C#

Question 0

What are the 2 floating point types in C#?

Answer 0

Float - 32 bits. Literal suffixes with f, e.g. 3.5f
Double - 64 bits. By defaultreal numeric literal treated as double. To force integer to be treated as double literal suffix with d.

Question 1

What is a floating point number?

Answer 1

A floating point number is a method of representing an approximation of a real number that can support a wide range of values. It is represented by a fixed number of digits and a scaled using an exponent. They can be stored efficiently in binary allowing great range but less precision.

Question 2

How do you declare an array of 10 integers named numbers?

Answer 2

int[] numbers = new int[10];

Question 3

What is the property of strings that mean that they cannot be modified?

Answer 3

Immutable. Strings objects cannot be modified after they have been created. Concatenation strings results in a new string object being created.

Question 4

What abstract base type of all arrays types?

Answer 4

Array. Arrays are objects in C#, unlike in C and C++ where they are just addressable contiguous areas of memory. It has several useful properties and methods defined on it.

Question 5

What is a delegate?

Answer 5

A delegate is a type-safe function pointer. The delegate type defines a method signature. Delegates do that type can reference methods with a matching signature.

Question 6

Define a delegate Add, that takes in two integer values as parameters

Answer 6

private delegate int add(int a, int b);

Question 7

How can delegates be used?

Answer 7

Callback methods
Event handlers
Anonymous methods

Question 8

What are events?

Answer 8

Events enable a class or object to notify other classes or objects when something of interest happens. The event pattern is closely related to the Observer pattern.

Question 9

Name all 4 access modifiers

Answer 9

private - restricted to containing type
public - access not restricted
protected - restricted to containing type or types that derive from it
internal - restricted to current assembly

Question 10

What is a constant?

Answer 10

Constants are immutable types whose value is known at compile-time. Only the C# built-in types can be declared as const.

Question 11

Declare a constant for the value of pie

Answer 11

const decimal pie = 3.14;

Question 12

What does the static keyword do and how can it be used?

Answer 12

The static keyword can be used to declare a member static. It can be used on classes, fields, methods, properties, operators, events, and constructors.

Question 13

What is a static class and what are its properties?

Answer 13

Static classes are classes defined with the static keyword. Static classes cannot be instantiated. All of its members must be static also.

Question 14

What is a popular example of a static class in the .NET framework?

Answer 14

The Math class. It provides utility methods for performing mathematical operations.

Question 15

What design pattern is commonly used to ensure that only a single instance of a class can be created?

Answer 15

Singleton. There are a few ways to implement this, each of which has trade-offs. It involves making the constructor private so that instantiation can be controlled via a static method.

Question 16

What is a field?

Answer 16

A field is variable of any type that is declared directly in a class or struct. They are members of their containing type.

Question 17

How do you prevent a class from being inherited?

Answer 17

The sealed keyword prohibits a class from being inherited

Question 18

What is the execution entry point for a C# console application?

Answer 18

The Main method

Question 19

How do you initiate a string without escaping each backslash?

Answer 19

You put an @ sign in front of the double-quoted string.

String ex = @”This has a carriage return\r\n”;

Question 20

What is the difference between a struct and a class?

Answer 20

Structs cannot be inherited. Structs are passed by value and not by reference. Structs are stored on the stack not the heap. The result is better performance with Structs.

Question 21

What are namespaces, and how they are used?

Answer 21

Namespaces are used to organize classes within the .NET Framework. They dictate the logical structure of the code. They are analogous to Java packages, with the key difference being Java packages define the physical layout of source files (directory structure) while .NET namespaces do not. However, many developers follow this approach and organize their C# source files in directories that correlate with namespaces. The .NET Framework has namespaces defined for its many classes, such as System.Xml–these are utilized via the using statement. Namespaces are assigned to classes via the namespace keyword.

Question 22

What is garbage collection?

Answer 22

Garbage collection is a feature of most modern object oriented languages. It is a feature of the runtime that disposes of objects that are no longer referenced

Question 23

What does type safe mean? Why is C# type safe where JavaScript for example isn’t?

Answer 23

Type safety relates to how a language enforces constraints on the kinds of operations that can be performed on a type such that only valid operations can be performed. For example, the method quack() can’t be called on a swan object that does not implement this method. JavaScript does not enforce this and would not generate an error until runtime.

Question 24

What are exceptions?

Answer 24

A feature of many modern object oriented languages it is a mechanism for handling errors. Classes can throw exceptions (objects) that stop the normal execution of code and instead get passed up the calling hierarchy until they are either handled by a try catch finally block, or reach the top of the calling hierarchy.

Question 25

How do you pass an object by reference?

Answer 25

Use either ref or out keyword.

Question 26

What is the difference between ref and out?

