C# interview Questions and Answers

What are the new features introduced in c# 4.0?
This question is basically asked to check, if you are passionate about catching up with latest technological advancements. The list below shows a few of the new features introduced in c# 4.0. If you are aware of any other new features, please submit those using the from at the end of this post.
1. Optional and Named Parameters
2. COM Interoperability Enhancements
3. Covariance and Contra-variance
4. Dynamic Type Introduction

What's the difference between IEnumerable<T> and List<T> ?
1. IEnumerable is an interface, where as List is one specific implementation of IEnumerable. List is a class.
2. FOR-EACH loop is the only possible way to iterate through a collection of IEnumerable, where as List can be iterated using several ways. List can also be indexed by an int index, element can be added to and removed from and have items inserted at a particular index.
3. IEnumerable doesn't allow random access, where as List does allow random access using integral index.
4. In general from a performance standpoint, iterating thru IEnumerable is much faster than iterating thru a List.

Difference between EXE and DLL?
1. .EXE is an executable file and can run by itself as an application, where as .DLL is usullay consumed by a .EXE or by another .DLL and we cannot run or execute .DLL directly.
2. For example, In .NET, compiling a Console Application or a Windows Application generates .EXE, where as compiling a Class Library Project or an ASP.NET web application generates .DLL. In .NET framework, both .EXE and .DLL are called as assemblies.

What are the difference between interfaces and abstract classes?
1. Abstract classes can have implementations for some of its members, but the interface can't have implementation for any of its members.
2. Interfaces cannot have fields where as an abstract class can have fields.
3. An interface can inherit from another interface only and cannot inherit from an abstract class, where as an abstract class can inherit from another abstract class or another interface.
4. A class can inherit from multiple interfaces at the same time, where as a class cannot inherit from multiple classes at the same time.
5. Abstract class members can have access modifiers where as interface members cannot have access modifiers.

What is a delegate?
A delegate is a type safe function pointer. Using delegates you can pass methods as parameters. To pass a method as a parameter, to a delegate, the signature of the method must match the signature of the delegate. This is why, delegates are called type safe function pointers.

What is the main use of delegates in C#?
Delegates are mainly used to define call back methods.

What do you mean by chaining delegates?
Or
What is a multicast delegate?
The capability of calling multiple methods on a single event is called as chaining delegates. Let me give you an example to understand this further.
1. Create a new asp.net web application
2. Drag and drop a button control and leave the ID as Button1. 
3. On the code behind file, add the code shown below.

When you click the Button now, both Method1 and Method2 will be executed. So, this capability of calling multiple methods on a single event is called as chaining delegates. In the example, we are using EventHandler delegate, to hook up Method1 and Method2 to the click event of the button control. Since, the EventHandler delegate is now pointing to multiple methods, it is also called as multicast delegate.

What are the advantages of using interfaces?
Interfaces are very powerful. If properly used, interfaces provide all the advantages as listed below. 
1. Interfaces allow us to implement polymorphic behaviour. Ofcourse, abstract classes can also be used to implement polymorphic behaviour.
2. Interfaces allow us to develop very loosely coupled systems.
3. Interfaces enable mocking for better unit testing.
4. Interfaces enables us to implement multiple class inheritance in C#.
5. Interfaces are great for implementing Inverson of Control or Dependancy Injection.
6. Interfaces enable parallel application development.

What are the advantages and disadvantages of using arrays?
Advantages of using arrays:
1. Arrays are strongly typed, meaning you can only have one type of elements in the array. The strongly typed nature of arrays gives us 
2 advantages. One, the performance will be much better because boxing and unboxing will not happen. Second, run time errors can be prevented because of type mis matches. Type mis matches and runtime errors are most commonly seen with collection classes like ArrayList, Queue, Stack etc, that are present in System.Collections namespace. 
Disadvantages of using arrays:
1. Arrays are fixed in size and cannot grow over time, where ArrayList in System.Collections namespace can grow dynamically.
2. Arrays are zero index based, and hence a little difficult to work with. The only way to store or retrieve elements from arrays, is to use integral index. Arrays donot provide convinient methods like Add(), Remove() etc provided by collection classes found in System.Collections or System.Collections.Generics namespaces, which are very easy to work with.

Explain what is an Interface in C#?
An Interface in C# is created using the interface keyword. An example is shown below.

Code: [Select all]

using System;
namespace Interfaces
{
interface IBankCustomer
{
void DepositMoney();
void WithdrawMoney();
}
public class Demo : IBankCustomer
{
public void DepositMoney()
{
Console.WriteLine("Deposit Money");
}

public void WithdrawMoney()
{
Console.WriteLine("Withdraw Money");
}

public static void Main()
{
Demo DemoObject = new Demo();
DemoObject.DepositMoney();
DemoObject.WithdrawMoney();
}
}
}

Can an Interface contain fields?
No, an Interface cannot contain fields.

Can an interface inherit from another interface?
Yes, an interface can inherit from another interface. It is possible for a class to inherit an interface multiple times, through base classes or interfaces it inherits. In this case, the class can only implement the interface one time, if it is declared as part of the new class. If the inherited interface is not declared as part of the new class, its implementation is provided by the base class that declared it. It is possible for a base class to implement interface members using virtual members; in that case, the class inheriting the interface can change the interface behavior by overriding the virtual members.

Can you create an instance of an interface?
No, you cannot create an instance of an interface.

What is a partial class. Give an example?
A partial class is a class whose definition is present in 2 or more files. Each source file contains a section of the class, and all parts are combined when the application is compiled. To split a class definition, use the partial keyword as shown in the example below. Student class is split into 2 parts. The first part defines the study() method and the second part defines the Play() method. When we compile this program both the parts will be combined and compiled. Note that both the parts uses partial keyword and public access modifier.

