9 Apr, 2007
VB Interview Questions
- What is IUnknown? What methods are provided by IUnknown? It is a generally good idea to have an answer for this question if you claim you know COM in your resume. Otherwise, you may consider your interview failed at this point. IUnknown is the base interface of COM. All other interfaces must derive directly or indirectly from IUnknown. There are three methods in that interface: AddRef, Release and QueryInterface.
- What are the purposes of AddRef, Release and QueryInterface functions? AddRef increments reference count of the object, Release decrements reference counter of the object and QueryInterface obtains a pointer to the requested interface.
- What should QueryInterface functions do if requested object was not found? Return E_NOINTERFACE and nullify its out parameter.
- How can would you create an instance of the object in COM? Well, it all depends on your project. Start your answer from CoCreateInstance or CoCreateInstanceEx, explain the difference between them. If interviewer is still not satisfied, you’ll have to explain the whole kitchen behind the scenes, including a difference between local server and inproc server, meaning and mechanism of class factory, etc. You may also mention other methods of object creation like CoGetInstanceFromFile, but discussion will likely turn to discussion of monikers then.
- What happens when client calls CoCreateInstance? Again, all depends on the level of detail and expertise of interviewer. Start with simple explanation of class object and class factory mechanism. Further details would depend on a specific situation.
- What the limitations of CoCreateInstance? Well, the major problems with CoCreateInstance is that it is only able to create one object and only on local system. To create a remote object or to get several objects, based on single CLSID, at the same time, one should use CoCreateInstanceEx.
- What is aggregation? How can we get an interface of the aggregated object? Aggregation is the reuse mechanism, in which the outer object exposes interfaces from the inner object as if they were implemented on the outer object itself. This is useful when the outer object would always delegate every call to one of its interfaces to the same interface in the inner object. Aggregation is actually a specialized case of containment/delegation, and is available as a convenience to avoid extra implementation overhead in the outer object in these cases. We can get a pointer to the inner interface, calling QueryInterface of the outer object with IID of the inner interface.
- C is aggregated by B, which in turn aggregated by A. Our client requested C. What will happen? QueryInterface to A will delegate request to B which, in turn, will delegate request for the interface to C. This pointer will be returned to the client.
- What is a moniker ? An object that implements the IMoniker interface. A moniker acts as a name that uniquely identifies a COM object. In the same way that a path identifies a file in the file system, a moniker identifies a COM object in the directory namespace.
- What’s the difference, if any, between OLE and COM? OLE is build on top of COM. The question is not strict, because OLE was built over COM for years, while COM as a technology was presented by Microsoft a few years ago. You may mention also that COM is a specification, while OLE is a particular implementation of this specification, which in today’s world is not exactly true as well, because what people call COM today is likely implementation of COM spec by Microsoft.