Answer 26

Both keywords indicate passing a parameter by reference. Ref is to be used for passing value in and out, whereas out is just for method to set value.

Question 27

What is the affect of modifying a value type within a method where it has been passed by reference?

Answer 27

Within the method the parameter is the sae memory location as outside so any modifications to it are reflected outside too.

Question 28

What happens when within a method you assign a new object to a reference parameter where it is a reference type?

Answer 28

As the parameter is a effectively a pointer to a pointer, assigning a new object to it causes the variable outside of the method to point to the new object.

Question 29

Which access modifiers should fields generally be given?

Answer 29

Private or protected. Access to fields should be controlled via properties, methods, and indexers.

Question 30

Declare a field called size of type int with private accessibility

Answer 30

private int size;

Question 31

What is the order in which fields and constructors are initialized/called?

Answer 31

Fields are initialized first. Constructors are then called beginning with the base class.

Question 32

What is a default parameter? How are they useful?

Answer 32

Default parameters are specified with the optional keyword. They can be used in some cases instead of overloading a method. We can omit optional parameter from the method call

Question 33

What keyword must be present in the definition of a method in the base class in order for a derived class to override the method?

Answer 33

The virtual keyword is specified in the base class. The derived method is specified with the overrides keyword.

Question 34

What is this code of the ColorCoOrds class which derives from CoOrds class and what does it do? ``` public ColorCoOrds(int x, int y) : base (x, y) { color = System.Drawing.Color.Red; } ```

Answer 34

The ColourCoOrds class calls the constructor on its base class which accepts two integer parameters before it's own constructor code gets executed.

Question 35

What are optional/default parameters?

Answer 35

Optional parameters are parameters that can be omitted from method calls. In the method declaration optional parameters are declared by assigning a default value in method signature. Default values must be able to be determined at run-time, e.g. constants, null values.

Question 36

Where must optional/default parameters be specified in a method declaration?

Answer 36

Optional parameters must be specified after all required parameters

Question 37

How does intellisense indicate an optional parameter?

Answer 37

Intellisense displays optional parameters in [square brackets] along with their default value.

Question 38

Declare a void method countdown which takes in an optional int parameter with a default of 10

Answer 38

public void Countdown(int timer = 10)

Question 39

What is a named argument?

Answer 39

A named argument is where the client has specified the name of the parameter in the method call.

Question 40

Use named arguments to call this method: public decimal CalculateBMI(int height, int weight) {...}

Answer 40

CalculateBMI(weight: 76, height: 180);

Question 41

Why are named and optional/default arguments useful together?

Answer 41

In methods where there are many parameters the use of optional/default and named arguments allow method calls without specifying values for every parameter. Especially useful with COM interfaces such as Microsoft Office

Question 42

What are the disadvantages of optional parameters?

Answer 42

Optional parameters are implemented by the compiler by inserting the default value into the calling code when no value is supplied. This means that if we make any changes to our parameters then our client code needs to be recompiled to reflect the updates values.

Question 43

What is the alternative to using overloaded methods?

Answer 43

Using optional parameters to provide default values for parameters that are not supplied by the calling code.

Question 44

What is a sealed class?

Answer 44

A sealed class is a class that cannot be inherited. It is specified using the sealed keyword. System.String is an example of sealed class. It is often done for performance or security reasons.

Question 45

What does the break statement do inside a loop?

Answer 45

It breaks out of the loop and continues at the next line following the loop

Question 46

What are the 4 iteration statements available in C#?

Answer 46

while do...while for foreach

Question 47

What does a continue statement do inside a loop?

Answer 47

It stops executing the code in the current iteration and starts a new iteration.

Question 48

What are the advantages of generic collections over non-generic collections?

Answer 48

Generic collections are type-safe. They also provide better performance for value types as they do not require boxing and unboxing values.

Question 49

What are Generic Constraints?

Answer 49

Generic Constraints are constraints that allow you to specify constraints on the types that the generic class/struct can be used with. For example, a generic class be declared such that it can be created with only types that implement an interface IAnimal.

