This topic pertains only to applications written in C++. For an introduction to programming for DirectX in Visual Basic, see Visual Basic Programming Topics.

COM dictates that objects update their functionality, not by changing the methods within existing interfaces, but by extending new interfaces that encompass new features. By keeping existing interfaces static, an object built on COM can freely extend its services, while maintaining compatibility with older applications.

DirectX components follow this practice. For example, the DirectDraw component supports several interfaces to access a DirectDrawSurface object: IDirectDrawSurface, IDirectDrawSurface2, IDirectDrawSurface3, IDirectDrawSurface4, and IDirectDrawSurface7. Each version of the interface supports the methods provided by its ancestor but adds new methods to support new features.

If your application does not need these new features, it does not need to retrieve newer interfaces. To take advantage of features provided by a new interface, you must call the object's IUnknown::QueryInterface method, specifying the globally unique identifier (GUID) of the interface that you want to retrieve. Interface GUIDs are declared in the corresponding header file.

The following example shows how to query for a new interface:

LPDIRECTDRAW    lpDD1;
LPDIRECTDRAW2   lpDD2;
 
ddrval = DirectDrawCreate( NULL, &lpDD1, NULL );
if( FAILED(ddrval))
    goto ERROROUT;
 
// Query for the IDirectDraw2 interface.
ddrval = lpDD1->QueryInterface(IID_IDirectDraw2, (void **)&lpDD2);
if( FAILED(ddrval))
    goto ERROROUT;
 
// Now that you have an IDirectDraw2, release the original interface.
lpDD1->Release();

In some rare cases, a new interface does not support some methods provided in a previous interface version. The IDirect3DDevice2 interface is an example of this type of interface. If your application requires features provided by an earlier version of an interface, you can query for the earlier version as shown in the preceding example, using the GUID of the older interface to retrieve it.