C# INTERVIEW QUESTIONS & ANSWERS PART-2

What is Method Hiding in C#?

If the derived class doesn't want to use methods in the base class, derived class can implement it's own version of the same method with same signature. For example, in the classes given below, DriveType() is implemented in the derived class with same signature. This is called Method Hiding.

 class Car

{

 public void DriveType()

 {

  Console.WriteLine("Right Hand Drive");

 }

}

 class Ford : Car

{

 public void DriveType()

 {

  Console.WriteLine("Right Hand ");

 }

}

What is Abstract Class in C#?

If we don't want a class to be instantiated, define the class as abstract. An abstract class can have abstract and non-abstract classes. If a method is defined as abstract, it must be implemented in derived class. For example, in the classes given below, method Drive Type is defined as abstract. 

abstract class Car

{

 public Car()

 {

  Console.WriteLine("Base Class Car");

 }

 public abstract void DriveType();

}

 class Ford : Car

{

 public void DriveType()

 {

  Console.WriteLine("Right Hand ");

 }

}

Method DriveType get implemented in derived class.

What is a Constructor in C#?

Constructor is a special method that gets invoked/called automatically, whenever an object of a given class gets instantiated. In our class car, constructor is defined as shown below

public Car()

{

 Console.WriteLine("Base Class Car");

}

Whenever an instance of class car is created from the same class or it’s derived class (Except Few Scenarios), Constructor get called and sequence of code written in the constructor get executed.

interface Breaks

{

 void BreakType();

}

interface Wheels

{

 void WheelType();

}

class Ford : Breaks, Wheels

{

 public Ford()

 {

  Console.WriteLine("Derived Class Ford");

 }

 public void Price()

 {

  Console.WriteLine("Ford Price : 100K $");

 }

 public void BreakType()

 {

  Console.WriteLine("Power Break");

 }

 public void WheelType()

 {

  Console.WriteLine("Bridgestone");

 }

}

What is Delegate? And types of Delegates?

Delegate is an abstract pointer to a function or method. In other words you can create a pointer which points to a method or function and then pass that pointer wherever you wish and invoke the function/method.

Delegates are used in the following cases:

Delegates can be used to handle (call/invoke) multiple methods on a single event.

Delegates can be used to define callback (asynchronous) methods.

Delegates can be used for decoupling and implementing generic behaviors.

Delegates can be invoked method at runtime.

Delegates can be used in LINQ for parsing the Expression Tree.

Delegates can be used in different Design Pattern.

Types of Delegates

There are three types of delegates that can be used in C#.

       1)Single Delegate

 2)Multicast Delegate

 3)Generic Delegate

            1. Func

            2. Action

            3. Predicate

Can we use delegates for asynchronous method calls in C#?

Yes. We can use delegates for asynchronous method calls.

What are the uses of delegates in C#?

Below are the lists of uses of delegates in C# -

·   Callback Mechanism

·   Asynchronous Processing

·   Abstract and Encapsulate method

·   Multicasting

Define Multicast Delegate in C#?

A delegate with multiple handlers are called as multicast delegate. The example to demonstrate the same is given below

public delegate void CalculateMyNumbers(int x, int y);

int x = 6;

int y = 7;

CalculateMyNumbers addMyNumbers = new CalculateMyNumbers(FuncForAddingNumbers);

CalculateMyNumbers multiplyMyNumbers = new CalculateMyNumbers(FuncForMultiplyingNumbers);

CalculateMyNumbers multiCast = (CalculateMyNumbers)Delegate.Combine (addMyNumbers,

multiplyMyNumbers);

multicast.Invoke(a,b);

What are the differences between events and delegates in C#?

Main difference between event and delegate is event will provide one more of encapsulation over delegates. So when you are using events destination will listen to it but delegates are naked, which works in subscriber/destination model.

What is nested class?

A Nested classes are classes within classes.

A nested class is any class whose declaration occurs within the body of another class or interface.

What is a base class?

A class declaration may specify a base class by following the class name with a colon and the name of the base class. Omitting a base class specification is the same as deriving from type object.

Where are the types of arrays in C#? 

· Single-Dimensional.

· Multidimensional.

· Jagged arrays.

What is Jagged Arrays?

The array which has elements of type array is called jagged array. The elements can be of different dimensions and sizes. We can also call jagged array as Array of arrays.

                                                        (or) 

A jagged array is an array whose elements are arrays.

The elements of a jagged array can be of different dimensions and sizes.

A jagged array is sometimes called an array–of–arrays.

What is the difference between methods – “System.Array.Clone()” and “System.Array.CopyTo()” in C#?

