C# .Net Interview Questions

Posted by mauliksoni on June 6, 2007 · 58 Comments 

===> C# Interview Questions PDF Download <===

 

1. What standard types does C# use? C# supports a very similar range of basic types to C++, including int, long, float, double, char, string, arrays, structs and classes. In C# Types The names may be familiar, but many of the details are different. For example, a long is 64 bits in C#, whereas in C++ the size of a long depends on the platform (typically 32 bits on a 32-bit platform, 64 bits on a 64-bit platform). Also classes and structs are almost the same in C++ – this is not true for C#. Finally, chars and strings in .NET are 16-bit (Unicode/UTF-16), not 8-bit like C++.

2.What is the syntax to inherit from a class in C#?

 

 

 

Place a colon and then the name of the base class.

Public class derivedclass:baseclass

3.How can I make sure my C# classes will interoperate with other .Net languages?

 

 

 

Make sure your C# code conforms to the Common Language Subset (CLS). To help with this, add the [assembly: CLSCompliant (true)] global attribute to your C# source files. The compiler will emit an error if you use a C# feature which is not CLS-compliant.

　4.Does C# support variable argument on method?The params keyword can be applied on a method parameter that is an array. When the method is invoked, the elements of the array can be supplied as a comma separated list.So, if the method parameter is an object array,

void paramsExample(object arg1, object arg2, params object[] argsRest)

{

foreach (object arg in argsRest)

{

/* .... */

}

}

then the method can be invoked with any number of arguments of any type.paramsExample(1, 0.0f, “a string”, 0.0m, new UserDefinedType());

5.What’s the difference between const and readonly?

Readonly fields are delayed initalized constants. However they have one more thing different is that; When we declare a field as const it is treated as a static field. where as the Readonly fields are treated as normal class variables.const keyword used ,when u want’s value constant at compile time but in case of readonly ,value constant at run timeForm the use point of view if we want a field that can have differnet values between differnet objects of same class, however the value of the field should not change for the life span of object; We should choose the Read Only fields rather than constants.Since the constants have the same value accross all the objects of the same class; they are treated as static.

 

 

6.What is the difference about Switch statement in C#?No fall-throughs allowed. Unlike the C++ switch statement, C# does not support an explicit fall through from one case label to another. If you want, you can use goto a switch-case, or goto default.

case 1:

cost += 25;

break;

case 2:

cost += 25;

goto case 1;

7. What is the difference between a static and an instance constructor?

An instance constructor implements code to initialize the instance of the class. A static constructor implements code to initialize the class itself when it is first loaded.

 

 

8. Assume that a class, Class1, has both instance and static constructors. Given the code below, how many times will the static and instance constructors fire?

Class1 c1 = new Class1();

Class1 c2 = new Class1();

Class1 c3 = new Class1();

By definition, a static constructor is fired only once when the class is loaded. An instance constructor on the other hand is fired each time the class is instantiated. So, in the code given above, the static constructor will fire once and the instance constructor will fire three times.

9. In which cases you use override and new base?

Use the new modifier to explicitly hide a member inherited from a base class. To hide an inherited member, declare it in the derived class using the same name, and modify it with the new modifier.

 

 

10.You have one base class virtual function how will you call the function from derived class?

class a

{

public virtual int m()

{

return 1;

}

}

class b:a

{

public int j()

{

return m();

}

}

　

11. Can we call a base class method without creating instance?

It is possible if it’s a static method.

 

It is possible by inheriting from that class also.It is possible from derived classes using base keyword.

12. What is Method Overriding? How to override a function in C#?

Method overriding is a feature that allows you to invoke functions (that have the same signatures) that belong to different classes in the same hierarchy of inheritance using the base class reference. C# makes use of two keywords: virtual and overrides to accomplish Method overriding. Let’s understand this through small examples.

class BC

{

public void Display()

{

System.Console.WriteLine("BC::Display");

}

}

class DC : BC

{

new public void Display()

{

System.Console.WriteLine("DC::Display");

}

}

class Demo

{

public static void

Main()

{

BC b;

b = new BC();

b.Display();

}

}

Output : BC::Display

13. What is an Abstract Class?

A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.

