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C# Interview Questions

1. What is C#?
C# is an object oriented, type safe and managed language that is compiled by .Net framework to generate Microsoft Intermediate Language.

2. What are the types of comment in C#?
Single line,Multiline,XML document

3. Can multiple catch blocks be executed?
No, Multiple catch blocks can’t be executed. Once the proper catch code executed, the control is transferred to the finally block and then the code that follows the finally block gets executed.

4. What is the difference between public, static and void?
Public declared variables or methods are accessible anywhere in the application. Static declared variables or methods are globally accessible 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. And Void is a type modifier that states that the method or variable does not return any value.

5. What is an Object?
An object is an instance of a class through which we access the methods of that class. “New” keyword is used to create an object. A class that creates an object in memory will contain the information about the methods, variables and behavior of that class.

6. Define Constructors?
A constructor is a member function in a class that has the same name as its class. The constructor is automatically invoked whenever an object class is created. It constructs the values of data members while initializing the class.

7. 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.

8. What is the difference between ref & out parameters?
An argument passed as ref must be initialized before passing to the method whereas out parameter needs not to be initialized before passing to a method.

9. What is the use of using statement in C#?
The using block is used to obtain a resource and use it and then automatically dispose of when the execution of block completed.

10. What is serialization?
When we want to transport an object through network then we have to convert the object into a stream of bytes. The process of converting an object into a stream of bytes is called Serialization. For an object to be serializable, it should inherit ISerialize Interface.
De-serialization is the reverse process of creating an object from a stream of bytes.

11. Can “this” be used within a static method?
We can’t use ‘This’ in a static method because we can only use static variables/methods in a static method.

12. What is difference between constants and read-only?
Constant variables are declared and initialized at compile time. The value can’t be changed after wards. Read-only variables will be initialized only from the Static constructor of the class. Read only is used only when we want to assign the value at run time.

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

14. What are value types and reference types?
Value types are stored in the Stack whereas reference types stored on heap.
Value types:

C#
int, enum , byte, decimal, double, float, long
1 int, enum , byte, decimal, double, float, long

Reference Types:

C#
string , class, interface, object
1 string , class, interface, object

15. What are Custom Control and User Control?
Custom Controls are controls generated as compiled code (Dlls), those are easier to use and can be added to toolbox. Developers can drag and drop controls to their web forms. Attributes can be set at design time. We can easily add custom controls to Multiple Applications (If Shared Dlls), If they are private then we can copy to dll to bin directory of web application and then add reference and can use them.
User Controls are very much similar to ASP include files, and are easy to create. User controls can’t be placed in the toolbox and dragged – dropped from it. They have their design and code behind. The file extension for user controls is ascx.

16. What are sealed classes in C#?
We create sealed classes when we want to restrict the class to be inherited. Sealed modifier used to prevent derivation from a class. If we forcefully specify a sealed class as base class then a compile-time error occurs.

17. What is method overloading?
Method overloading is creating multiple methods with the same name with unique signatures in the same class. When we compile, the compiler uses overload resolution to determine the specific method to be invoke.

18. What is the difference between Array and Arraylist?
In an array, we can have items of the same type only. The size of the array is fixed. An arraylist is similar to an array but it doesn’t have a fixed size.

19. Can a private virtual method be overridden?
No, because they are not accessible outside the class.

20. Describe the accessibility modifier “protected internal”?
Protected Internal variables/methods are accessible within the same assembly and also from the classes that are derived from this parent class.

21. What are the differences between System.String and System.Text.StringBuilder classes?
System.String is immutable. When we modify the value of a string variable then a new memory is allocated to the new value and the previous memory allocation released. System.StringBuilder was designed to have concept of a mutable string where a variety of operations can be performed without allocation separate memory location for the modified string.

22. What’s the difference between the System.Array.CopyTo() and System.Array.Clone() ?
Using Clone() method, we creates a new array object containing all the elements in the original array and using CopyTo() method, all the elements of existing array copies into another existing array. Both the methods perform a shallow copy.

