Top 100 C# interview questions and answers 2020

C# is a modern, high level, general purpose object oriented programming language. This language is principal language of .Net framework. The design goal of the language was software robustness, durability & programmer productivity. It can be used to create a console application, GUI application, web application both on PC or embedded systems.

1. Single line
   //This is a Single line comment
   
   
2. Multiple line
   /*This is a multiple line comment
   Here you can use more than 1 line*/
   
   
3. XML Comments
   /// This is XML code Comment.
   

A class describes all attributes of objects as well as the method that implements the behavior of member object. Basically, it represents a blue print of object. It is a template of an object. A class contains data & behavior of an entity.
Example: Aircraft.
Data: Model number of aircraft, Color of aircraft, category of aircraft etc.
Behavior/Method: Duration of flight, number of passengers etc.

An object is an instance of the class. It is a basic unit of the system. An object is an entity that has attributes, behavior & identity. Attributes & behavior of an object is defined by the class definition.

They both look very much same but they are not same. The class is definition while the object is the instance of the class created. The class is blue print while objects are actual objects existing in the world.
Example: Car has attributes & method like speed, brake, type of car etc.

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Following are the advantages of C#

1. Easy to learn
2. Object oriented
3. Component oriented
4. Part of .NET framework

Following are the IDE’s used for C# development

1. Visual Studio Express (VCE)
2. Visual Studio (VS)
3. Visual Web Developer

Struct & Class both are the user defined data type but have some major differences:

1. Both can have a constructor, method/functions, properties, fields, constants, enumeration, events & event handler.
2. Structure & classes can implement the interface.
3. Both of them can have a constructor with & without parameters.
4. Both can have delegates & events.

Oops stands for object oriented programming system. It is a problem-solving technique to think real world problems in terms of objects.

Following are different properties provided by oops

1. Inheritance: It is a process of forming new class from the existing class.
   Example: A scientific calculator is an extended form of a calculator. Here, the calculator is the parent & scientific calculator is child object.
   
   
2. Polymorphism: The ability to use an operator or functions in different ways giving different meaning to function to the operator is called polymorphism.
   Example: A person is behaving like a Child, professor, student and then as a clerk. This is the power of polymorphism, a single object behaves in different forms.
   
   
3. Encapsulation: Encapsulate means to hide also called as data hiding. Encapsulation is used to hide the code & data in a single unit. It is a process of hiding all the internal details of an object from the outside world.
   Example: Capsule: Capsule which hides medicine inside it.
   
   
4. Abstraction: Abstraction means showing only necessary information & hiding unnecessary information from the user. Abstraction let you focus on what object does instead of how it does it.
   Example: Suppose you have a car, and u are riding car, now when you apply break you know that car is going to stop but you don't know what is the process happened when you applied break. Abstraction means putting all the variables and methods in a class that is necessary.
   
   

The sealed class is used to stop the class from being inherited from other classes. If you define a class in C# as "Sealed" & "nonheritable" in VB.NET, then you can not inherit the class further.

Value types directly contain data and derived from System.ValueType.
Value types use the stack to store value.
Example: int, float, char, Bool, short

reference types stored the address of memory where data is stored
reference types use the heap to store value.
Example: Class, Interface, String, Object, Delegate, Array

using SizeOf operator.
Example: Console.WriteLine("The size of long is {0}.", sizeof(long));
output: The size of long is 8.

Following are different types of parameter in c#
1. Value parameter(in)
2. Reference parameter (ref)
3. Output parameter(out)
4. Parameter Array(param)

"out" parameter can be passed to a method and it need not be initialized where as “ref” parameter has to be initialized before it is used.

Array
1. Arrays are strongly typed collection of same datatype and these arrays are fixed length that cannot be changed during runtime.
2. Arrays belong to System.Array namespace.

ArrayList
1. Array lists are not strongly type collection. It will store values of different datatypes or same datatype. Array list size will increase or decrease dynamically it can take any size of values from any data type.
2. Arraylist belongs to System.Collection namespaces

"Const" keyword is used for making an entity constant.

Example: const string _name = "techstudy"; Once we define a variable as a const then we can’t reassign the value of that variable(string _name).

