Question 91 – What is an Interface?

• An interface looks like a class, but has no implementation.
• An interface is a named set of method signatures.
• An Interface is a reference type and it contains only abstract members.
• An interface is an array of related function that must be implemented in derived type.
• Members of an interface are implicitly public & abstract.
• It can contain definitions of events, indexers, methods parameter less and parameterful properties.
• The interface can’t contain constants, data fields, constructors, destructors and static members.
• All the member declarations inside interface are implicitly public. 
• Interfaces are great for putting together plug-n-play like architectures where components can be interchanged at will. Since all interchangeable components implement the same interface, they can be used without any extra programming.

Question 92 – What is a difference between Abstract Class and Interface?

Abstract Class

Interface

Question 93 – What is a Constructor?

• Constructor is used to initialize an object (instance) of a class.
• Constructor is a like a method without any return type.
• Constructor has same name as class name.
• Constructor follows the access scope (Can be private, protected, public, Internal and external).
• Constructor can be overloaded, means we can have constructors with different set of parameters.
• We can always make the call to one constructor from within the other constructor. 
• Only this and base keywords allowed in initializing constructors, other method calls will raise error.

Question 94 – What is a Constructor chaining?

Overloading the constructor using the this and base keywords so that it overload is called constructor chaining

Question 95 – What are the Types of constructors?

• Static Constructor
• Default Constructor
• Private Constructor
• Copy Constructor 
• Parameterized Constructor

Question 96 – What is a Private Constructor?

• Used to prevent the user to instantiate the class directly.
• Used to prevent the creation of instances of a class when there are no instance fields or methods
• A private constructor is a special instance constructor.
• It is commonly used in classes that contain static members only.
• If a class has one or more private constructors and no public constructors, then other classes (except nested classes) are not allowed to create instances of this class.
• Note that if you don’t use an access modifier with the constructor it will still be private by default.
• Private constructors are used to restrict the instantiation of object using ‘new’ operator.
• This type of constructors is mainly used for creating singleton object.
• Can use nested class (Inner Class) or static method to initialize a class having private constructor. 
• Example of Private Constructor – Math class

Question 97 – What is a Static Constructors?

• Special constructor and gets called before the first object is created of the class.
• The time of execution cannot be determined, but it is definitely before the first object creation – could be at the time of loading the assembly.
• Static constructors might be convenient, but they are slow. The runtime is not smart enough to optimize them in the same way it can optimize inline assignments.
• The static constructor for a class executes before any of the static members for the class are referenced.
• The static constructor for a class executes after the static field initializers (if any) for the class.
• A static constructor cannot be called directly.
• The user has no control on when the static constructor is executed in the program. 
• Example – When the class is using a log file and the constructor is used to write entries to this file.

Question 98 – What are the features of Static Constructor?

• Only one Static constructor – Overloading needs the two methods to be different in terms to methods definition, so you can have at the most one static constructor 
• Without parameters – It is going to be called by CLR, who can pass the parameters to it, if required, No one, so we cannot have parameterized static constructor. 
• Access only static members – If allowed to work on non-static members, will reflect the changes in all the object instances, which is impractical. Non-static members in the class are specific to the object instance 
• No access modifier – The call to the static method is made by the CLR and not by the object, so we do not need to have the access modifier to it.

Question 99 – What is a Default Constructor?

• A default constructor is a constructor in both that has no parameters or where it has parameters they are all defaulted.
• If no constructor is supplied then the compiler will supply a default constructor. 
• This default constructor is a parameter less constructor with no body, which calls the parameter less constructor of the base class.

Question 100 – What is a COPY Constructor?

• C# does not provide a copy constructor.
• A copy constructor is a special constructor used to create a new object as a copy of an existing object.
• This constructor takes a single argument: a reference to the object to be copied.
• It is a great convenience to create copy constructor for C# classes using Reflection.
• If you create a new object and want to copy the values from an existing object, you have to write the appropriate method yourself.

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