23. How can we sort the elements of the array in descending order?
Using Sort() methods followed by Reverse() method.

24. Write down the C# syntax to catch exception?
To catch an exception, we use try catch blocks. Catch block can have parameter of system.Exception type.

In the above example, we can omit the parameter from catch statement.

25. What’s the difference between an interface and abstract class?
Interfaces have all the methods having only declaration but no definition. In an abstract class, we can have some concrete methods. In an interface class, all the methods are public. An abstract class may have private methods.

26. What is the difference between Finalize() and Dispose() methods?
Dispose() is called when we want for an object to release any unmanaged resources with them. On the other hand Finalize() is used for the same purpose but it doesn’t assure the garbage collection of an object.

27. What are circular references?
Circular reference is situation in which two or more resources are interdependent on each other causes the lock condition and make the resources unusable.

28. What are generics in C#.NET?
Generics are used to make reusable code classes to decrease the code redundancy, increase type safety and performance. Using generics, we can create collection classes. To create generic collection, System.Collections.Generic namespace should be used instead of classes such as ArrayList in the System.Collections namespace. Generics promotes the usage of parameterized types.

29. What is an object pool in .NET?
An object pool is a container having objects ready to be used. It tracks the object that is currently in use, total number of objects in the pool. This reduces the overhead of creating and re-creating objects.

30. List down the commonly used types of exceptions in .Net?
ArgumentException, ArgumentNullException , ArgumentOutOfRangeException, ArithmeticException, DivideByZeroException ,OverflowException , IndexOutOfRangeException ,InvalidCastException ,InvalidOperationException , IOEndOfStreamException , NullReferenceException , OutOfMemoryException , StackOverflowException etc.

31. What are Custom Exceptions?
Sometimes there are some errors that need to be handeled as per user requirements. Custom exceptions are used for them and are used defined exceptions.

32. What are delegates?
Delegates are same are function pointers in C++ but the only difference is that they are type safe unlike function pointers. Delegates are required because they can be used to write much more generic type safe functions.

33. How do you inherit a class into other class in C#
Colon is used as inheritance operator in C#. Just place a colon and then the class name.

34. What is the base class in .net from which all the classes are derived from?


System.Object

35. What is the difference between method overriding and method overloading?
In method overriding, we change the method definition in the derived class that changes the method behavior. Method overloading is creating a method with the same name within the same class having different signatures.

36. What are the different ways a method can be overloaded?
Methods can be overloaded using different data types for parameter, different order of parameters, and different number of parameters.

37. Why can’t you specify the accessibility modifier for methods inside the interface?
In an interface, we have virtual methods that do not have method definition. All the methods are there to be overridden in the derived class. That’s why they all are public.

38. How can we set class to be inherited, but prevent the method from being over-ridden?
Declare the class as public and make the method sealed to prevent it from being overridden.

39. What happens if the inherited interfaces have conflicting method names?
Implement is up to you as the method is inside your own class. There might be problem when the methods from different interfaces expect different data, but as far as compiler cares you’re okay.

40. What is the difference between a Struct and a Class?
Structs are value-type variables and classes are reference types. Structs stored on the stack, causes additional overhead but faster retrieval. Structs cannot be inherited.

41. How to use nullable types in .Net?
Value types can take either their normal values or a null value. Such types are called nullable types.

42. How we can create an array with non-default values?
We can create an array with non-default values using Enumerable.Repeat.

43. What is difference between is and as operators in c#?
“is” operator is used to check the compatibility of an object with a given type and it returns the result as Boolean.
“as” operator is used for casting of object to a type or a class.

44. What’s a multicast delegate?
A delegate having multiple handlers assigned to it is called multicast delegate. Each handler is assigned to a method.

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

Eg:

46. What is difference between the “throw” and “throw ex” in .NET?
“Throw” statement preserves original error stack whereas “throw ex” have the stack trace from their throw point. It is always advised to use “throw” because it provides more accurate error information.

47. What are C# attributes and its significance?
C# provides developers a way to define declarative tags on certain entities eg. Class, method etc. are called attributes. The attribute’s information can be retrieved at runtime using Reflection.