Code: [Select all]

using System;
namespace PartialClass
{
  public partial class Student
  {
    public void Study()
    {
      Console.WriteLine("I am studying");
    }
  }
  public partial class Student
  {
    public void Play()
    {
      Console.WriteLine("I am Playing");
    }
  }
  public class Demo
  {
    public static void Main()
    {
      Student StudentObject = new Student();
      StudentObject.Study();
      StudentObject.Play();
    }
  }
}

It is very important to keep the following points in mind when creating partial classes.
1. All the parts must use the partial keyword.
2. All the parts must be available at compile time to form the final class.
3. All the parts must have the same access modifiers - public, private, protected etc.
4. Any class members declared in a partial definition are available to all the other parts. 
5. The final class is the combination of all the parts at compile time.

What are the advantages of using partial classes?
1. When working on large projects, spreading a class over separate files enables multiple programmers to work on it at the same time.

2. When working with automatically generated source, code can be added to the class without having to recreate the source file. Visual Studio uses this approach when it creates Windows Forms, Web service wrapper code, and so on. You can create code that uses these classes without having to modify the file created by Visual Studio.

Is it possible to create partial structs, interfaces and methods?
Yes, it is possible to create partial structs, interfaces and methods. We can create partial structs, interfaces and methods the same way as we create partial classes.

Can you create partial delegates and enumerations?
No, you cannot create partial delegates and enumerations.

Can different parts of a partial class inherit from different interfaces?
Yes, different parts of a partial class can inherit from different interfaces. 

Can you specify nested classes as partial classes?
Yes, nested classes can be specified as partial classes even if the containing class is not partial. An example is shown below.

Code: [Select all]

class ContainerClass
{
  public partial class Nested
  {
    void Test1() { }
  }
  public partial class Nested
  {
    void Test2() { }
  }
}

How do you create partial methods?
To create a partial method we create the declaration of the method in one part of the partial class and implementation in the other part of the partial class. The implementation is optional. If the implementation is not provided, then the method and all the calls to the method are removed at compile time. Therefore, any code in the partial class can freely use a partial method, even if the implementation is not supplied. No compile-time or run-time errors will result if the method is called but not implemented. In summary a partial method declaration consists of two parts. The definition, and the implementation. These may be in separate parts of a partial class, or in the same part. If there is no implementation declaration, then the compiler optimizes away both the defining declaration and all calls to the method.

The following are the points to keep in mind when creating partial methods.
1. Partial method declarations must begin partial keyword.
2. The return type of a partial method must be void.
3. Partial methods can have ref but not out parameters.
4. Partial methods are implicitly private, and therefore they cannot be virtual.
5. Partial methods cannot be extern, because the presence of the body determines whether they are defining or implementing.

What is the use of partial methods?
Partial methods can be used to customize generated code. They allow for a method name and signature to be reserved, so that generated code can call the method but the developer can decide whether to implement the method. Much like partial classes, partial methods enable code created by a code generator and code created by a human developer to work together without run-time costs.

What is a nested type. Give an example?
A type(class or a struct) defined inside another class or struct is called a nested type. An example is shown below. InnerClass is inside ContainerClass, Hence InnerClass is called as nested class.

Code: [Select all]

using System;
namespace Nested
{
  class ContainerClass
  {
    class InnerClass
    {
      public string str = "A string variable in nested class";
    }

    public static void Main()
    {
      InnerClass nestedClassObj = new InnerClass();
      Console.WriteLine(nestedClassObj.str);
    }
  }
}

What is a Destructor?
A Destructor has the same name as the class with a tilde character and is used to destroy an instance of a class.

Can a class have more than 1 destructor? 
No, a class can have only 1 destructor.

Can structs in C# have destructors?
No, structs can have constructors but not destructors, only classes can have destructors.

Can you pass parameters to destructors? 
No, you cannot pass parameters to destructors. Hence, you cannot overload destructors.

Can you explicitly call a destructor?
No, you cannot explicitly call a destructor. Destructors are invoked automatically by the garbage collector.

Why is it not a good idea to use Empty destructors? 
When a class contains a destructor, an entry is created in the Finalize queue. When the destructor is called, the garbage collector is invoked to process the queue. If the destructor is empty, this just causes a needless loss of performance.

Is it possible to force garbage collector to run?
Yes, it possible to force garbage collector to run by calling the Collect() method, but this is not considered a good practice because this might create a performance over head. Usually the programmer has no control over when the garbage collector runs. The garbage collector checks for objects that are no longer being used by the application. If it considers an object eligible for destruction, it calls the destructor(if there is one) and reclaims the memory used to store the object.

Usually in .NET, the CLR takes care of memory management. Is there any need for a programmer to explicitly release memory and resources? If yes, why and how?
If the application is using expensive external resource, it is recommend to explicitly release the resource before the garbage collector runs and frees the object. We can do this by implementing the Dispose method from the IDisposable interface that performs the necessary cleanup for the object. This can considerably improve the performance of the application.

When do we generally use destructors to release resources?
If the application uses unmanaged resources such as windows, files, and network connections, we use destructors to release resources.

What is a constructor in C#?
Constructor is a class method that is executed when an object of a class is created. Constructor has the same name as the class, and usually used to initialize the data members of the new object. 

In C#, What will happen if you do not explicitly provide a constructor for a class?
If you do not provide a constructor explicitly for your class, C# will create one by default that instantiates the object and sets all the member variables to their default values.

Structs are not reference types. Can structs have constructors?
Yes, even though Structs are not reference types, structs can have constructors.