Access Modifiers are basically used to control the accessibility of types and members with in the types. In C# there are 5 different types of Access Modifiers.

1. Public: Public is the most common access specifier in C#. It can be accessed from anywhere, that means there is no restriction on accessibility. It is accessible to all classes & projects.

2. Private: The range of the accessibility is limited only within the classes or struct in which they are declared.

3. Protected: All members in the current class & in derived classes can access the variables.

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

5. Protected Internal: The protected internal access specifier allows a class to hide its member functions & member variables from other class objects and functions, except a child class within the same application. This is also used while implementing inheritance.

In C# there is a "try...catch & finally" block to handle the error.

try {
   // statements causing exception
} catch( ExceptionName e1 ) {
   // error handling code
}  finally {
   // statements to be executed
}

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

using keyword is used to include a namespace in the program. A program generally has multiple using statements.

Using Array.sort(array) function.

No! C# does not support multiple inheritance.

1) While loop checks for the condition first so it may not even enter into the loop if the condition is false.
2) do while loop executes the content of the loop once before checking the condition of the while.
3) For loop is similar to while loop except, the initialization of variable, condition check & increment happens at one place.

"Const" keyword is used for making an entity constant. We cannot modify the value later in the code. Value assigning is always mandatory to constant variables.
"readonly" variable value can be changed during runtime and value to readonly variables can be assigned in the constructor or at the time of declaration.

Dispose: This method uses interface – "IDisposable" interface & it will free up both managed and unmanaged codes like - database connection, files etc. It belongs to the IDisposable interface. Implement this method when you are writing a custom class that will be used by other users.

Finalize: This method is called internally unlike Dispose method which is called explicitly. It is called by the garbage collector and can’t be called from the code. Finalize belongs to System.Object class. Implement this method when you have unmanaged resources in your code, and make sure that these resources are freed when the Garbage collection happens.

The string is immutable. Immutable means once we create string object we cannot modify. Any operation like insert, replace or append happened to change string simply it will discard the old value and it will create a new instance in memory to hold the new value.
String belongs to System namespace

=>Example
string str = "hi";
//It creates a new string instance instead of changing the old one
str += "help";
str += "test";

StringBuilder
String builder is mutable. Mutable means once we create string builder object we can perform any operation like insert, replace or append without creating new instance for every time.
StringBuilder belongs to System.Text namespace

=> Example
StringBuilder sb = new StringBuilder("");
//It does not creats new instance for every time.
sb.Append("hi");
sb.Append("help ");
string str = sb.ToString();

1. ArgumentNullException
2. NullReferenceException
3. IndexOutOfRangeException
4. DivideByZeroException
5. StackOverflowException
6. InvalidOperationException etc.

Delegates are type safe pointers, unlike function pointers as in C++. A delegate is used to represent the reference to the methods of some return type and parameters.
Delegates are required because they can be used to write much more generic type safe functions.

Following are the types of delegates in C# -
1. Single Delegate
2. Multicast Delegate
3. Generic Delegate

A delegate having multiple handlers assigned to it is called multicast delegate. Each handler is assigned to a method.

Variable types do not hold null values so to hold the null values we have to use nullable types. nullable types can have values either null or other values as well.
Eg: int? nullvariable = null;

Boxing: This is the process of converting from value type to reference type.

Example
int i = 67; // i is a value type
object o = i; // i is boxed
System.Console.WriteLine(i.ToString()); // i is boxed

UnBoxing: It’s the process of converting reference type to value type.

System.Collections.ArrayList list = new System.Collections.ArrayList(); // list is a reference type
int n = 67; // n is a value type
list.Add(n); // n is boxed
n = (int)list[0]; // list[0] is unboxed

We can't use "this" in a static method because we can only use static variables/methods in a static method.

Interface class is an abstract class which has only public abstract methods & the methods only have the declaration and not the definition. These abstract methods must be implemented in the inherited classes.

We create an abstract class when we define a template that needs to be followed by all the derived classes. Abstract class can have implementation which should be implemented in the child class.

Methods can be overloaded using the different order of parameters, data types for the parameter, and using a different number of parameters.

