What is the differnece between Abstract class & Interface?

In an abstract class, we can create the functionality and that needs to be implemented by the derived class. The interface allows us to define the functionality or functions but cannot implement that. The derived class extends the interface and implements those functions.

Abstract Class	Interface
Abstract classes are inherited.	Interfaces are Implemented.
It is a half-defined parent class.	It is a contract.
Share some common logic in child classes	Planning Abstraction.
It contains both declaration and definition parts.	It contains only a declaration part.
It contains both declaration and definition parts.	It contains only a declaration part.
Multiple inheritance is not achieved by abstract class.	Multiple inheritance is achieved by interface.
It contain constructor.	It does not contain constructor.
It can contain static members.	It does not contain static members.
It can contain different types of access modifiers like public, private, protected etc.	It only contains public access modifier because everything in the interface is public.
The performance of an abstract class is fast.	The performance of interface is slow because it requires time to search actual method in the corresponding class.
It is used to implement the core identity of class.	It is used to implement peripheral abilities of class.
A class can only use one abstract class.	A class can use multiple interface.
If many implementations are of the same kind and use common behavior, then it is superior to use abstract class.	If many implementations only share methods, then it is superior to use Interface.
Abstract class can contain methods, fields, constants, etc.	Interface can only contain methods .
It can be fully, partially or not implemented.	It should be fully implemented.

Sr. No.	Key	Abstract Class	Interface
1	Definition	In terms of standard definition, an Abstract class is, conceptually, a class that cannot be instantiated and is usually implemented as a class that has one or more pure virtual (abstract) functions.	On other hand an Interface is a description of what member functions must a class, which inherits this interface, implement. In other words, an interface describes behaviour of the class.
2	Implementation	As like of other general class design in C# Abstract class also have its own implementation along with its declaration.	On other hand an Interface can only have a signature, not the implementation. While its implementation is being provided by the class which implements it.
3	Inheritance	As per specification in C# a class can extends only one other class hence multiple inheritance is not achieved by abstract class.	On other hand in case of Interface a class can implements multiple interfaces and hence multiple inheritance is achieved by interface.
4	Constructor	Like other classes in C# for instantiation abstract class also have constructor which provide an instance of abstract class to access its non-static methods.	On other hand Interface do not have constructor so we can't instantiate an interface directly although its method could get accessed by creating instance of class which implementing it.
5	Modifiers	As abstract class is most like of other ordinary class in C# so it can contain different types of access modifiers like public, private, protected etc.	On other hand as Interface needs to be get implemented in order to provide its methods implementation by other class so can only contains public access modifier.
6	Performance	As abstract class have its method as well as their implementations also for its abstract methods implementation it have reference for its implementing class so performance is comparatively faster as compare to that of Interface.	On other hand the performance of interface is slow because it requires time to search actual method in the corresponding class.

Watch video to clear the concept-

https://www.youtube.com/watch?v=0EnSPBVrbG0

Difference between Viewstate and Sessionstate ?

 Session state is saved on the server, View State is saved in the page.

View State: 

1.View state is maintained in page level only .

2.View state information stored in client only.

3.View state persist the values of particular page in the client when post back operation done.  

4.Simple implementation View state does not require any custom programming to use.

5.No server resources are required The view state is contained in a structure within the page code.

6.If ViewState grows larger, it affects the performance of garbage collection. 

Session State:

1.Session state is maintained in session level .

2.Sessioan state value is available in all pages within a user session.

3.Session stae information stored in server.

4.A session is defined as a period of time that is shared between the web application and the user. Each user that is using the web application has their own session. 

5.SessionState variables are cleared, when the user session times out. The default is 20 minutes. This is configurable in web.config

 Usage: If you're going to store information that you want to access on different web pages, you can use SessionState If you want to store information that you want to access from the same page, then you can use Viewstate.   

Storage The Viewstate is stored within the page itself (in encrypted text), while the Sessionstate is stored in the server.

Interface in C# using an example

http://www.onlinebuff.com/article_implementing-interface-in-c-with-an-example_18.html

http://www.codeproject.com/Articles/18743/Interfaces-in-C-For-Beginners 

What is an Interface?

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments. The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class. Since C# doesn’t support multiple inheritance, interfaces are used to implement multiple inheritance.

What is Functional Dependency (Normalization)?

http://www.emunix.emich.edu/~khailany/files/Normalization.htm

Difference between char varchar and nvarchar in sql server?

Char DataType

Char datatype which is used to store fixed length of characters. Suppose if we declared char(50) it will allocates memory for 50 characters. Once we declare char(50) and insert only 10 characters of word then only 10 characters of memory will be used and other 40 characters of memory will be wasted.

varchar DataType

Varchar means variable characters and it is used to store non-unicode characters. It will allocate the memory based on number characters inserted. Suppose if we declared varchar(50) it will allocates memory of 0 characters at the time of declaration. Once we declare varchar(50) and insert only 10 characters of word it will allocate memory for only 10 characters.

nvarchar DataType

nvarchar datatype same as varchar datatype but only difference nvarchar is used to store Unicode characters and it allows you to store multiple languages in database. nvarchar datatype will take twice as much space to store extended set of characters as required by other languages.

