Interview - 2

Front 1

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

Back 1

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.

Front 2

What’s the difference between const and readonly?

Back 2

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.

Front 3

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

Back 3

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.

Front 4

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();

Back 4

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.

Front 5

In which cases you use override and new base?

Back 5

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.

Front 6

Can we call a base class method without creating instance?

Back 6

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.

Front 7

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

Back 7

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.

Front 8

What is an Abstract Class?

Back 8

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.

Front 9

When do you absolutely have to declare a class as abstract?

Back 9

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.

Front 10

What is an interface class?

Back 10

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.

Front 11

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

Back 11

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.

Front 12

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

Back 12

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

Front 13

Describe the accessibility modifier “protected internal”.

Back 13

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

Front 14

What is an indexer in C#?

Back 14

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.

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

</strong>

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.

Front 15

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

Back 15

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.

Front 16

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

Back 16

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.

Front 17

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

Back 17

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).

Front 18

What is the difference between shadow and override?

Back 18

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.

Front 19

Should I make my destructor virtual?

Back 19

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

Front 20

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

Back 20

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.

Front 21

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

Back 21

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

<strong>
class Person

{

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

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

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

}

</strong>

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.

Front 22

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

Back 22

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.

Front 23

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

Back 23

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:

<strong>
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!

</strong>

Front 24

What is the purpose of the finally block?

Back 24

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.

Front 25

Can I use exceptions in C#?

Back 25

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.

Front 26

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

Back 26

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.

Front 27

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

Back 27

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 {}

Front 28

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

Back 28

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

Front 29

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

Back 29

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);

Front 30

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

Back 30

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.

Front 31

How does one compare strings in C#?

Back 31

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:

<strong>
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

</strong>

Front 32

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

Back 32

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.

Front 33

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

Back 33

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.

Front 34

Does C# has its own class library?

Back 34

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