface specified by riid.

也就是说，这个初始化是必要的，否则将会导致Prompt对这个无效的指针进行操作，自然会使程序崩溃。