COM DCOM Interview Preparation Guide
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DCOM COM frequently Asked Questions by expert members with experience in COM DCOM. So get preparation for the DCOM COM job interview

25 COM DCOM Questions and Answers:

1 :: How do you make a NET component talk to a COM component?

RCW is used for making a NET compnent talk to a com component.
CCW is used for Com to .NET communication

2 :: What are queued components?

Queued Components, a key feature of COM+ and based on
Microsoft Message Queuing Services (MSMQ), provides an easy
way to invoke and execute components asynchronously.
Processing can occur without regard to the availability or
accessibility of either the sender or receiver. A home
shopping network is an example of the type of application
that might benefit from asynchronous processing. In this
asynchronous, disconnected scenario where viewers phone in
to several operators, orders are taken en masse and are
then queued for later retrieval and processing by the

3 :: 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.

4 :: How does a DCOM component know where to instantiate itself?

While accessing the DCOM component you have to provide
COSERVERINFO structure. This structure is having the
information about where the component is.

5 :: What kind of components can be used as DCOM servers?

There are two kind components
1. InProc
2. OutProc

We can use both as DCOM servers, its upto our design.

6 :: What are the different compatibility types when we create a COM component?

No Compatibility
Project Compatibility
Binary Compatibility

7 :: How to Use structs in COM interfaces when Automation compatibility is not an issue?

Structs, also known as User Defined Types (UDTs), can be
used in Automation- compatible interfaces
An Automation- compatible struct may contain only primitive
Automation types as its members.

Nesting structs is not allowed, but VARIANT is allowed thus
enabling nested structs (you can store a struct in a

In order for a struct to be usable for Automation-
compatible interfaces, the struct must be described in a
type library and it must be declared with its own GUID:

struct MyStruct
[helpstring("A long value")]
long nLongValue;

[helpstring("A string")]
BSTR bstrStringValue;

// Later in the IDL file
[uuid(...), version(...), helpstring(...)]
library MyLib
struct MyStruct;

8 :: Do COM keep track of all the object references (Accounting)?

Object references in COM is accounted using two methods of
IUnknown Interface (AddRef and Release).

AddRef: Increments a reference count whereas "Release"
decrements the count.
When the count of the reference is zero the DLL is unloaded
from memory.

9 :: how to call a dll as a COM dll?

Inorder to call your DLL as COM dll, you have to follow
some rules to develop the dll. If those rules are there
then only your DLL will be called as COM dll.

COM is a specification set of rules to develope binaries,
COM is not a language.

10 :: Define and explain about COM?

COM (Component Object Model) technology in the Microsoft
Windows-family of Operating Systems enables software
components to communicate. COM is used by developers to
create re-usable software components, link components
together to build applications, and take advantage of
Windows services.

11 :: Differentiate normal DLL to COM DLL?

COM Dll exposes Interface on the contrary to normal DLL
that exports functions. Clients create the pointer to
COMDLL's interface to call the methods defined by the
component that implements the interface.
That results in isolation of implementation and definition
of method in the interface. Client doesnt need to relink or
recompile the code if method in the com dll changes as far
as the definition of the interface remains same.

12 :: When you call CoInitialize(NULL) function how it works internally?

CoInitialize will initialize the COM library and will move
the executing thread to a STA or Single Threaded Apartment
Apartment is the logical entity where threads live.

13 :: Explain Futures of COM?

1. Defines a binary standard for component interoperability
2. Is programming language-independent
3. Is provided on multiple platforms (Microsoft® Windows®,
4. Microsoft Windows NT™, Apple® Macintosh®, UNIX®)
5. Provides for robust evolution of component-based
applications and systems
6. Is extensible

14 :: How do you know it is general dll or com dll?

Open the dll in dependency walker application
(depends.exe). If the dll is having following functions

It is a COM DLL otherwise it is not.

17 :: What are the purposes of AddRef, Release and QueryInterface functions?

Query Interface method is used to get the pointer to the
interface specified in one of the parameters of this
method. Client then uses this pointer to call the method of
the component.
AddRef and Release are used to increase and decrease the
count of the instance of component loaded in memory
respectively. when the count reaches zero the component is

18 :: What is IUnknown? What methods are provided by IUnknown?

IUnknown is a type of COM Interface.

Every COM class implements an interface named IUnKnown.

IUnKnown contains three methods:
1) HRESULT QueryInterface()
2) ULONG AddRef()
3) ULONG Release()

19 :: What should QueryInterface functions do if requested object was not found?

eturns a pointer to the current interface if successful or
E_NOINTERFACE otherwise.

20 :: What happens when client calls CoCreateInstance?

- Reads Root\Classes\ProgId for the matching ProgID
- Reads Root\Classes\ProgId for the matching CLSID. The
CLSID is read from the above step.
- From the CLSID key, the server type and image filename is
- Depending upon the server type, it starts the server.
- Calls CoGetClassObject function to get a handle to the
factory object.
- Then calls createinstance on the factory interface to get
the pointer to the derived object.