Question 50

Derive a generic class MagicHat from Queue

Answer 50

``` class MagicHat : Queue { ... } ```

Question 51

Describe the difference between a Thread and a Process?

Answer 51

Each process provides the resources needed to execute a program. Each process is started with a single thread, often called the primary thread, but can create additional threads from any of its threads. A thread is the entity within a process that can be scheduled for execution. All threads of a process share its virtual address space and system resources.

Question 52

What is a Windows Service and how does its lifecycle differ from a "standard" EXE?

Answer 52

A windows service always runs when a computer starts up (if configured). A standard exe only runs when a user logs in and stops when the user logs out.

Question 53

What is the maximum amount of memory any single process on Windows can address? Is this different than the maximum virtual memory for the system? How would this affect a system design?

Answer 53

Limits depending on OS and processor architecture. 32-bit OS and processor: 2GB. 64-bit OS and processor: 8TB.

Question 54

What is the difference between an EXE and a DLL?

Answer 54

An EXE has an entry point to start execution, a DLL doesn't.

Question 55

What is strong-typing versus weak-typing? Which is preferred? Why?

Answer 55

These terms don't have widely agreed definitions.

Question 56

Juxtapose the use of override with new. What is shadowing?

Answer 56

Override is used to replace the definition of a method with another in a derived class. The derived class implementation will be used no matter what. New on the other hand creates a new method which shadows the method in the base class. This new method will be called when an object of the derived class is called using it's type, but if called via the base class type will use the base class version of the method.

Question 57

Explain the use of virtual, sealed, override, and abstract.

Answer 57

``` virtual - method can be overriden sealed - class cannot be derived from override - override a virtual method in a derived class abstract - used on method or class. abstract class cannot be instantiated. abstract method within abstract class provides default implementation. abstract method within non-abstract class can only provide signature, no implementation. ```

Question 59

Explain the importance and use of each component of this string: Foo.Bar, Version=2.0.205.0, Culture=neutral, PublicKeyToken=593777ae2d274679d

Answer 59

This is an example of an assembly full-qualified name. The assembly name, version, culture and public key token. Each assembly is signed with a private key, the public key is used to decrypt and ensures that assembly hasn't been tampered with.

Question 59

By what mechanism does NUnit know what methods to test?

Answer 59

Using custom attributes

Question 60

What benefit do you get from using a Primary Interop Assembly (PIA)?

Answer 60

A Primary Interop Assembly is an assembly that contains metadata about a COM library which it wraps.

Question 61

What is the difference between: catch(Exception e){throw e;} and catch(Exception e){throw;}

Answer 61

When you use throw e, the original stack trace is lost. When you use throw; the original stack trace is preserved.

Question 62

Explain what’s happening in the first constructor: public class c{ public c(string a) : this() {;}; public c() {;} } How is this construct useful?

Answer 62

The first constructor is calling the parameter-less constructor. It is useful for providing multiple constructors with different parameters.

Question 63

What is the difference between typeof(foo) and myFoo.GetType()?

Answer 63

typeof(foo) - returns System.Type. typeof takes a type name. GetType - returns System.Type of current instance. Is - checks whether type is in same inheritance tree

Question 64

What is this? Can this be used within a static method?

Answer 64

this is a keyword that refers to the current instance of the object.

Question 65

How many processes can listen on a single TCP/IP port?

Answer 65

One.

Question 66

What is the GAC? What problem does it solve?

Answer 66

Global Assembly Cache.

Question 67

Describe the difference between Interface-oriented, Object-oriented and Aspect-oriented programming.

Answer 67

?

Question 68

Describe what an Interface is and how it’s different from a Class.

Answer 68

?

Question 69

What is Reflection?

Answer 69

?

Question 70

Can DateTimes be null?

Answer 70

DateTimes are structs so are value types and therefore cannot be null. DateTimes that have not been initialized will have the minimum value. Nullable types allow us to assign null.

Question 71

How does the generational garbage collector in the .NET CLR manage object lifetime? What is non-deterministic finalization?

Answer 71

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Question 72

What is the difference between Finalize() and Dispose()?

Answer 72

?

Question 74

How is the using() pattern useful? What is IDisposable? How does it support deterministic finalization?

Answer 74

?

Question 75

What can an interface define signatures for?

Answer 75

methods properties indexers events

Question 76

What is polymorphism?

Answer 76

It means being able to use different forms of a type without regard to implementation details.

Question 77

What is the Liskov Substitution Principle?

Answer 77

Simply put, this means that an instance of a class should function as an instance of its super class—it should be possible to use it in methods instead of the super class without any exceptions or side effects. It also reflects how a class hierarchy should ideally operate, in that a base class should capture the behaviour and properties that all its subclasses have in common.

Question 78

We have a Workshop class that has 2 methods Repair Coachwork and Repair Wheels that take in a HorseDrawnVehicle class. We have Carriage, Wagon, and Sleigh that all derive from HorseDrawnVehicle however Sleigh doesn't have wheels but HorseDrawnVehicle does. What principle does this break and why?

Answer 78

Liskov Subsitution Principle. Sleigh does not fit into the model as it does not have wheels and therefore has undesirable side effects. To fix, modify class hierarchy to include HorseDrawnWheeledVehicle which the others derive from. Repair Wheels method take in this type as parameter.

Question 79

What should we favour, inheritance or delegation?

Answer 79

Delegation. Often more flexible.

Question 80

How can events allow us to cancel actions?