We cannot create instances of static classes. Can we have constructors for static classes?
Yes, static classes can also have constructors.

Can you prevent a class from being instantiated?
Yes, a class can be prevented from being instantiated by using a private constructor as shown in the example below.

Code: [Select all]

using System;
namespace TestConsole
{
  class Program
  {
    public static void Main()
    {
      //Error cannot create instance of a class with private constructor
      SampleClass SC = new SampleClass();
    }
  }
  class SampleClass
  {
    double PI = 3.141;
    private SampleClass()
    {
    }
  }
}

Can a class or a struct have multiple constructors?
Yes, a class or a struct can have multiple constructors. Constructors in csharp can be overloaded.

Can a child class call the constructor of a base class?
Yes, a child class can call the constructor of a base class by using the base keyword as shown in the example below.

Code: [Select all]

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }

  class ChildClass : BaseClass
  {
    public ChildClass(string str): base(str)
    {
    }

    public static void Main()
    {
      ChildClass CC = new ChildClass("Calling base class constructor from child class");
    }
  }
}

If a child class instance is created, which class constructor is called first - base class or child class?
When an instance of a child class is created, the base class constructor is called before the child class constructor. An example is shown below.

Code: [Select all]

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass()
    {
      Console.WriteLine("I am a base class constructor");
    }
  }
  class ChildClass : BaseClass
  {
    public ChildClass()
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

Can a class have static constructor?
Yes, a class can have static constructor. Static constructors are called automatically, immediately before any static fields are accessed, and are generally used to initialize static class members. It is called automatically before the first instance is created or any static members are referenced. Static constructors are called before instance constructors. An example is shown below.

Code: [Select all]

using System;
namespace TestConsole
{
  class Program
  {
    static int I;
    static Program()
    {
      I = 100;
      Console.WriteLine("Static Constructor called");
    }
    public Program()
    {
      Console.WriteLine("Instance Constructor called");
    }
    public static void Main()
    {
      Program P = new Program();
    }
  }
}

Can you mark static constructor with access modifiers?
No, we cannot use access modifiers on static constructor.

Can you have parameters for static constructors?
No, static constructors cannot have parameters.

What happens if a static constructor throws an exception?
If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.

Give 2 scenarios where static constructors can be used?
1. A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
2. Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.

Does C# provide copy constructor?
No, C# does not provide copy constructor.

What is .NET Remoting?
.NET Remoting allows objects to interact with one another across application domains.

What are the 2 message encoding formats supported by .NET Remoting and when do you choose one over the other?
Message Encoding Formats:
1. Binary encoding.
2. XML encoding.

Applications can use binary encoding where performance is critical, or XML encoding where interoperability with other remoting frameworks is essential.

What are the two types of .NET remote objects?
1. Client-activated objects - Client-activated objects are under the control of a lease-based lifetime manager that ensures that the object is garbage collected when its lease expires.
2. Server-activated objects - In the case of server-activated objects, developers have a choice of selecting either a "single call" or "singleton" model. The lifetime of singletons are also controlled by lease-based lifetime.

What is considered as Remote Object?
Any object outside the application domain of the calling appication is considered remote object, even if the objects are executing on the same machine.

Can you treat every object as a remote object?
Objects that cannot be serialized cannot be passed to a different application domain and are therefore nonremotable.

What are the ways in which an object can be serialized?
1. Mark your class with serializable attribute.
2. Make your class implement ISerializable interface.

How can you change an object into a remote object?
Any object can be changed into a remote object by deriving it from MarshalByRefObject. 

What happens when a client activates a remote object?
When a client activates a remote object, it receives a proxy to the remote object. All operations on this proxy are appropriately indirected to enable the remoting infrastructure to intercept and forward the calls appropriately.

What are proxy objects and what is the use of these proxy objects?
Proxy objects are created when a client activates a remote object. The proxy object acts as a representative of the remote object and ensures that all calls made on the proxy are forwarded to the correct remote object instance.

What is the difference between method parameters and method arguments. Give an example?
In the example below FirstNumber and SecondNumber are method parameters where as FN and LN are method arguments. The method definition specifies the names and types of any parameters that are required. When calling code calls the method, it provides concrete values called arguments for each parameter. The arguments must be compatible with the parameter type but the argument name (if any) used in the calling code does not have to be the same as the parameter named defined in the method.

Code: [Select all]

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   int FN = 10;
   int SN = 20;
   //FN and LN are method arguments
   int Total = Sum(FN, SN);
   Console.WriteLine(Total);
  }
  //FirstNumber and SecondNumber are method parameters
  public static int Sum(int FirstNumber, int SecondNumber)
  {
   int Result = FirstNumber + SecondNumber;
   return Result;
  }
}
}

Can you pass value types by reference to a method?
Yes, we can pass value types by by reference to a method. An example is shown below.

Code: [Select all]

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   int I = 10;
   Console.WriteLine("Value of I before passing to the method = " + I);
   Function(ref I);
   Console.WriteLine("Value of I after passing to the method by reference= " + I);
  }
  public static void Function(ref int Number)
  {
   Number = Number + 5;
  }
}
}

What is the difference between static class and class with static methods? In which case I should use either of them?
If your class has only static members you never need an instance of that class so you should make the class itself static but if your class has instance members (non static) then you have to make your class an instance class to access its instance members via instances of your class.

If a method's return type is void, can you use a return keyword in the method?
Yes, Even though a method's return type is void, you can use the return keyword to stop the execution of the method as shown in the example below.

