COM Interview Questions II
- What’s the difference between COM and DCOM? Again, the question does not require strict answer. Any DCOM object is yet a COM object (DCOM extends COM) and any COM object may participate in DCOM transactions. DCOM introduced several improvements/optimizations for distributed environment, such as MULTI_QI (multiple QueryInterface()), security contexts etc. DCOM demonstrated importance of surrogate process (you cannot run in-proc server on a remote machine. You need a surrogate process to do that.) DCOM introduced a load balancing.
- What is a dual interface? Dual interface is one that supports both - IDispatch interface and vtbl-based interface. Therefore, it might be used in scripting environment like VBScript and yet to use power and speed of vtbl-based interface for non-scripting environment. Discussion then may easily transform into analyzing of dual interface problems - be prepared to this twist.
- Can you have two dual interfaces in one class? Yes. You may have two dual interfaces in one class, but only one of them may be default. The bottom line is that you cannot work with two dual interfaces at the same time due to nature of dual interface! To support two dual interfaces in VB you would write something like:
dim d1 as IDualInterface1 dim d2 as IDualInterface2 set d1 = new MyClassWithTwoDuals set d2 = d1
In ATL’s class you would have to use macro COM_INTERFACE_ENTRY2(IDispatch,
IDualInterface1), to distinguish between different dual interfaces. - What is marshalling by value? Some objects can essentially be considered static: regardless of which methods are called, the state of the object does not change. Instead of accessing such an object remotely, it is possible to copy the static state of the object and create a new object with the same state information on the caller side. The caller won’t be able to notice the difference, but calls will be more efficient because they do not involve network round trips. This is called “marshaling by value”.
- What is a multi-threaded apartment (MTA)? Single-threaded apartment (STA)? This is pretty difficult question to describe shortly. Anyway, apartments were introduced by Microsoft in NT 3.51 and late Windows 95 to isolate the problem of running legacy non-thread safe code into multithreaded environment. Each thread was “encapsulated” into so called single-threaded apartment. The reason to create an object in apartment is thread-safety. COM is responsible synchronize access to the object even if the object inside of the apartment is not thread-safe. Multithreaded apartments (MTA, or free threading apartment) were introduced in NT 4.0. Idea behind MTA is that COM is not responsible to synchronize object calls between threads. In MTA the developer is responsible for that. See “Professional DCOM Programming” of Dr. Grimes et al. or “Essential COM” of Don Box for the further discussion on this topic.
- Let’s assume we have object B and aggregated object C (in-proc server), created by B. Can you access any interface of B from C? What’s the difference between aggregated and contained objects? Yes, you can. This is fundamental postulate of COM: “If you can get there from here, you can get there from anywhere”, i.e. QI’ing for IUnknown you may proceed and to get a pointer to any other interface, supported by the object. Aggregated object exposes its interface directly, without visible intervention of the object container. Contained object is created within the object container and its interfaces might be altered or filtered by the object container.
- What is ROT ? GIT ? Count pros and cons of both. By definition, running object table (ROT) is a globally accessible table on each computer that keeps track of all COM objects in the running state that can be identified by a moniker. Moniker providers register an object in the table, which increments the object’s reference count. Before the object can be destroyed, its moniker must be released from the table. Global Interface Table (GIT) allows any apartment (either single- or multi-threaded) in a process to get access to an interface implemented on an object in any other apartment in the process.
- If you have an object with two interfaces, can you custom marshal one of them? No! The decision to use custom marshaling is an all-or-nothing decision; an object has to custom marshal all its interfaces or none of them.
- Is there a way to register in-proc server without regsvr32.exe? Yes. Call DllRegisterServer() from the client. Do not forget to call DLLUnregisterServer() from the same client. You may also use Registrar object for the same purpose or use direct manipulation of the windows registry.
- What is VARIANT? Why and where would you use it? VARIANT is a huge union containing automation type. This allows easy conversion of one automation type to another. The biggest disadvantage of VARIANT is size of the union.