 

14.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.

15. What is an interface class?

Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.

 

16.Can you inherit multiple interfaces?

Yes. .NET does support multiple interfaces.

 

17.What happens if you inherit multiple interfaces and they have conflicting method names?

It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.

 

18. What’s the difference between an interface and abstract class?

In an interface class, all methods are abstract – there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.

 

19. Why can’t you specify the accessibility modifier for methods inside the interface?

They all must be public, and are therefore public by default.

 

20. Describe the accessibility modifier “protected internal”.

It is available to classes that are within the same assembly and derived from the specified base class.

 

21. If a base class has a number of overloaded constructors and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to specific base constructor?

Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

 

22. What are the different ways a method can be overloaded?

Different parameter data types, different number of parameters, different order of parameters.

 

23. How do you mark a method obsolete?

[Obsolete]

public int Foo()

{…}

//or

[Obsolete(\"This is a message describing why this method is obsolete\")]

public int Foo()

{…}

24. What is a sealed class?

It is a class, which cannot be subclassed. It is a good practice to mark your classes as sealed, if you do not intend them to be subclassed.

 

25. How do you prevent a class from being inherited?

Mark it as sealed.

 

26. Can you inherit from multiple base classes in C#?

No. C# does not support multiple inheritance, so you cannot inherit from more than one base class. You can however, implement multiple interfaces.

 

27. What is an indexer in C#?

The indexers are usually known as smart arrays in C# community. Defining a C# indexer is much like defining properties. We can say that an indexer is a member that enables an object to be indexed in the same way as an array.

class MyClass
{
private string []data = new string[5];
public string this [int index]
{
get
{
return data[index];
}
set
{
data[index] = value;
}
}
}

Where the modifier can be private, public, protected or internal. The return type can be any valid C# types. The ‘this’ is a special keyword in C# to indicate the object of the current class. The formal-argument-list specifies the parameters of the indexer.

28. What is the use of fixed statement?

The fixed statement sets a pointer to a managed variable and “pins” that variable during the execution of statement.

Without fixed, pointers to managed variables would be of little use since garbage collection could relocate the variables unpredictably. (In fact, the C# compiler will not allow you to set a pointer to a managed variable except in a fixed statement.)

Class A

{

public int i;

}

A objA = new A; // A is a .net managed type

fixed(int *pt = &objA.i) // use fixed while using pointers with managed

// variables

{

*pt=45; // in this block use the pointer the way u want

}

29. What is the order of destructors called in a polymorphism hierarchy?

Destructors are called in reverse order of constructors. First destructor of most derived class is called followed by its parent’s destructor and so on till the topmost class in the hierarchy.

You don’t have control over when the first destructor will be called, since it is determined by the garbage collector. Sometime after the object goes out of scope GC calls the destructor, then its parent’s destructor and so on.

When a program terminates definitely all object’s destructors are called.

30. What is a virtual method?

Ans.In C#, virtual keyword can be used to mark a property or method to make it overrideable. Such methods/properties are called virtual methods/properties.By default, methods and properties in C# are non-virtual.

 

31. Is it possible to Override Private Virtual methods?

No, First of all you cannot declare a method as ‘private virtual’.

 

32. Can I call a virtual method from a constructor/destructor?

Yes, but it’s generally not a good idea. The mechanics of object construction in .NET are quite different from C++, and this affects virtual method calls in constructors.C++ constructs objects from base to derived, so when the base constructor is executing the object is effectively a base object, and virtual method calls are routed to the base class implementation. By contrast, in .NET the derived constructor is executed first, which means the object is always a derived object and virtual method calls are always routed to the derived implementation. (Note that the C# compiler inserts a call to the base class constructor at the start of the derived constructor, thus preserving standard OO semantics by creating the illusion that the base constructor is executed first.)The same issue arises when calling virtual methods from C# destructors. A virtual method call in a base destructor will be routed to the derived implementation.