“CopyTo()” method can be used to copy the elements of one array to other. 

“Clone()” method is used to create a new array to contain all the elements which are in the original array.

Explain sealed class in C#?

Sealed class is used to prevent the class from being inherited from other classes. So “sealed” modifier also can be used with methods to avoid the methods to override in the child classes.

Give an example of using sealed class in C#?

Below is the sample code of sealed class in C# -

Class X {}

Sealed class Y: X {}

Sealed Methods-

Class A

{

Protected virtual void First () { }

 Protected virtual void second () { }

}

Class B: A

{

Sealed protected override void First () {}

 Protected override void second () { }

}

If any class inherits from class “B” then method – “First” will not be overridable as this method is sealed in class B.

How the exception handling is done in C#?

In C# there is a “try… catch” block to handle the error.

Can we execute multiple catch blocks in C#?

No. Once any exception is occurred it executes specific exception catch block and the control comes out.

Can we have only “try” block without “catch” block in C#?

Yes we can have only try block without catch block.

What is the use of enumerated data type?

An enumerated data type is another user defined type which provides a way for attaching names to numbers thereby increasing comprehensibility of the code. The enum keyword automatically enumerates a list of words by assigning them values 0,1,2, and so on.

What is enum in C#?

enum keyword is used for declaring an enumeration, which consists of named constants and it is called as enumerator lists. Enums are value types in C# and these can’t be inherited. Below is the sample code of using Enums

Eg: enum Fruits { Apple,Orange,Banana,WaterMelon};

What is an interface class? 

It is an abstract class with public abstract methods all of which must be implemented in the inherited classes.

What are the different types of Caching?

There are three types of Caching:

Output Caching: stores the responses from an asp.net page.

Fragment Caching: Only caches/stores the portion of page (User Control).

Data Caching: is Programmatic way to Cache objects for performance.

What are literals and their types?  

Literals are value constants assigned to variables in a program. C# supports several types of literals are

· Integer literals: Used to write values of types Int, ulnt, long, and ulong.

· Real literals: Used to write values of types float, double, and dedmal.

· Boolean literals: True and false are literals of the Boolean type that map to the true and false state, respectively.

· Single character literals: Represents a single character and usually consists of a character in quotes, such as 'a'.

  Str

· String literals

· Backslash character literals

· The Null literal: Represents the null–type.

What is the difference between “dispose” and “finalize” variables in C#?

· Dispose - This method uses interface – “IDisposable” interface and it will free up both managed and unmanaged codes like – database connection, files etc.

· Finalize - This method is called internally unlike Dispose method which is called explicitly. It is called by garbage collector and can’t be called from the code.

Why to use “finally” block in C#?

“Finally” block will be executed irrespective of exception. So while executing the code in try block when exception is occurred, control is returned to catch block and at last “finally” block will be executed. So closing connection to database / releasing the file handlers can be kept in “finally” block.

What is the difference between “finalize” and “finally” methods in C#?

· Finalize – This method is used for garbage collection. So before destroying an object this method is called as part of clean up activity.

· Finally – This method is used for executing the code irrespective of exception occurred or not.

What is the difference between “throw ex” and “throw” methods in C#?

· “throw ex” will replace the stack trace of the exception with stack trace info of re throw point.

· “throw” will preserve the original stack trace info.

Do we get error while executing “finally” block in C#?

Yes. We may get error in finally block.

Mention the assembly name where System namespace lies in C#?

Assembly Name – mscorlib.dll

Can we override private virtual method in C#?

No. We can’t override private virtual methods as it is not accessible outside the class.

In try block if we add return statement whether finally block is executed in C#?

Yes. Finally block will still be executed in presence of return statement in try block.

What you mean by inner exception in C#?

Inner exception is a property of exception class which will give you a brief insight of the exception i.e, parent exception and child exception details.

Explain String Builder class in C#?

This will represent the mutable string of characters and this class cannot be inherited. It allows us to Insert, Remove, Append and Replace the characters. “ToString()” method can be used for the final string obtained from StringBuilder. For example,

StringBuilder TestBuilder= new StringBuilder(“Hello”);

TestBuilder.Remove(2,3); // result- “He”

TestBuilder.Insert(2,”Ip”); // result- “Help”

TestBuilder.Replace(‘I’,’a’); // result- “Heap”

How we can sort the array elements in descending order in C#?

“Sort()” method is used with “Reverse()” to sort the array in descending order.

 Explain circular reference in C#?

This is a situation where in, multiple resources are dependent on each other and this causes a lock condition and this makes the resource to be unused.