The virtual keyword is used while defining a class to specify that the method and properties of that class can be overridden in derived class.

Generics are used to make reusable code classes to decrease the code redundancy, increase type safety and performance. Using generics, we can create a collection of classes. To create a generic collection, System.Collections.Generic namespace should be used.

Colon is used as inheritance operator in C#. Just place a colon and then the class name.

Example: public class DerivedClass : BaseClass

"is" operator is used to checking the compatibility of an object with a given type and it returns the result as Boolean.

"as" operator is used for casting of an object to a type or a class.

Yes! C# is managed code because it uses Common language runtime(CLR) to compile C# code.

Public: The keyword public is an access modifier that tells the C# compiler that the Main method is accessible by anyone.

Static: The keyword static declares that the Main method is a global one and can be called without creating an instance of the class. The compiler stores the address of the method as the entry point and uses this information to begin execution before any objects are created.

Void: The keyword void is a type modifier that states that the Main method does not return any value.

“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.

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

1. ArrayList
2. Stack
3. Queue
4. SortedList
5. HashTable
6. Bit Array

It is a feature of modern OS with which we can run multiple programs at the same time.
Example: Word, Excel etc.

It is a subset of multitasking. Instead of switching between programs this feature switches between different part of the same program.
Example: Writing in MS-Word and It also checking spelling using spell-checker in the background

It is a basic unit to which OS allocates processor time.
Example: .Net Framework program has at least 2 threads running one is the main program & Second is Garbage collector(GC).

Syntax to change thread priority

Threading.Priority = Threadpriority.priorityUnit;
Levels of priority
1) Threadpriority.Highest (10)
2) Threadpriority.Abovenormal (7)
3) Threadpriority.Normal(5)
4) Threadpriority.Belownormal(3)
5) Threadpriority.Lowest(1)

"Thread.Currentthread" refers to the current thread running the method.
It is public static property.

Thread execution can be paused by calling the Thread.Sleep method. This method takes on integer value that determines how long the thread should sleep
Example: Thread.currenttime.sleep(2000) // 2 Seconds

Thread.sleep(System.Threading.Timeout.Infinite)

To interrupt this sleep method you can call-
Thread.interrupt method

It is similar to sleep & interrupts. Suspend allow you to block thread until another thread calls thread.resume.

The difference between sleep & suspend is that suspend does not immediately place a thread in a wait state. the thread does not suspend until the .Net runtime determines that it is in a safe place to suspend it. Sleep will immediately place a thread in a wait state.

Thread.Abort stops the thread execution at the moment itself.

There are two versions of Thread.join
1) Thread.join()
2) Thread.join(integer) this returns a bool value.

Example: If we have two threads-> Thread1 & thread2 while executing "Thread1" you call "Thread2.Join()". So "Thread1" will wait until "Thread2" has completed its execution and again invoke "Thread1".

Thread.join(integer) ensures that thread does not wait for a long time. once it exceeds the provided specific time, It will start the waiting thread.

If you want to transfer an object through network then you have to convert the object into a stream of bytes. The process of converting an object into a stream of bytes is called serialization.

Object pool is a container of ready to use objects. It reduce the overhead of creating new objects.

Circular reference is state in which 2 or more resources are interdependent on each other causes the lock condition and make the resources unusable.

A Hashtable is a collection of key-value pairs. It contains values based on the key.

In Constructor overloading, n number of constructors can be created for the same class. But the signatures of each constructor should always vary

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public class Student
{
                            public Student() 
 { }
                            public Student(String fullname) 
 { }
}

Indexers are known as the smart arrays in C#. It allows the instances of a class to be indexed in the same way as array.

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public int this[int index] 

In singleton pattern, a class can only have one instance and provides access point to it globally.

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Public sealed class Singleton
{
Private static readonly Singleton _instance = new Singleton();
}

Name of C# Compiler is – CSC.

“Concat” method of String class is used to concatenate two strings

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string.Concat(firstnamestr, lastnamestr)

Using break statement, you can 'jump out of a loop' while by using continue statement, you can 'jump over one iteration' and then resume your loop execution.