Answer 80

We can have a class that raises an event prior to the main event which has a class that derives from CancelEventArgs. Based on info in this class the client can cancel the event by setting cancel parameter to true.

Question 81

Where is this pattern commonly used and why? Action temp = Foo; if (temp != null) temp();

Answer 81

When we are raising events. It does 2 things; ensures it is not a null delegate and ensures that it is thread safe.

Question 82

How does this pattern avoid a race condition? Action temp = Foo; if (temp != null) temp();

Answer 82

Delegates are immutable reference types and therefore assigning to a temp variables creates a copy of the delegate objects. If we didn't take a copy another thread could set to null after checking it is non-null.

Question 83

What are the 2 basic ways to handle an event in .NET?

Answer 83

Attach an event handler through use of delegation | Override a virtual method of the base class

Question 84

What is "Move Accumulation to Collecting Parameter"?

Answer 84

This is a refactoring pattern where there is a single bulky method that accumulates information to a local variable. This refactoring pattern refactors to instead use several private methods that gather discrete pieces of info which we call passing in our variable as a Collecting Parameter to collect our data.

Question 85

What is a constructor initializer? What is the syntax?

Answer 85

It is an optional clause defined on a constructor. It is used to call either another constructor within the same class using : this(...) or a constructor in the base class using : base(...). It is executed prior to the constructor and if not specified an implicit initializer of the form base() is provided.

Question 86

What is Razor?

Answer 86

Razor is a view-engine. A view engine is a pluggable template syntax engine.

Question 87

What does the acronym SOLID represent?

Answer 87

5 Main OO Design Principles: ``` S - Single Responsibility Principle O - Open/Closed Principle L - Liskov Substitution Principle I - Interface Segregation Principle D - Dependency Inversion Principle ```

Question 88

What does the principle "Encapsulate what varies" mean?

Answer 88

Those areas where there is likely to be change we should spend time in designing a solution that encapsulates those areas such that they are the only area of code that require change when it does happen

Question 89

Why should we favour composition over inheritance?

Answer 89

Composition is more flexible....

Question 90

Why should we program to interfaces rather than implementations?

Answer 90

Interfaces define behaviour rather than implement it. If we define the behaviour correctly we can swap the implementations in and out without having to change other areas of code.

Question 91

What does "don't call us, we'll call you" mean to you as a design principle?

Answer 91

...

Question 92

Explain Abstraction

Answer 92

...

Question 93

Explain Encapsulation

Answer 93

...

Question 94

Explain Polymorphism

Answer 94

...

Question 95

Explain Inheritance

Answer 95

...

Question 96

Explain the Open-Closed Principle

Answer 96

Classes should be open for extension, but closed for modification. Classes should be easily extendable to incorporate new behaviour without modifying existing code. The Observer pattern achieves this by allowing us to add new Observers at any time without having to change the Subject.

Question 97

What is the Dependency Inversion Principle?

Answer 97

Depend upon abstractions. Do not depend upon concrete classes. High-level components should not rely on low-level components, they should both depend on abstractions. Where Pizza is an abstract class, concrete classes that derive from it depend on the abstraction as does PizzaStore. PizzaStore should not create the concrete classes otherwise this would violate the principle. Factory pattern can fix. Similar to "Program to an interface..." but relates specifically to abstraction both ways.

Question 98

What guidelines can help you avoid violating the Dependency Inversion Principle?

Answer 98

``` No variable should hold a reference to a concrete class. No class should derive from a concrete class No method should override an implemented method of any of its base classes. (It's completely ok to break these guidelines as every single program does but prompt to think about design) ```

Question 99

What is the Simple Factory idiom?

Answer 99

It's not actually the factory pattern but is commonly used.

Question 100

What is the Factory Method Pattern?

Answer 100

Factory Method lets subclasses decide what objects to create. With the abstract PizzaStore class, it is the Creator where NYPizzaStore and other stores implement their own version of the createPizza() method. They create Pizza objects, the Product. Pizza is an abstract class. Class hierarchies are parallel.

Question 101

What is common to all factory patterns?

Answer 101

They encapsulate object creation.

Question 102

What is the Abstract Factory Pattern?

Answer 102

...

Question 103

What is an Extension Method?

Answer 103

An extension method is a method defined outside of a given type but can be called as if it was part of the type. They are defined as static methods but can be used only on instances of the type they are defined for.

Question 104

Define an extension method WordCount for String class in a static class MagicString.

Answer 104

``` public static class MagicString { public static int WordCount(this String str) { // return word count } } ```

Question 105

Can extension methods be used to add methods to sealed classes?

Answer 105

Yes.

Question 106

How are Extension Methods implemented by the compiler?

Answer 106

The compiler passes in the object that the extension method is being called on to the static method that is defined as the extension method.