Explain object pool in C#?

Object pool is used to track the objects which are being used in the code. So object pool reduces the object creation overhead.

 What is Nullable Types in C#?

Variable types does not hold null values so to hold the null values we have to use nullable types. So, nullable types can have values either null or other values as well.

 Why to use “Nullable Coalescing Operator” (??) in C#?

Nullable Coalescing Operator can be used with reference types and nullable value types. So if the first operand of the expression is null then the value of second operand is assigned to the variable. For example,

Double? myFirstno= null;

double mySecno;

mySecno = myFirstno ?? 10.11;

 What is the difference between “as” and “is” operators in C#?

· “as” operator is used for casting object to type or class.

· “is” operator is used for checking the object with type and this will return a Boolean value.

Why to use lock statement in C#?

Lock will make sure one thread will not intercept the other thread which is running the part of code. So lock statement will make the thread wait, block till the object is being released.

 Explain Hashtable in C#?

It is used to store the key/value pairs based on hash code of the key. Key will be used to access the element in the collection. 

For example,

Hashtable myHashtbl = new Hashtable();

myHashtbl.Add("1", "TestValue1");

myHashtbl.Add("2", "TestValue2");

How to check whether hash table contains specific key in C#?

Method – “ContainsKey” can be used to check the key in hash table. Below is the sample code for the same –

Eg: myHashtbl.ContainsKey(“1”);

What is the difference between “continue” and “break” statements in C#?

· “continue” statement is used to pass the control to next iteration. This statement can be used with – “while”, “for”, “foreach” loops.

· “break” statement is used to exit the loop.

Write a sample code to write the contents to text file in C#?

Below is the sample code to write the contents to text file –

Using System.IO;

File.WriteAllText(“mytextfilePath”,”MyTestContent”);

Explain Partial Class in C#?

Partial classes’ concept added in .Net Framework 2.0 and it allows us to split the business logic in multiple files with the same class name along with “partial” keyword.

Explain Anonymous type in C#?

This is being added in C# 3.0 version. This feature enables us to create an object at compile time. Below is the sample code for the same –

Var myTestCategory = new { CategoryId = 1, CategoryName = “Category1”};

 Name the compiler of C#?

C# Compiler is – CSC.

Explain the types of unit test cases?

Below are the lists of unit test case types –

·   Positive Test cases

·   Negative Test cases

·   Exception Test cases

 Explain Copy constructor in C#?

If the constructor contains the same class in the constructor parameter then it is called as copy constructor.

class MyClass

{

public string prop1, prop2;

 public MyClass(string a, string b)

 {

   prop1 = a;

   prop2 = b;

 }

 public MyClass(MyClass myobj) // Copy Constructor

 {

 prop1 = myobj.prop1;

 prop2 = myobj.prop2;

}

}

Explain Static constructor in C#?

If the constructor is declared as static then it will be invoked only once for all number of instances of a class. Static constructor will initialize the static fields of a class.

class MyClass

{

 public string prop1, prop2;

 public MyClass(string a, string b)

 {

 prop1 = a;

 prop2 = b;

 }

Static MyClass()

 {

 Console.WriteLine(“Static Constr Test”);

 }

 public MyClass(MyClass myobj) // Copy Constructor

 {

 prop1 = myobj.prop1;

 prop2 = myobj.prop2;

 }

}

Which string method is used for concatenation of two strings in c#?

“Concat” method of String class is used to concatenate two strings. For example,

string.Concat(firstStr, secStr)

 Explain Indexers in C#?

Indexers are used for allowing the classes to be indexed like arrays. Indexers will resemble the property structure but only difference is indexer’s accessors will take parameters. For example,

class MyCollection<T>

{

 private T[] myArr = new T[100];

 public T this[int t]

 {

 get

 {

 return myArr[t];

 }

 set

 {

 myArr[t] = value;

 }

 }

}

What are the collection types can be used in C#?

Below are the collection types in C# -

·     ArrayList

·     Stack

·     Queue

·     SortedList

·     HashTable

·     Bit Array

Explain Attributes in C#?

·   Attributes are used to convey the info for runtime about the behavior of elements like – “methods”, “classes”, “enums” etc.

·   Attributes can be used to add metadata like – comments, classes, compiler instruction etc.
List out the pre-defined attributes in C#?

Below are the predefined attributes in C# -

· Conditional

· Obsolete

· Attribute Usage

 What is Thread in C#?

Thread is an execution path of a program. Thread is used to define the different or unique flow of control. If our application involves some time-consuming processes then it’s better to use Multithreading. Which involve in multiple threads.

