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# QueryInterface
Although there are mechanisms by which an object can express the functionality it provides statically (before it is instantiated), the fundamental COM mechanism is to use the **IUnknown** method called [QueryInterface](http://msdn.microsoft.com/library/windows/desktop/ms682521).  
  
 Every interface is derived from **IUnknown**, so every interface has an implementation of `QueryInterface`. Regardless of implementation, this method queries an object using the IID of the interface to which the caller wants a pointer. If the object supports that interface, `QueryInterface` retrieves a pointer to the interface, while also calling `AddRef`. Otherwise, it returns the **E_NOINTERFACE** error code.  
  
 Note that you must obey [Reference Counting](../atl/reference-counting.md) rules at all times. If you call **Release** on an interface pointer to decrement the reference count to zero, you should not use that pointer again. Occasionally you may need to obtain a weak reference to an object (that is, you may wish to obtain a pointer to one of its interfaces without incrementing the reference count), but it is not acceptable to do this by calling `QueryInterface` followed by **Release**. The pointer obtained in such a manner is invalid and should not be used. This more readily becomes apparent when [_ATL_DEBUG_INTERFACES](reference/debugging-and-error-reporting-macros.md#_atl_debug_interfaces) is defined, so defining this macro is a useful way of finding reference counting bugs.  
  
## See Also  
 [Introduction to COM](../atl/introduction-to-com.md)   
 [QueryInterface: Navigating in an Object](http://msdn.microsoft.com/library/windows/desktop/ms687230)