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DotNet Interoperability
How can we use COM Components in .NET?
.NET components communicate with COM using RCW (Runtime Callable Wrapper).
Once I have developed the COM wrapper do I have to still register the COM in registry?
How can we use .NET components in COM?
.NET components can not be used in straight forward way with COM. You will need to create CCW in order that COM components communicate with .NET assemblies.
When we use windows API in .NET is it managed or unmanaged code?
Windows API in .NET is unmanaged code.
What is COM?
Microsoft's COM is a technology for component software development. It is a binary standard which is language independent. DCOM is a distributed extensio of COM.
What is Reference Counting in COM?
Reference counting is a memory management technique used to count how many times an object has a pointer referring to it. The first time it is created, the reference count is set to one. When the last reference to the object is nulled, the reference count is set to zero and the object is deleted. Care must be exercised to prevent a context switch from changing the reference count at the time of deletion.
Can you describe IUKNOWN interface in short?
Every COM object supports at least one interface, the IUNKNOWN interface. All interfaces are classes derived from the base class IUnknown. Each interface supports methods access data and perform operations transparently to the programmer. For Example, IUnknown supports three methods, AddRef,Release(), and QueryInterface().
Suppose that pinterf is a pointer to an IUnknown. pinterf->AddRef() increments the referene count.
pinterf->Release() decrements the reference count,deleting the object when the reference count reaches zero.
pinterf->QueryInterface(IDesired,pDesired) checks to see if the current interface (IUnknown) supports another interface, IDesired, creates an instance of the object if the reference count is zero and then calls pDesired->AddRef() to increment the reference count and returns the pointer to caller.
Can you Explain what is DCOM?
DCOM differs from COM in that it allows for creating objects distributed across a network, a protocol for invoking that object's methods, and secures access to the object. DCOM provides a wrapper around COM,hence it is a backwards compatible extension. DCOM uses Remote Procedure Calls(RPC) using Open Software Foundation's Distributed Computing Environment.

These RPC are implemented over TCP/IP and named pipes. The protocol which is actually being used is registered just prior to use, as opposed to being registered at initialization time. The reason for this is that if a protocol is not being used,it willnot be loaded.

In order to inform an object that the client is still alive, periodic pinging is used. Hence, when the client has died and no ping has been received before the expiration time, the server object will perform some clean up tasks.

Since RPC across a network are typically slow,DCOM sends multiple requests in the same call. For example, in COM, the program performs a QueryInterface, one interface at a time. In DCOM, multiple QueryInterfaces are all clustered into one call.

This clustering optimization trick is also used when creating an instance of the object and serializing it with data. Since these two operations usually occur together,DCOM allows one method which will perform both operations in one call without waiting for an acknowlegment from the first task before performing the second one.

Similarly, when a client pings its server object, he can do it in one call. Moreover, if there are multiple clients sending pings to multiple servers,an optimization is made where the multiple pings going to the same object are consolidated into just one ping. This is to cut down on the use of precious bandwidth used only for pinging.

The client has the control to set the computer which will be responsible for the lifetime of the object. That is to say, these objects are not created just somewhere where the system resources and access previleges allow for it.

Call Security is implemented in all four ways: authentication (to prevent false clients from impersonating the true client),authorization(to insure that a client only does what it is authorized to do),data integrity(to insure that data was not tampered with during transit) and data privacy(to insure that only designated sources can read it). The security issues are handled as they are on operating systems. The client gives the server various access privileges to access memory or disk space.

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