List out the states of a thread in C#?

Below are the states of thread –

·   Unstarted State

·   Ready State

·   Not Runnable State

·   Dead State

      Explain the methods and properties of Thread class in C#?

Below are the methods and properties of thread class –

·   CurrentCulture

·   CurrentThread

·   CurrentContext

·   IsAlive

·   IsThreadPoolThread

·   IsBackground

·   Priority

What are an ExecuteScalar, ExecuteReader and ExecuteNonQuery?

ExecuteScalar is typically used when your query returns a single value. If it returns more, then the result is the first column of the first row. An example might be SELECT @@IDENTITY AS 'Identity'.

ExecuteReader : ExecuteReader used for getting the query results as a DataReader object. It is readonly forward only retrieval of records and it uses select command to read through the table from the first to the last.

  SqlDataReader reader;

  reader = Command.ExecuteReader();

  while (reader.Read()) {

               MessageBox.Show(reader.Item(0));

  }

  reader.Close();

 ExecuteNonQuery : ExecuteNonQuery used for executing queries that does not return any data. It is used to execute the sql statements like update, insert, delete etc. ExecuteNonQuery executes the command and returns the number of rows affected.

  int retValue = 0;

  Command = new SqlCommand(Sql, Connection);

  retValue = Command.ExecuteNonQuery();

What is an assembly and types of assemblies?

Assembly is a unit of deployment like EXE or a DLL.

Types of assembly: - There are two types of assemblies

1) Private assembly: - It is normally used by a single application and is stored in the application’s directory.

2) Public or Shared assembly: - It is stored in the global assembly cache, which is a repository of assemblies maintained by the .net runtime.

 What is variable and dynamic keyword?

Var:-

Introduced in C#3.0

Statically typed – This means the type of variable declared is decided by the compiler at compile time.

Dynamic:-

Introduced in C# 4.0

Dynamically typed - This means the type of variable declared is decided by the compiler at runtime time.

 What is the difference between “Convert.ToString ()”and “.ToString ()” methods?

Example: - int i=0;

                   MessageBox.Show(i.ToString());

                   MessageBox.Show(Convert.ToString(i));

We can convert the integer “i” using “i.ToString()” or “Convert.ToString” so what is the difference. The basic difference between them is “Convert” function which handles Nulls while “i.ToString()” does not do. It will throw a Null referencing exception error. So as a good coding practice using “Convert” is always safe.

What is the difference between “Enumeration” and “Enum”?

Enumeration: - An enumeration is used in any programming language to define a constant set of values. The enumeration is defined with the help of the keyword 'enum'.
Enum: - Is a value type data type. In other words, enumeration contains its own values and cannot inherit or cannot pass inheritance.

The enum is used to declare a list of named integer constants using the enum keyword directly inside a namespace, class, or structure.

The enum is used to give a name to each constant so that the constant integer can be referred using its name.

What is the difference between “Dictionary” and “Hash table”?

Dictionary collection is a generic collection equivalent for hash table.

Hash table allows you to add key and value of any type (i.e., Objects). This leads to two problems one is boxing and unboxing issues and second it’s not strongly typed.

What is the use of IEnumerable, ICollection and IDictionary?

IEnumerable: - All collection classes use this interface and it helps to iterate through all the collection elements.

ICollection: - This interface has the count property and it is implemented by all collection elements.

IDictionary: - This interface is implemented by hash table and dictionary.

What is IEnumerable and IQuerable?

IEnumerable: - It is suitable just for iterate through collection and you can not modify (Add or Remove) data IEnumerable bring ALL data from server to client then filter them, assume that you have a lot of records so IEnumerable puts overhead on your memory.

·    IQuerable: - Whenever we encounter to huge data with so many records so we have to reduce overhead from application. IQueryable prepares high performance in such situations (huge data) by filtering data firstly and then sending filtered data to client.

IEnumerable	IQueryable
IEnumerable exists in System.Collections Namespace.	IQueryable exists in System. Linq Namespace.
IEnumerable doesn’t support custom query.	IQueryable supports custom query using CreateQuery and Execute methods.
IEnumerable doesn’t support lazy loading. Hence not suitable for paging like scenarios.	IQueryable support lazy loading. Hence it is suitable for paging like scenarios.
IEnumerable supports deferred execution.	IQueryable supports deferred execution.
IEnumerable is suitable for LINQ to Object and LINQ to XML queries.	IQueryable is suitable for LINQ to SQL queries.
IEnumerable can move forward only over a collection, it can’t move backward and between the items.	IQueryable can move forward only over a collection, it can’t move backward and between the items.