Code: [Select all]

using System;
namespace Demo
{
class Program
{
  public static void Main()
  {
   SayHi();
  }
  public static void SayHi()
  {
   Console.WriteLine("Hi");
   return;
   Console.WriteLine("This statement will never be executed");
  }
}
}

How do you create empty strings in C#? 
Using string.empty as shown in the example below.
string EmptyString = string.empty;

What is the difference between System.Text.StringBuilder and System.String?
1. Objects of type StringBuilder are mutable where as objects of type System.String are immutable. 
2. As StringBuilder objects are mutable, they offer better performance than string objects of type System.String.
3. StringBuilder class is present in System.Text namespace where String class is present in System namespace.

What is the difference between int.Parse and int.TryParse methods? 
Parse method throws an exception if the string you are trying to parse is not a valid number where as TryParse returns false and does not throw an exception if parsing fails. Hence TryParse is more efficient than Parse.

What is the difference between string keyword and System.String class? 
string keyword is an alias for Syste.String class. Therefore, System.String and string keyword are the same, and you can use whichever naming convention you prefer. The String class provides many methods for safely creating, manipulating, and comparing strings.

Are string objects mutable or immutable? 
String objects are immutable.

What do you mean by String objects are immutable?
String objects are immutable means, they cannot be changed after they have been created. All of the String methods and C# operators that appear to modify a string actually return the results in a new string object. In the following example, when the contents of s1 and s2 are concatenated to form a single string, the two original strings are unmodified. The += operator creates a new string that contains the combined contents. That new object is assigned to the variable s1, and the original object that was assigned to s1 is released for garbage collection because no other variable holds a reference to it.

Code: [Select all]

string s1 = "First String ";
string s2 = "Second String";

// Concatenate s1 and s2. This actually creates a new
// string object and stores it in s1, releasing the
// reference to the original object.
s1 += s2;

System.Console.WriteLine(s1);
// Output: First String Second String

What is an array? 
An array is a data structure that contains several variables of the same type.

What are the 3 different types of arrays?
1. Single-Dimensional
2. Multidimensional
3. Jagged

What is Jagged Array? 
A jagged array is an array of arrays.

Are arrays value types or reference types?
Arrays are reference types.

What is the base class for Array types? 
System.Array

Can you use foreach iteration on arrays in C#?
Yes,Since array type implements IEnumerable, you can use foreach iteration on all arrays in C#.

What is Boxing and Unboxing? 
Boxing - Converting a value type to reference type is called boxing. An example is shown below.

Code: [Select all]

int i = 101;
object obj = (object)i; // Boxing

Unboxing - Converting a reference type to a value typpe is called unboxing. An example is shown below.

Code: [Select all]

obj = 101;
i = (int)obj; // Unboxing

Is boxing an implicit conversion?
Yes, boxing happens implicitly.

Is unboxing an implicit conversion? 
No, unboxing is an explicit conversion.

What happens during the process of boxing?
Boxing is used to store value types in the garbage-collected heap. Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by this value type. Boxing a value type allocates an object instance on the heap and copies the value into the new object. Due to this boxing and unboxing can have performance impact.

What are the 4 pillars of any object oriented programming language?
1. Abstraction
2. Inheritance
3. Encapsulation
4. Polymorphism

Do structs support inheritance?
No, structs do not support inheritance, but they can implement interfaces.

What is the main advantage of using inheritance? 
Code reuse

Does C# support multiple class inheritance?
No, C# supports single class inheritance only. However classes can implement multiple interfaces at the same time.

What is an abstract class? 
An abstract class is an incomplete class and must be implemented in a derived class.

Can you create an instance of an abstract class?
No, abstract classes are incomplete and you cannot create an instance of an abstract class.

What is a sealed class? 
A sealed class is a class that cannot be inherited from. This means, If you have a class called Customer that is marked as sealed. No other class can inherit from Customer class. For example, the below code generates a compile time error "MainClass cannot derive from sealed type Customer.

Code: [Select all]

using System;
public sealed class Customer
{
}
public class MainClass : Customer
{
public static void Main()
{
}
}

What are abstract methods? 
Abstract methods are methods that only the declaration of the method and no implementation.

Can a sealed class be used as a base class? 
No, sealed class cannot be used as a base class. A compile time error will be generated.

Explain polymorphism in C# with a simple example? 
Polymorphism allows you to invoke derived class methods through a base class reference during run-time. An example is shown below.

Code: [Select all]

using System;
public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I am a drawing object.");
}
}
public class Triangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Triangle.");
}
}
public class Circle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Circle.");
}
}
public class Rectangle : DrawingObject
{
public override void Draw()
{
Console.WriteLine("I am a Rectangle.");
}
}
public class DrawDemo
{
public static void Main()
{
DrawingObject[] DrawObj = new DrawingObject[4];

DrawObj[0] = new Triangle();
DrawObj[1] = new Circle();
DrawObj[2] = new Rectangle();
DrawObj[3] = new DrawingObject();

foreach (DrawingObject drawObj in DrawObj)
{
drawObj.Draw();
}
}
}

When can a derived class override a base class member? 
A derived class can override a base class member only if the base class member is declared as virtual or abstract.

What is the difference between a virtual method and an abstract method? 
A virtual method must have a body where as an abstract method should not have a body.

Can fields inside a class be virtual?
No, Fields inside a class cannot be virtua. Only methods, properties, events and indexers can be virtual.

What are Access Modifiers in C#? 
In C# there are 5 different types of Access Modifiers.
Public 
The public type or member can be accessed by any other code in the same assembly or another assembly that references it.

Private
The type or member can only be accessed by code in the same class or struct.

Protected
The type or member can only be accessed by code in the same class or struct, or in a derived class.

Internal
The type or member can be accessed by any code in the same assembly, but not from another assembly.

