C# and OOP

Question 1

Printing a line

Answer 1

Console.WriteLine(iValue);

Question 1

Converting number to string

Answer 1

string strValue = floatValue.ToString();

Question 2

Casting float to int

Answer 2

int iValue = (int)fValue;
(rounds down)

Question 3

Converting string to number

Answer 3

string strValue = “13.8”;
float fValue = float.Parse(strValue);

Question 4

Getting a substring of a string

Answer 4

string.Substring(a, b) gives you the b characters which come after character a

“vge1974”.Substring(2, 4) = “e197”

Question 5

Conditional statement

Answer 5

if (condition)
{
code
}
else
{
more code
}

Question 6

Comparison operators

Answer 6

==, !=, <, >, <=, >=

Question 7

Logical operators

Answer 7

&&, ||, !
“true”, “false”

Question 8

Expressing x^y

Answer 8

Math.Pow(x, y)

Question 9

While loop

Answer 9

while (condition)
{
code
}

(can control flow using “break” and “continue”)

Question 10

Do while loop

Answer 10

do
{
code
}
while (condition)

Question 11

For loop

Answer 11

for (setup; condition; increment)
{
code
}

Question 12

Foreach loop

Answer 12

foreach (<type> currentItem in listOfItems)
{
code
}</type>

Question 13

What is OOP?

Answer 13

A programming framework that facilitates the reuse and extensibility of code. Bits of source code with relevant purposes are grouped together as classes, managed as a hierarchy with inheritance. Objects are instances of a class, which is like a blueprint.

Question 14

Advantages of OOP

Answer 14

With OOP, source code becomes easier to maintain and follow
Code can be reused through inheritance, improving efficiency and productivity
Security through encapsulation

Question 15

Drawing classes in UML

Answer 15

Attributes
———–
Methods

The constructor is always there but not listed

Question 16

Drawing the main program itself in UML

Answer 16

Attributes
———–
Methods

Question 17

Creating a new object using the constructor

Answer 17

Cow cowName = new Cow(“Moo”, 8);

Question 18

What is method overloading?

Answer 18

When a method has different declarations for different types of input

Question 19

What are static functions and variables?

Answer 19

Ones which belong to the class, rather than the instances of the class (the objects). They are usually used to denote functions/variables that are the same across all objects of that class. They can be called without creating an object of such a class. They have to be changed from the class, not the object.
They are underlined in UML.

Question 20

Example of using a static variable

Answer 20

Student s1 = new Student();
s1.studentName = “Henry”;

Console.WriteLine(s1.studentName); // instance variable
Console.WriteLine(Student.universityName); // static variable

Question 21

What is encapsulation and why is it useful?

Answer 21

This is where a class keeps all relevant information, but only exposes necessary information to other classes.

Issues avoided by encapsulation: human errors, intentional attacks, classes which are difficult to follow

Question 22

public vs private vs protected

Answer 22

public: accessible everywhere
private: accessible only within the same class
protected: accessible only within the same class AND child classes

Question 23

Old encapsulation method using setters and getters

Answer 23

public class Cow
{
private float _fWeight;
private string _strName;

public void SetFWeight(float fWeightIn)
{
    if (fWeightIn > 0.0f)
        _fWeight = fWeightIn;
}

public float GetFWeight()
{
    return _fWeight;
} }

Question 24

New encapsulation method using get/set

Answer 24

public class Cow
{
private float _fWeight;
private string _strName;

public float WeightPublic
{
set
{
if (value > 0.0f)
_fWeight = value;
}
get
{
return _fWeight;
}
}

Question 25

The “this” keyword

Answer 25

Refers to the current instance of the class

Question 26

What is inheritance and why is it useful?

Answer 26

Inheritance is where a child class can be derived from a parent class to retain and update the variables and methods of the parent class.
The UML symbol is an arrow with a white head.

Question 27

How to show inheritance in C#?

Answer 27

Syntax:
“public class Cow : Animal”

Question 28

Using “base”

Answer 28

Allows a child class to add code to its parent’s constructor

public Cow(string strName, float fWeight, bool bMakeMilk) : base(strName, fWeight)

Question 29

UML association arrow

Answer 29

A class creates an object of another class

Question 30

UML composition arrow

Answer 30

Black diamond
Type of association, where a class A creates an object B of another class, and B is destroyed when A is destroyed. A is called the container and B is called the content.

Question 31

UML aggregation arrow

Answer 31

White diamond
Opposite of composition, where the content is not destroyed when the container is destroyed.

Question 32

Using lists in C#

Answer 32

List<type> ListName = new List<type>();</type></type>

Question 33

List operations in C#

Answer 33

lstData.Count
lstData.Add(value)
lstData.Remove(value) // removes first occurrence
lstData.RemoveAt(k) // Removes the item at this position

Question 34

What is abstraction?

Answer 34

Where a parent class designs the inputs and outputs of the methods, leaving the child class to implement the actual contents of the methods

Question 35

Abstract classes vs abstract methods

Answer 35

Abstract classes are ones which cannot be used to create objects, only to derive new classes using inheritance.

Abstract methods are unfinished methods which only specify input/output values, and are expected to be provided by the derived class. Abstract methods can only be used in abstract classes, but an abstract class does not need to contain abstract methods.

Question 36

What is an interface?

Answer 36

An EU version of an abstract class, which only has public abstract methods. A child class can implement as many interfaces as needed, as opposed to only being able to inherit one abstract class

Declaration: public interface IDrawable

Question 37

What is polymorphism?

Answer 37

Polymorphic objects can be treated as different classes, meaning they can be cast to their parent’s class (e.g. a Cow can be cast to an animal)

Polymorphic methods can be overridden or hidden: the system decides which method to call with reference to the class of the object.