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IDBPromptInitialize::PromptDataSource 的最后一个参数

Posted by on 2004 年 11 月 29 日

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为了调用“数据连接属性”对话框,需要类似如下的代码:

C++代码
  1. IDBPromptInitialize *pDBPrompt;   
  2. IUnknown *pDataSource = NULL;   
  3. CoCreateInstance(CLSID_DataLinks, NULL, CLSCTX_INPROC_SERVER, IID_IDBPromptInitialize, (LPVOID *)&pDBPrompt);   
  4. pDBPrompt->PromptDataSource(NULL, hParent, DBPROMPTOPTIONS_PROPERTYSHEET, 0, NULL, NULL, IID_IUnknown, (IUnknown **)&pDataSource);  

但是,如果将

C++代码
  1. 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。

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