Protected Internal 
The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.

What are Access Modifiers used for?
Access Modifiers are used to control the accessibilty of types and members with in the types.

Can you use all access modifiers for all types? 
No, Not all access modifiers can be used by all types or members in all contexts, and in some cases the accessibility of a type member is constrained by the accessibility of its containing type.

Can derived classes have greater accessibility than their base types?
No, Derived classes cannot have greater accessibility than their base types. For example the following code is illegal.

Code: [Select all]

using System;
internal class InternalBaseClass
{
   public void Print()
   {
      Console.WriteLine("I am a Base Class Method");
   }
}
public class PublicDerivedClass : InternalBaseClass
{
   public static void Main()
   {
      Console.WriteLine("I am a Public Derived Class Method");
   }
}

When you compile the above code an error will be generated stating "Inconsistent accessibility: base class InternalBaseClass is less accessible than class PublicDerivedClass".To make this simple, you cannot have a public class B that derives from an internal class A. If this were allowed, it would have the effect of making A public, because all protected or internal members of A are accessible from the derived class.

Can you declare struct members as protected? 
No, struct members cannot be declared protected. This is because structs do not support inheritance.

Can the accessibility of a type member be greater than the accessibility of its containing type?
No, the accessibility of a type member can never be greater than the accessibility of its containing type. For example, a public method declared in an internal class has only internal accessibility.

Can destructors have access modifiers? 
No, destructors cannot have access modifiers.

What does protected internal access modifier mean?
The protected internal access means protected OR internal, not protected AND internal. In simple terms, a protected internal member is accessible from any class in the same assembly, including derived classes. To limit accessibility to only derived classes in the same assembly, declare the class itself internal, and declare its members as protected.

What is the default access modifier for a class,struct and an interface declared directly with a namespace? 
internal

Can you specify an access modifier for an enumeration? 
Enumeration members are always public, and no access modifiers can be specified.

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What's the difference between a primary key and a unique key?

Primary key and Unique key enforces uniqueness of the column on which they are defined.The difference between primary key and unique are  below.

1. Primary Key can't accept null values but where as unique key can accept null value.
2. By default, Primary key is clustered index but By default, Unique key is a unique non-clustered index.
3. We can have only one Primary key in a table where as we can have more than one unique key in table.

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ASP.Net Interview Question and Answer

1.) What is asp.net life cycle ? 

Life Cycle Events

PreInit

The properties like IsPostBack have been set at this time.

This event will be used when we want to:

1. Set master page dynamically.
2. Set theme dynamically.
3. Read or set profile property values.
4. This event is also preferred if want to create any dynamic controls.

Init

1. Raised after all the controls have been initialized with their default values and any skin settings have been applied.
2. Fired for individual controls first and then for page.

LoadViewState

1. Fires only if IsPostBack is true.
2. Values stored in HiddenField with id as _ViewState decoded and stored into corresponding controls.

LoadPostData

Some controls like:

1. Fires only if IsPostBack is true.
2. Some controls like Textbox are implemented from IPostBackDataHandler and this fires only for such controls.
3. In this event page processes postback data included in the request object pass it to the respective controls.

PreLoad

• Used only if want to inject logic before actual page load starts.

Load

• Used normally to perform tasks which are common to all requests, such as setting up a database query.

Control events

1. This event is fired when IsPostBack is true.
2. Use these events to handle specific control events, such as a Button control's Click event or a TextBox control's TextChanged event.

PreRenderRaised after the page object has created all the controls that are required for rendering which includes child controls and composite controls.

1. Use the event to make final changes to the contents of the page or its controls before the values are stored into the viewstate and the rendering stage begins.
2. Mainly used when we want to inject custom JavaScript logic.

SaveViewState

• All the control values that support viewstate are encoded and stored into the viewstate.

RenderGenerates output (HTML) to be rendered at the client side.

• We can add custom HTML to the output if we want here.

Unload

1. Fired for individual controls first and then for page.

   2. Used to perform cleanup work like closing open files and database connections.  


2.) How the request is handled by IIS ?

We give an URL to an aspx page in the browser address bar and press enter. What happens next? We get the response in terms of rendered HTML but how?

1. We are requesting something from the browser, which means indirectly we are requesting something from the Web Server, that means IIS. IIS, based on the file extension, decides which ISAPI extension can serve the request.
   
   And in case of ASP.Net (.aspx) it will be aspnet_isapi_dll so the request is passed to it for processing.
    
2. When the first request comes to the website,
   
   an application domain is created by the ApplicationManager class where exactly the website runs, and which creates an isolation between 2 web applications.
   Within the application domain an instance of the HostingEnvironment class is created which provides access information about the application such as the name of the folder where the application is stored.
    
3. Next ASP.Net creates core objects like HttpContext, HttpRequest,HttpResponse.
    
4. Finally the application starts by creating an instance of the HttpApplication Class (which can be reused for multiple requests to maximize performance).

3.) What is Difference between Session and Cookies  ? 

The basic and main difference between cookie and session is that cookies are stored in the user's browser but sessions can't store in user's browser. This specifies which is best used for.

• A cookie can keep all the information in the client's browser until deleted. If a person has a login and password, this can be set as a cookie in their browser so they do not have to re-login to your website every time they visit. You can store almost anything in a browser cookie.

• Sessions are not reliant on the user allowing a cookie. They work like a token in the browser which allowing access and passing information while the user has opened his browser. The problem in sessions is when you close the browser the session will automatically lost. So, if you had a site requiring a login, this couldn't be saved as a session but it can be saved as a cookie, and the user has to re-login every time they visit.
cookies are nothing but a small piece of information on the client machine. before we create a cookies we should check whether the cookies are allowed at the browser side. They are limited in a size 4k.(they are 2 types of cookies peristant cookie , and session cookies)

Sessions cookies are stored in a server memory during the client browser session.When the browser is closed the session cookies are lost. 