48. How to implement singleton design pattern in C#?
In singleton pattern, a class can only have one instance and provides access point to it globally.

Eg:

49. What is the difference between directcast and ctype?
DirectCast is used to convert the type of an object that requires the run-time type to be the same as the specified type in DirectCast.
Ctype is used for conversion where the conversion is defined between the expression and the type.

50. Is C# code is managed or unmanaged code?
C# is managed code because Common language runtime can compile C# code to Intermediate language.

51. What is static constructor?
Static constructor is used to initialize static data members as soon as the class is referenced first time, whereas an instance constructor is used to create an instance of that class with keyword. A static constructor does not take access modifiers or have parameters and can’t access any non-static data member of a class.

52. What is the use of Monitor in C#?
It provides a mechanism that synchronizes access to objects.
The Monitor class controls access to objects by granting a lock for an object to a single thread. Object locks provide the ability to restrict access to a block of code, commonly called a critical section. While a thread owns the lock for an object, no other thread can acquire that lock. You can also use Monitor to ensure that no other thread is allowed to access a section of application code being executed by the lock owner, unless the other thread is executing the code using a different locked object.

For more visit: http://msdn2.microsoft.com/en-us/library/system.threading.monitor.aspx

 

53. What is lock statement in C#?
Lock ensures that one thread does not enter a critical section of code while another thread is in the critical section. If another thread attempts to enter a locked code, it will wait, block, until the object is released.

54. How to loop through all rows of the DataTable?
ForEach loop

55.What is an Array?
An array is a collection of related instance either value or reference types. Array posses an immutable structure in which the number of dimensions and size of the array are fixed at instantiation.
C# Supports Single, Mult dimensional and Jagged Array.

Single Dimensional Array: it is sometimes called vector array consists of single row.
Multi-Dimensional Array: are rectangular & consists of rows and columns.
Jagged Array: also consists of rows & columns but in irregular shaped (like row 1 has 3 column and row 2 has 5 column)

56. What is an ArrayList?
ArrayList is a dynamic array. Elements can be added & removed from an arraylist at the runtime. In this elements are not automatically sorted.

57. What is BitArray?
The BitArray collection is a composite of bit values. It stores 1 or 0 where 1 is true and 0 is false. This collection provides an efficient means of storing and retrieving bit values.

58. What is HashTable?
A Hashtable is a collection of key-value pairs. Entries in this are instance of DictionaryEntry type. It implements IDictionary, ISerilizable, IDeserializable collback interface.

59. What is Queue?
This is a collection that abstracts FIFO (First In First Out) data structure. The initial capacity is 32 elements. It is ideal for messaging components.

60. What is Stack?
This is a collection that abstracts LIFO (Last In First Out) data structure in which initial capacity is 32.

61. What is SortedList?
This is a collection and it is a combination of key/value entries and an ArrayList collection. Where the collection is sorted by key.

62. What is a collection?
A collection serves as a container for instances of other classes. All classes implement ICollection interface which intern implement IEnumerable interface.

63. What is reflection?
Reflection is the ability to find the information about types contained in an assembly at runtime.

64. How can you sort the elements of the array in descending order?
By calling Array.Sort() and then Array.Reverse() methods.

65. What class is underneath the SortedList class?
A sorted HashTable.

66. Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.

67. Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.

68. What are the different ways a method can be overloaded?
Different parameter data types, different number of parameters, different order of parameters.

69. What is the use of CommandBehavior.CloseConnection?
We can use pass it with ExecuteReader method of Command object like

This will make sure that when we are calling reader.Close() the associated connection object will also be closed.

70. What does the term immutable mean?
The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.

71. What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.

72. Are private class-level variables inherited?
Yes, but they are not accessible. Although they are not visible or accessible via the class interface, they are inherited.

73. What class is underneath the SortedList class?
A sorted HashTable

74. Types of Errors ?
Configuration Errors
Parser Errors
Compilation Errors
Run-Time Errors

75. Difference between classes and structures?
A struct is a value type and a class is a reference type.
When we instantiate a class, memory will be allocated on the heap and when struct gets initiated it gets memory on the stack.

