COM DCOM Questions And Answers

1⟩ 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.

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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

server.

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3⟩ 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.

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4⟩ 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.

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5⟩ How to create an instance of the object in COM?

To create the instance of COM componet use the following

WIN32 APIs

To access the component that was there in local system use

following API

CoCreateInstance(clsid,NULL(used for

aggregation),CLSCTX_ALL,Interface_GUID,(void**)

&pRequestedInterface );

To access the COM componet remotely use the following API

CoCreateInstanceEx

(CLSID,NULL,CLSCTX_ALL,COSERVERINFO,Interface_GUID,MULTI_QI*

);

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6⟩ 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

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7⟩ What happens when client calls CoCreateInstance?

- Reads RootClassesProgId for the matching ProgID

- Reads RootClassesProgId for the matching CLSID. The

CLSID is read from the above step.

- From the CLSID key, the server type and image filename is

known.

- 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.

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8⟩ What are the different compatibility types when we create a COM component?

No Compatibility

Project Compatibility

Binary Compatibility

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9⟩ 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

VARIANT).

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:

[uuid(21602F40-CC62-11d4-AA2B-00A0CC39CFE0)]

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;

};

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10⟩ 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.

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11⟩ 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()

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12⟩ What should QueryInterface functions do if requested object was not found?

eturns a pointer to the current interface if successful or

E_NOINTERFACE otherwise.

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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

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14⟩ 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

unloaded.

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15⟩ Which tool is used to configure the port range and protocols for DCOM communications?

use DCOMCONFIG.EXE

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16⟩ Can a COM interface be implemented by more than one COM class in the same COM component?

NO

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17⟩ 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.

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18⟩ 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.

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19⟩ 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

DLLRegisterServer

DLLUnRegisterServer

DLLCanUnloadNow

DLLGetClassObject

It is a COM DLL otherwise it is not.

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20⟩ 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.

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COM DCOM

Home Computer Programming COM DCOM

25 COM DCOM Questions And Answers

1⟩ C is aggregated by B, which in turn aggregated by A. Our client requested C. What will happen?

2⟩ What are queued components?

3⟩ How does a DCOM component know where to instantiate itself?

4⟩ Do COM keep track of all the object references (Accounting)?

5⟩ How to create an instance of the object in COM?

6⟩ How do you make a NET component talk to a COM component?

7⟩ What happens when client calls CoCreateInstance?

8⟩ What are the different compatibility types when we create a COM component?

9⟩ How to Use structs in COM interfaces when Automation compatibility is not an issue?

10⟩ What kind of components can be used as DCOM servers?

11⟩ What is IUnknown? What methods are provided by IUnknown?

12⟩ What should QueryInterface functions do if requested object was not found?

13⟩ Explain Futures of COM?

14⟩ What are the purposes of AddRef, Release and QueryInterface functions?

15⟩ Which tool is used to configure the port range and protocols for DCOM communications?

16⟩ Can a COM interface be implemented by more than one COM class in the same COM component?

17⟩ When you call CoInitialize(NULL) function how it works internally?

18⟩ Differentiate normal DLL to COM DLL?

19⟩ How do you know it is general dll or com dll?

20⟩ how to call a dll as a COM dll?

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COM DCOM

Home Computer Programming COM DCOM

25 COM DCOM Questions And Answers

1⟩ C is aggregated by B, which in turn aggregated by A. Our client requested C. What will happen?

2⟩ What are queued components?

3⟩ How does a DCOM component know where to instantiate itself?

4⟩ Do COM keep track of all the object references (Accounting)?

5⟩ How to create an instance of the object in COM?

6⟩ How do you make a NET component talk to a COM component?

7⟩ What happens when client calls CoCreateInstance?

8⟩ What are the different compatibility types when we create a COM component?

9⟩ How to Use structs in COM interfaces when Automation compatibility is not an issue?

10⟩ What kind of components can be used as DCOM servers?

11⟩ What is IUnknown? What methods are provided by IUnknown?

12⟩ What should QueryInterface functions do if requested object was not found?

13⟩ Explain Futures of COM?

14⟩ What are the purposes of AddRef, Release and QueryInterface functions?

15⟩ Which tool is used to configure the port range and protocols for DCOM communications?

16⟩ Can a COM interface be implemented by more than one COM class in the same COM component?

17⟩ When you call CoInitialize(NULL) function how it works internally?

18⟩ Differentiate normal DLL to COM DLL?

19⟩ How do you know it is general dll or com dll?

20⟩ how to call a dll as a COM dll?

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