4.) Advantages and disadvantages of Session?
 
Following are the basic advantages and disadvantages of using session. I have describe in details with each type of session at later point of time.

Advantages:

• It helps maintain user state and data all over the application.
• It is easy to implement and we can store any kind of object.
• Stores client data separately.
• Session is secure and transparent from the user.

Disadvantages:

• Performance overhead in case of large volumes of data/user, because session data is stored in server memory.
• Overhead involved in serializing and de-serializing session data, because in the case of StateServerand SQLServer session modes, we need to serialize the objects before storing them.

Besides these, there are many advantages and disadvantages of session that are based on the session type. I have discussed all of them in the respective sections below.

5). What is state management?
Ans: State management is the process by which you maintain state and page information over multiple requests for the same or different pages.

6). Http is stateless, What does this mean?
Ans: Stateless protocol is a communications protocol that treats each request as an independent transaction that is unrelated to any previous request so that the communication consists of independent pairs of requests and responses.

7). What is Session?
Ans: We know that Http is stateless, means when we open a webpage and fill some information and then move to next page then the data which we have entered will lost.
It happed do to Http protocol stateless nature. So here session come into existence, Session provide us the way of storing data in server memory. So you can store your page data into server
memory and retrieve it back during page postbacks.

8).  What are the Advantage and disadvantage of Session?
Ans: Advantages:
Session provide us the way of maintain user state/data.
It is very easy to implement.
One big advantage of session is that we can store any kind of object in it. :eg, datatabe, dataset.. etc
By using session we don't need to worry about data collesp, because it store every client data separately.
Session is secure and transparent from the user.
Disadvantages:
Performance overhead in case of large volumes of data/user, because session data is stored in server memory.
Overhead involved in serializing and de-serializing session data, because in the case of StateServer and SQLServer session modes, we need to serialize the objects before storing them.

9). What is Session ID in Asp.net?
Ans: Asp.Net use 120 bit identifier to track each session. This is secure enough and can't be reverse engineered. When client communicate with server, only session id is transmitted, between them. When client request for data, ASP.NET looks on to session ID and retrieves corresponding data.

10). By default where the sessions ID's are stored ?
Ans: By default, the unique identifier for a session is stored in a non-expiring session cookie in the browser. You can specify that session identifiers not be stored in a cookie by setting the cookieless attribute to true in the sessionState configuration element.
We can also configure our application to store it in the url by specifying a "cookieless" session
The ASP Session cookie has this format:-
ASPSESSIONIDACSSDCCC=APHELKLDMNKNIOJONJACDHFN


11). Where does session stored if cookie is disabled on client’s machine?
Ans: If you want to disable the use of cookies in your ASP.NET application and still make use of session state, you can configure your application to store the session identifier in the URL instead of a cookie by setting the cookieless attribute of the sessionState configuration element to true, or to UseUri, in the Web.config file for your application.
The following code example shows a Web.config file that configures session state to use cookieless session identifiers.

Code:

<configuration>
  <system.web>
    <sessionState
      cookieless="true"
      regenerateExpiredSessionId="true"
      timeout="30" />
  </system.web>
</configuration>

12). Can you describe all the property set in web.config under session state?
Ans: 

Code:

<configuration>
  <sessionstate
      mode="inproc"
      cookieless="false"
      timeout="20"
      sqlconnectionstring="data source=127.0.0.1;user id=<user id>;password=<password>"
      server="127.0.0.1"
      port="42424"
  />
</configuration>

Mode: The mode setting supports three options: inproc, sqlserver, and stateserver. As stated earlier, ASP.NET supports two modes: in process and out of process. There are also two options for out-of-process state management: memory based (stateserver), and SQL Server based (sqlserver). We'll discuss implementing these options shortly.
Cookieless: The cookieless option for ASP.NET is configured with this simple Boolean setting.
Timeout: This option controls the length of time a session is considered valid. The session timeout is a sliding value; on each request the timeout period is set to the current time plus the timeout value
Sqlconnectionstring: The sqlconnectionstring identifies the database connection string that names the database used for mode sqlserver.
Server: In the out-of-process mode stateserver, it names the server that is running the required Windows NT service: ASPState.
Port: The port setting, which accompanies the server setting, identifies the port number that corresponds to the server setting for mode stateserver.

13). What are Session Events?
Ans: There are two types of session events available in ASP.NET:
Session_Start
Session_End
You can handle both these events in the global.asax file of your web application. When a new session initiates, the session_start event is raised, and the Session_End event raised when a session is abandoned or expires.

14). How you can disable session?
Ans: If we set session Mode="off" in web.config, session will be disabled in the application. For this, we need to configure web.config the following way:

Code:

<configuration>
  <sessionstate  Mode="off"/>
</configuration>

15). If I have more than one version of one assemblies, then how will I use old version (how/where to specify version number?) in my application?

Ans.The version number is stored in the following format: …. The assembly manifest can then contain a reference to which version number we want to use.

16). How do you create threading in.NET? What is the namespace for that?
Ans.
System.Threading;

//create new thread using the thread class’s constructor

Thread myThread = new Thread(new ThreadStart (someFunction));

17). What do you mean by Serialize and MarshalByRef?