 

33. How do I declare a pure virtual function in C#?

Use the abstract modifier on the method. The class must also be marked as abstract (naturally). Note that abstract methods cannot have an implementation (unlike pure virtual C++ methods).

 

34. Are all methods virtual in C#?

No. Like C++, methods are non-virtual by default, but can be marked as virtual.

 

35. What is the difference between shadow and override?

When you define a class that inherits from a base class, you sometimes want to redefine one or more of the base class elements in the derived class. Shadowing and overriding are both available for this purpose.

Comparison

It is easy to confuse shadowing with overriding. Both are used when a derived class inherits from a base class, and both redefine one declared element with another. But there are significant differences between the two.The following table compares shadowing with overriding.

Point of comparison Shadowing OverridingPurposeShadowing

Protecting against a subsequent base class modification that introduces a member you have already defined in your derived classAchieving polymorphism by defining a different implementation of a procedure or property with the same calling sequence1Redefined elementShadowing

Any declared element typeOnly a procedure (Function, Sub, or Operator) or propertyRedefining elementShadowing

Any declared element typeOnly a procedure or property with the identical calling sequence1Access level of redefining elementShadowing

Any access levelCannot change access level of overridden elementReadability and writability of redefining elementShadowing

Any combinationCannot change readability or writability of overridden propertyControl over redefiningShadowing

Base class element cannot enforce or prohibit shadowingBase class element can specify MustOverride, NotOverridable, or OverridableKeyword usageShadowing

Shadows recommended in derived class; Shadows assumed if neither Shadows nor Overrides specified2Overridable or MustOverride required in base class; Overrides required in derived classInheritance of redefining element by classes deriving from your derived classShadowing

Shadowing element inherited by further derived classes; shadowed element still hidden3Overriding element inherited by further derived classes; overridden element still overridden

1 The calling sequence consists of the element type (Function, Sub, Operator, or Property), name, parameter list, and return type. You cannot override a procedure with a property, or the other way around. You cannot override one kind of procedure (Function, Sub, or Operator) with another kind.

2 If you do not specify either Shadows or Overrides, the compiler issues a warning message to help you be sure which kind of redefinition you want to use. If you ignore the warning, the shadowing mechanism is used.

3 If the shadowing element is inaccessible in a further derived class, shadowing is not inherited. For example, if you declare the shadowing element as Private, a class deriving from your derived class inherits the original element instead of the shadowing element.

36. Should I make my destructor virtual?

C# destructor is really just an override of the System.Object Finalize method, and so is virtual by definition

 

37. Are C# destructors the same as C++ destructors?

No. They look the same but they are very different. The C# destructor syntax (with the familiar ~ character) is just syntactic sugar for an override of the System.Object Finalize method. This Finalize method is called by the garbage collector when it determines that an object is no longer referenced, before it frees the memory associated with the object. So far this sounds like a C++ destructor. The difference is that the garbage collector makes no guarantees about when this procedure happens. Indeed, the algorithm employed by the CLR garbage collector means that it may be a long time after the application has finished with the object. This lack of certainty is often termed ‘non-deterministic finalization’, and it means that C# destructors are not suitable for releasing scarce resources such as database connections, file handles etc.To achieve deterministic destruction, a class must offer a method to be used for the purpose. The standard approach is for the class to implement the IDisposable interface. The user of the object must call the Dispose() method when it has finished with the object. C# offers the ‘using’ construct to make this easier.