Classes can have explicit parameter less constructors. But structs dosn’t have this.
Classes support inheritance. No inheritance for structs.

A struct cannot inherit from another struct or class, and it cannot be the base of a class. Like classes, structures can implement interfaces.

76. Write a single line of code to read the entire content of the text file.

77. Write a single line of code to create a text file and write contents into it.

78. What is garbage collection?
Garbage collection is a mechanism that allows the computer to detect when an object can no longer be accessed. It then automatically releases the memory used by that object (as well as calling a clean-up routine, called a “finalizer,” which is written by the user). Some garbage collectors, like the one used by .NET, compact memory and therefore decrease your program’s working set.

79.What is nested class?
A Nested classes are classes within classes.

80.Constructor and Destructor?

Constructor:

  1. The Constructor is the first method that is run when an instance of a type is created. In visual basic a constructor is always Sub new ().
  2. Constructor are use to initialize class and structure data before use. Constructor never returns a value and can be overridden to provide custom initialization functionality.
  3. The constructor provides a way to set default values for data or perform other necessary functions before the object available for use.

Destructor:

Destructors are called just before an object is destroyed and can be used to run clean-up code. You can’t control when a destructor is called.

81. what is Static?
Static:
1)A Static memberdata or a memberfunction can be called without the help of an object.
2)Static members can be called with the help of Classname.

Syntax: classname.staticmember
3)’this ‘ keyword cannot be used inside a static memberfunction
4)Nonstatic members cannot be used inside a static memberfunction

82.When do you absolutely have to declare a class as abstract?
1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
2. When at least one of the methods in the class is abstract.

83. What is UDP and how does it work?
The User Datagram Protocol (UDP ) provides an unreliable datagram-oriented
protocol on top of IP.The delivery and order of datagrams are not guaranteed.
It is connection-less, that is, a UDP application does not have to connect
explicitly to another. Datagrams are simply sent or received.

84.Difference Between String and StringBuilder
String Class
Once the string object is created, its length and content cannot be modified.It is Slower.
StringBuilder
Even after object is created, it can be able to modify length and content. It is Faster.

85. What are the Access Modifiers in C# ?
Different Access Modifier are – Public, Private, Protected, Internal, Protected Internal
Public – When a method or attribute is defined as Public, it can be accessed from any code in the project. For example, in the above Class “Employee” getName() and setName() are public.
Private – When a method or attribute is defined as Private, It can be accessed by any code within the containing class only. For example, in the above Class “Employee” attributes name and salary can be accessed within the Class Employee Only. If an attribute or class is defined without access modifiers, it’s default access modifier will be private.
Protected – When attribute and methods are defined as protected, it can be accessed by any method in the inherited classes and any method within the same class. The protected access modifier cannot be applied to classes and interfaces. Methods and fields in a interface can’t be declared protected.
Internal – If an attribute or method is defined as Internal, access is restricted to classes within the current project assembly.
Protected Internal – If an attribute or method is defined as Protected Internal, access is restricted to classes within the current project assembly and types derived from the containing class.

86. What is Constructor Overloading in C# .net ?
In Constructor overloading, n number of constructors can be created for the same class. But the signatures of each constructor should be different.

87. What is Data Encapsulation ?
Data Encapsulation is defined as the process of hiding the important fields from the end user.

88. What is Inheritance?
In object-oriented programming (OOP), inheritance is a way to reuse code of existing objects. In inheritance, there will be two classes – base class and derived classes. A class can inherit attributes and methods from existing class called base class or parent class. The class which inherits from a base class is called derived classes or child class

89. What is Polymorphism in C# ?

The ability of a programming language to process objects in different ways depending on their data type or class is known as Polymorphism. There are two types of polymorphism.
Compile time polymorphism. Best example is Overloading
Runtime polymorphism. Best example is Overriding

90. Explain the use of Virtual Keyword in C# ?
When we want to give permission to a derived class to override a method in base class, Virtual keyword is used

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