Serialization is the act of saving the state of an object so that it can be recreated (i.e deserialized) at a later date.
The MarshalByRef class is part of the System.Runtime.Remoting namespace and enables us to access and use objects that reside in different application
domains. It is the base class for objects that need to communicate across application domains. MarshalByRef objects are accessed directly within their own
application domain by using a proxy to communicate. With MarshalByValue the a copy of the entire object is passed across the application domain

 18). What is the difference between Array and LinkedList?
An array is a collection of the same type. The size of the array is fixed in its declaration.
A linked list is similar to an array but it doesn’t have a limited size.

19). What is Asynchronous call and how it can be implemented using delegates?

A synchronous call will wait for a method to complete before program flow is resumed. With an asynchronous call the program flow continues whilst the method executes.

//create object
SomeFunction objFunc = new SomeFunction();

//create delegate
SomeDelegate objDel = new SomeDelegate(objFunc.FunctionA);

//invoke the method asynchronously (use interface IAsyncResult)
IAsyncResult asynchCall = SomeDelegate.Invoke();

 20). How to create events for a control? What is custom events? How to create it?
Ans.
An event is a mechanism used in a class that can be used to provide a notification when something interesting happens. (typical evens in a windows application
include: change text in textbox, double click or click a button, select an item in dropdown box).
A custom event is an event created by the user that other developers can use. For example assuming that we have a CashTransaction class and we have a bank
balance property in that class. We may want to set-up an event that provides a notification when the bank balance drops below a certain amount. In order to
produce an event the process would be roughly as follows:
Create the class for the event derived from EventArgs.
Create a delegate with a return type of void.
Create a class containing the method that will activate the event.
Create a class with methods to handle the event.

 21). If you want to write your own dot net language, what steps you will you take care?

We will need to ensure that the high level code is compiled to MSIL (Microsoft intermediate language) so that it can be interpreted by the CLR.

22). Describe the diffeerence between inline and code behind - which is best in a loosely coupled solution?

The term ‘code behind’ refers to application code that is not embedded within the ASPX page and is separated out into a separate file which is then referenced
from the ASPX page. Inline code is the traditional ASP architectural model where business logic code was embedded within the ASP page. Separating the business
logic code from the presentation layer offers several advantages:
1) It allows graphic designers and web developers to work on the presentation layer whilst the application developers concentrate on the business logic.
2) The codebehind file is compiled as a single dll increasing the efficiency of the application,
3) The codebehind model offers a true OO development platform,
4) It speeds up development time as it allows developers to fully maximise the features of the .NET framework such as Cahing, ViewState, Session, Smart Navigation etc.
5) Code is much easier to maintain and susceptible for change.
6) The compiler and VS.NET provides much better support for error checking, intellisense and debugging when using the code behind model.

23). How dot net compiled code will become platform independent?

The raison d’etre for .NET was to cater for multiples languages on a single windows platform whereas the aim of Java was to be a single language on multiple
platforms. The only way that .NET can be platform independent is if there is a version of the .NET framework installed on the target machine.

 24). Without modifying source code if we compile again, will it be generated MSIL again?
Ans.No.

25). How does you handle this COM components developed in other programming languages in.NET?

Ans.
use TlbImp.exe to import the COM types into your .NET project. If no type library for the COM component then use System.Runtime.InteropServices
use RegAsm.exe to call a .NET developed component in a COM application.

 26). How CCW (Com Callable Wrapper) and RCW (Runtime Callable Wrappers) works?

CCW: When a COM application calls a NET object the CLR creates the CCW as a proxy since the COM application is unable to directly access the .NET object.
RCW: When a .NET application calls a COM object the CLR creates the RCW as a proxy since the .NET application is unable to directly access the .COM object.

 27). What are the new thee features of COM+ services, which are not there in COM (MTS)?

Role based security.
Neutral apartment threading.
New environment called context which defines the execution environment

  28). What are the differences between COM architecture and.NET architecture?

.Net architecture has superseded the old COM architecture providing a flexible rapid application development environment which can be used to create windows,
web and console applications and web services. .NET provides a powerful development environment that can be used to create objects in any .NET compliant language.
.NET addresses the previous problems of dll hell with COM by providing strongly named assemblies and side-by-side execution where two assemblies with the same name can run on the same box.

29). Can we copy a COM dll to GAC folder?

Ans.
No. It only stores .NET assemblies.

30). What is Shared and Repeatable Inheritance?

Shared Inheritance-: Shared Inheritance-: ITt is multiple times using same class. The mechanism of deriving a new class from an existing class is called inheritance. Shared inheritance introduces a new opportunity of ambiguity and additional implementation complexity. Assume D inherits from B and C, both of which inherits from A. Here A in shared. Single copy made from both derived classes is called shared inheritance.

  31). Can you explain what inheritance is and an example of when you might use it?
 
Ans.
Inheritance is a fundamental feature of any OO language. It allows us to inherit the members and attributes from a base class to a new derived class. This
leads to increased code reusability and also makes applications easier to develop, maintain and extend as the new derived class can contain new features not
available in the base class whilst at the same time preserving the attributes inherited from the base class.

32). How can you write a class to restrict that only one object of this class can be created (Singleton class)?

Ans.
Use the singleton design pattern.

 public sealed class Singleton
 {
   static readonly Singleton Instance=new Singleton();
      static Singleton()
      {
      }
      Singleton()
      {
      }
      public static Singleton Instance
      {
           get
           {
                return Instance;
           }
      }
 }

 33). What are virtual destructures?


A constructor can not be virtual but a destructor may. Use virtual destructors when you want to implement polymorphic tearing down of an object.

  34). What is close method? How its different from Finalize and Dispose?
  
finalise is the process that allows the garbage collector to clean up any unmanaged resources before it is destroyed.
The finalise method can not be called directly; it is automatically called by the CLR. In order to allow more control over the release of unmanaged resources
the .NET framework provides a dispose method which unlike finalise can be called directly by code.
Close method is same as dispose. It was added as a convenience.