 

38. Are C# constructors the same as C++ constructors?

Very similar, but there are some significant differences. First, C# supports constructor chaining. This means one constructor can call another:

class Person

{

public Person( string name, int age ) { … }

public Person( string name ) : this( name, 0 ) {}

public Person() : this( "", 0 ) {}

}

Another difference is that virtual method calls within a constructor are routed to the most derived implementationError handling is also somewhat different. If an exception occurs during construction of a C# object, the destuctor (finalizer) will still be called. This is unlike C++ where the destructor is not called if construction is not completed.Finally, C# has static constructors. The static constructor for a class runs before the first instance of the class is created.Also note that (like C++) some C# developers prefer the factory method pattern over constructors.

39. Can you declare a C++ type destructor in C# like ~MyClass()?

Yes, but what’s the point, since it will call Finalize(), and Finalize() has no guarantees when the memory will be cleaned up, plus, it introduces additional load on the garbage collector. The only time the finalizer should be implemented, is when you’re dealing with unmanaged code.

 

40. What are the fundamental differences between value types and reference types?

C# divides types into two categories – value types and reference types. Most of the intrinsic types (e.g. int, char) are value types. Structs are also value types. Reference types include classes, arrays and strings. The basic idea is straightforward – an instance of a value type represents the actual data, whereas an instance of a reference type represents a pointer or reference to the data.The most confusing aspect of this for C++ developers is that C# has predetermined which types are represented as values, and which are represented as references. A C++ developer expects to take responsibility for this decision.For example, in C++ we can do this:

int x1 = 3; // x1 is a value on the stack

int *x2 = new int(3) // x2 is a pointer to a value on the heapbut in C# there is no control:int x1 = 3; // x1 is a value on the stack

int x2 = new int();

x2 = 3; // x2 is also a value on the stack!

41.How do you handle errors in VB.NET and C#?

C# and VB.NET use structured error handling (unlike VB6 and earlier versions where error handling was implemented using Goto statement). Error handling in both VB.NET and C# is implemented using Try..Catch..Finally construct (C# uses lower case construct – try…catch…finally).

 

42. What is the purpose of the finally block?

The code in finally block is guaranteed to run, irrespective of whether an error occurs or not. Critical portions of code, for example release of file handles or database connections, should be placed in the finally block.

 

43. Can I use exceptions in C#?

Yes, in fact exceptions are the recommended error-handling mechanism in C# (and in .NET in general). Most of the .NET framework classes use exceptions to signal errors.

 

44. Why is it a bad idea to throw your own exceptions?

Well, if at that point you know that an error has occurred, then why not write the proper code to handle that error instead of passing a new Exception object to the catch block? Throwing your own exceptions signifies some design flaws in the project.

 

45. What’s the C# syntax to catch any possible exception?

A catch block that catches the exception of type System. Exception. You can also omit the parameter data type in this case and just write catch {}

 

46. How to declare a two-dimensional array in C#?

Syntax for Two Dimensional Array in C Sharp is int[,] ArrayName;

 

47.How can you sort the elements of the array in descending order?

Using Array.Sort() and Array.Reverse() methods.

int[] arr = new int[3];

arr[0] = 4;

arr[1] = 1;

arr[2] = 5;

Array.Sort(arr);

Array.Reverse(arr);

48. What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?

The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element’s object, resulting in a different, yet identacle object.

　

49. Structs are largely redundant in C++.Why does C# have them?

In C++, a struct and a class are pretty much the same thing. The only difference is the default visibility level (public for structs, private for classes). However, in C# structs and classes are very different. In C#, structs are value types (instances stored directly on the stack, or inline within heap-based objects), whereas classes are reference types (instances stored on the heap, accessed indirectly via a reference). Also structs cannot inherit from structs or classes, though they can implement interfaces. Structs cannot have destructors. A C# struct is much more like a C struct than a C++ struct.