So if we are not using other languages then it’s better to use varchar datatype instead of nvarchar

Non-Unicode vs. Unicode Data Types: Comparison Chart

The primary difference between unicode and non-Unicode data types is the ability of Unicode to easily handle the storage of foreign language characters which also requires more storage space.

Non-Unicode	Unicode
(char, varchar, text)	(nchar, nvarchar, ntext)
Stores data in fixed or variable length	Same as non-Unicode
char: data is padded with blanks to fill the field size. For example, if a char(10) field contains 5 characters the system will pad it with 5 blanks	nchar: same as char
varchar: stores actual value and does not pad with blanks	nvarchar: same as varchar
requires 1 byte of storage	requires 2 bytes of storage
char and varchar: can store up to 8000 characters	nchar and nvarchar: can store up to 4000 characters
Best suited for US English: "One problem with data types that use 1 byte to encode each character is that the data type can only represent 256 different characters. This forces multiple encoding specifications (or code pages) for different alphabets such as European alphabets, which are relatively small. It is also impossible to handle systems such as the Japanese Kanji or Korean Hangul alphabets that have thousands of characters."1	Best suited for systems that need to support at least one foreign language: "The Unicode specification defines a single encoding scheme for most characters widely used in businesses around the world. All computers consistently translate the bit patterns in Unicode data into characters using the single Unicode specification. This ensures that the same bit pattern is always converted to the same character on all computers. Data can be freely transferred from one database or computer to another without concern that the receiving system will translate the bit patterns into characters incorrectly.

Abstract Classes and Abstract Methods in C#.NET?

http://www.programcall.com/27/csnet/abstract-classes-and-abstract-methods-in-csnet.aspx

http://www.dotnetperls.com/abstract

http://www.c-sharpcorner.com/uploadfile/annathurai/abstract-class-in-c-sharp/

http://www.dotnet-tricks.com/Tutorial/csharp/E0K2011113-A-Deep-Dive-into-C

http://www.onlinebuff.com/article_abstract-class-and-abstract-method-with-an-example-using-c_7.html

http://www.codeproject.com/Articles/6118/All-about-abstract-classes

Difference between a Value Type and a Reference Type?

The Types in .NET Framework are either treated by Value Type or by Reference Type. A Value Type holds the data within its own memory allocation and a Reference Type contains a pointer to another memory location that holds the real data. Reference Type variables are stored in the heap while Value Type variables are stored in the stack.

In a simple sentance the Value Type holds the data within its own memory allocation and a Reference Type contains a pointer to another memory location that holds the real data.

Value Type:

A Value Type stores its contents in memory allocated on the stack. When you created a Value Type, a single space in memory is allocated to store the value and that variable directly holds a value. If you assign it to another variable, the value is copied directly and both variables work independently. Predefined datatypes, structures, enums are also value types, and work in the same way. Value types can be created at compile time and Stored in stack memory, because of this, Garbage collector can't access the stack.

e.g. int x = 10;

Here the value 10 is stored in an area of memory called the stack.

Reference Type:

Reference Types are used by a reference which holds a reference (address) to the object but not the object itself. Because reference types represent the address of the variable rather than the data itself, assigning a reference variable to another doesn't copy the data. Instead it creates a second copy of the reference, which refers to the same location of the heap as the original value. Reference Type variables are stored in a different area of memory called the heap. This means that when a reference type variable is no longer used, it can be marked for garbage collection. Examples of reference types are Classes, Objects, Arrays, Indexers, Interfaces etc.

e.g. int[] iArray = new int[20];

In the above code the space required for the 20 integers that make up the array is allocated on the heap.

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Example:

Reference is usually associated with a pointer. Meaning the memory address where your variable reside is actually holding another memory address of the actual object in a different memory location.

Thus, when you do something like:

var a = "Hello";

the computer will do the following:

1. allocate memory (say starting at memory location 1000 for 5 bytes) and put H (at 1000), e (at 1001), l (at 1002), l (at 1003) and o (at 1004).
2. allocate somewhere in memory (say at location 0500) and assigned it as the variable a.
   So it's kind of like an alias (0500 is a).
3. assign the value at that memory location (0500) to 1000 (which is where the string Hello start in memory). Thus the variable a is holding a reference to the actual starting memory location of the "Hello" string.

Value type will hold the actual thing in its memory location.

Thus, when you do something like:

var a = 1;

the computer will do the following:

1. allocate a memory location say at 0500 and assign it to variable a (the same alias thing)
2. put the value 1 in it (at memory location 0500).
   Notice that we are not allocating extra memory to hold the actual value (1). Thus a is actually holding the actual value and that's why it's called value type.

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http://www.codeproject.com/Articles/32029/Reference-and-Value-Types-in-C