 35). What is Boxing and UnBoxing?

Ans.
Boxing is the process of converting a value type to a reference type. More specifically it involves encapsulating a copy of the object and moving it from
stack to heap. Unboxing is the reverse process.

  36). What is check/uncheck?

Ans.
checked: used to enable overflow checking for arithmetic and conversion functions.
unchecked: used to disable overflow checking for arithmetic and conversion functions

  37). What is the use of base keyword? Tell me a practical example for base keyword’s usage?

Ans.
The base keyword is used to access members of the base class from within a derived class:
* Call a method on the base class that has been overridden by another method.
* Specify which base-class constructor should be called when creating instances of the derived class.

A base class access is permitted only in a constructor, an instance method, or an instance property accessor.
It is an error to use the base keyword from within a static method.
Example:In this example, both the base class, Person, and the derived class, Employee, have a method named Getinfo. By using the base keyword,
it is possible to call the Getinfo method on the base class, from within the derived class.
// keywords_base.cs
// Accessing base class members

 using System;
 public class Person
 {
 protected string ssn = "444-55-6666";
 protected string name = "John L. Malgraine";
 public virtual void GetInfo()
 {
 Console.WriteLine("Name: {0}", name);
 Console.WriteLine("SSN: {0}", ssn);
 }
 }
 class Employee: Person
 {
 public string id = "ABC567EFG";
 public override void GetInfo()
 {
 // Calling the base class GetInfo method:
 base.GetInfo();
 Console.WriteLine("Employee ID: {0}", id);
 }
 }
 class TestClass {
 public static void Main()
 {
 Employee E = new Employee();
 E.GetInfo();
 }
 }
 

38). Difference Between Query String and Session

 

Querystring	Session
Querystring is client side state management technique.	Session is server side state management technique.
Querystring data is page specific i.e. can be accessed in that page only.	Session data can be accessed throughout the session.
Querystring data is visible to user and can be seen in browser url.	Session data is not visible to user.
Data is not secured and can be altered hence insensitive data is stored in querystring.	Data is secured hence sensitive data such as user information is stored.
Querystring has constraint of Maxlength.	Session does not have such constraint.

 

39). Difference between Query string and Cookies

cookies is a text file stored on client machine when we surf ant thing on internet by the server automatically we dont have to create it

query string is used to transfer data from 1 page to anothe but this is not safe s it shows in url what data we r sending
pen any site and see url after question mark tht is url

Cookies: - Cookies are little pieces of information that a server stores on a browser. They are of two types
1. Temporary cookie
2. Persistent cookie

Temporary cookie: - They are also known as session cookies. These are volatile in nature. When the browser is shutdown they are erased.

Persistent cookie:- These may be called as permanent cookies. These are especially saved in files. It may remain for a month or year. 

Properties of cookies 
Some properties of cookie
Name: - represent the name of cookie.
Name value: - represent a collection of key values of cookie
Domain: - represent the domain associated with a specific cookie.
Path: - the path associated with a cookie.
Expires: - expired time of cookie.
Hashkey: - identifies whether the cookie is a cookie dictionary.
Secure: - specifies whether the cookie is to be sent in an encrypted connection or not
Query string is the limited way to pass information to the web server while Transferring from one page to another page. This information is passed in url of the request. see below the code sample


Code Sample

//Retrieving values from query string
String name;
//Retrieving from query string
name = Request.Param["umar"].ToString();

But remember that many browsers impose a limit of 255 characters in query strings. You need to use HTTP-Get method to post a page to server otherwise query string values will not be available.

   

40). How can we identify that the Page is Post Back?

Page object has an "IsPostBack" property, which can be checked to know that is the page posted back.

41). What is the lifespan for items stored in ViewState?

PThe items stored in ViewState live until the lifetime of the current page expires including the postbacks to the same page

42). What is AutoPostBack?

If you want a control to postback automatically when an event is raised, you need to set the AutoPostBack property of the control to True

43). Why do you use the App_Code folder in ASP.NET?

The App_Code folder is automatically present in the project. It stores the files, such as classes, typed data set, text files, and reports. If this folder is not available in the application, you can add this folder. One of the important features of the App_Code folder is that only one dll is created for the complete folder, irrespective of how many files it contains.

44). In which event of page cycle is the ViewState available?

After the Init() and before the Page_Load().

45). How long the items in ViewState exists?

They exist for the life of the current page.

46). Where the viewstate is stored after the page postback?

ViewState is stored in a hidden field on the page at client side. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source.

  47). What are the different Session state management options available in ASP.NET?

1. In-Process
2. Out-of-Process.

In-Process stores the session in memory on the web server.

Out-of-Process Session state management stores data in an external server.  The external server may be either a SQL Server or a State Server.  All objects stored in session are required to be serializable for Out-of-Process state management.

48). What is the difference between web config and machine config?

Web config file is specific to a web application where as machine config is specific to a machine or server. There can be multiple web config files into an application where as we can have only one machine config file on a server.

49). What are the different types of cookies in ASP.NET?

Session Cookie – Resides on the client machine for a single session until the user does not log out.
Persistent Cookie – Resides on a user’s machine for a period specified for its expiry, such as 10 days, one month, and never.

50). What is the difference between ExecuteScalar and ExecuteNonQuery?

ExecuteScalar returns output value where as ExecuteNonQuery does not return any value but the number of rows affected by the query. ExecuteScalar used for fetching a single value and ExecuteNonQuery used to execute Insert and Update statements.

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