 

50. How does one compare strings in C#?

In the past, you had to call .ToString() on the strings when using the == or != operators to compare the strings’ values. That will still work, but the C# compiler now automatically compares the values instead of the references when the == or != operators are used on string types. If you actually do want to compare references, it can be done as follows: if ((object) str1 == (object) str2) { … } Here’s an example showing how string compares work:

using System;

public class StringTest

{

public static void Main(string[] args)

{

Object nullObj = null; Object realObj = new StringTest();

int i = 10;

Console.WriteLine(\"Null Object is [\" + nullObj + \"]\n\"

+ \"Real Object is [\" + realObj + \"]\n\"

+ \"i is [\" + i + \"]\n\");

// Show string equality operators

string str1 = \"foo\";

string str2 = \"bar\";

string str3 = \"bar\";

Console.WriteLine(\"{0} == {1} ? {2}\", str1, str2, str1 == str2 );

Console.WriteLine(\"{0} == {1} ? {2}\", str2, str3, str2 == str3 );

}

}Output:Null Object is []

Real Object is [StringTest]

i is [10]

foo == bar ? False

bar == bar ? True

51. Where we can use DLL made in C#.Net?

Supporting .Net, because DLL made in C#.Net semi compiled version. It’s not a com object. It is used only in .Net Framework As it is to be compiled at runtime to byte code.

 

52. If A.equals(B) is true then A.getHashcode & B.gethashcode must always return same hash code.

The answer is False because it is given that A.equals(B) returns true i.e. objects are equal and now its hashCode is asked which is always independent of the fact that whether objects are equal or not. So, GetHashCode for both of the objects returns different value.

 

53.Is it possible to debug the classes written in other .Net languages in a C# project?

It is definitely possible to debug other .Net languages code in a C# project. As everyone knows .net can combine code written in several .net languages into one single assembly. Same is true with debugging.

 

54. Does C# has its own class library?

Not exactly. The .NET Framework has a comprehensive class library, which C# can make use of. C# does not have its own class library.

 

55. IS it possible to have different access modifiers on the get/set methods of a property?

No. The access modifier on a property applies to both its get and set accessors. What you need to do if you want them to be different is make the property read-only (by only providing a get accessor) and create a private/internal set method that is separate from the property.

 

56. Is it possible to restrict the scope of a field/method of a class to the classes in the same namespace?

There is no way to restrict to a namespace. Namespaces are never units of protection. But if you’re using assemblies, you can use the ‘internal’ access modifier to restrict access to only within the assembly.

 

57. Is there an equivalent of exit() or quiting a C#.NET application?

Yes, you can use System.Environment.Exit(int exitCode) to exit the application or Application.Exit() if it’s a Windows Forms app.

 

58. What optimization does the C# compiler perform when you use the /optimize+compiler option?

The following is a response from a developer on the C# compiler team: We get rid of unused locals (i.e., locals that are never read, even if assigned). We get rid of unreachable code. We get rid of try-catch with an empty try. We get rid of try-finally with an empty try. We get rid of try-finally with an empty finally. We optimize branches over branches: gotoif A, lab1 goto lab2: lab1: turns into: gotoif !A, lab2 lab1: We optimize branches to ret, branches to next instruction, and branches to branches.

 

59. Does C# support multiple inheritance?

No, use interfaces instead.

 

60. IS goto statement supported in C#?How about Java?

Gotos are supported in C# to the fullest. In Java goto is a reserved keyword that provides absolutely no functionality.

 

61. What happens when you encounter a continue statement inside for loop?

The code for the rest of the loop is ignored, the control is transferred back to the beginning of the loop.

　62. Write one code example for compile time binding and one for run time binding?what is early/late binding? An object is early bound when it is assigned to a variable declared to be of a specific object type . Early bound objects allow the compiler to allocate memory and perform other optimizations before an application executes.

‘ Create a variable to hold a new object.

Dim FS As FileStream

‘ Assign a new object to the variable.

FS = New FileStream("C:\tmp.txt", FileMode.Open)

By contrast, an object is late bound when it is assigned to a variable declared to be of type Object. Objects of this type can hold references to any object, but lack many of the advantages of early-bound objects.

Dim xlApp As Object

xlApp = CreateObject("Excel.Application")

 

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