What is the difference between DLL and exe?

exe file is a excutable file which runs in a seperate process which is managed by OS,where as a dll file is a dynamic link library which can be used in exe files and other dll files.In .net frame work both are assemblies.

fullname of .exe is Extensible Execute File
fullname of .dll is Dynamic Link Liberary
.exe use by End User like-Client
.Dll can not use by End User.
We can Run the .exe
but we can not Run the .Dll

What is the difference between standard exe and windows services?

A Windows service is an application that starts when the Microsoft Windows operating system is booted and runs in the background as long as Windows is running. It is very similar in concept to a Unix daemon. Once a service is installed, its attributes can be modified by launching "Services" from the Windows Control Panel Administrative tools.

Can Date time can have Null values?

By default can not have null values.  But strongly type data set can have null values.

Dispose and Finalise method?

To help prevent resource leaks for unmanaged resources, the garbage collector calls a special method named Finalize on each object before it is removed from the memory.  The finalize method is called by  the garbage collector to perform termination housekeeoing on an object (such as releasing unmanaged resources used by the object) just before the garbage collector reclaims the object's memory.  Method finalize does not take parameters and does not return a value. 

finalize() method is automatically invoked by .net runtime environment to destroy objects which r out of scope. it works as a garbage collector. u donot give body to finalize() and u can not explicitly calll it.

What is managed and Unmanaged resources?

Every object uses various system resources, including the memory that holds the object itself.  We need a displined way to give resources back to the system when they are no longer needed. The common language runtime performs automatic garbege collection to reclaim the memory occupied by objects that are no longer in use. When there are no more references to an objevt that object is marked for garbage collection by CLR. The memory for such an object can be reclaimed when the runtime executes its garbage collectior.  There for memory leaks are much less likely in .NET.  Resources like memory that are allocated and reclaimed by the CLR are known as managed resources.

Unmanaged Resources.  An application opens a file on disk to modify files contents. If the pplication does not close the file, no other application may be allowed to use the file until the application that opened the file finishes. Resources like files, network connections and database connections that the programmer must manage are known as unmanaged resources as they are not managed by CLR.    

Dispose method is explicitly called to release resources used by objects, specially unmaged one. 

Explain the use of "Virtual, Override"

What is Normalization and Denormalisation? Why it is used?

To eliminate data redundancy and therefore to avoid data anomalies, normalization is used.  Data redundancy presents a special problem in database design because storing the same data  in different places can cause problems when updates or changes to the data are required.