C# Generics

Question 1

What is ildasm?

Answer 1

• ships with .net, can show intermediate language code of exes or DLLs

Question 2

Why generics?

Answer 2

• allow code reuse with type safety

- defer type specification to client

Question 3

What is a List?

Answer 3

• generic Collection
• basically an array that manages its own size
• List
• can pass in starting size in constructor
• will double in size as list grows … 0,4,8,16 … as items are added

Question 4

What is Queue?

Answer 4

• similar to list, generic collection
• no add method, use Enqueue
• use Dequeue - returns first in line and removes it from the queue
• FIFO data structure
• use Peek() to look at next item; doesn’t remove from Queue
• can search Queue with .Contains

Question 5

What is Stack?

Answer 5

• LIFO data structure
• stack.Push() - to add things to stack
• stack.Pop() - most recently added object, then removes from Stack
• stack.Peek() - get last item added but doesn’t remove from Stack

Question 6

What is a Hashset?

Answer 6

• similar to List
• order of items is not guaranteed
• set.Add() - returns true/false
• won’t accept duplicates
• you can use special operations on Hashsets
• set1.IntersectWith(set2)
• set1.UnionWith(set2)
• set1.SymmetricExceptWith(set2) - returns items in set1 and set2 but NOT in both
• Hashset by default doesn’t know how to compare complex objects; you have to tell it how to compare

Question 7

What is LinkedList?

Answer 7

• very concerned with order of items
• AddFirst, AddLast, AddBefore, AddAfter
• RemoveFirst, RemoveLast, etc
• very efficient in adjusting the list

Question 8

What is Dictionary?

Answer 8

• key and value store
• can’t add duplicate keys
• dict.ContainsKey
• there is a SortedDictionary, sorts on key

Question 9

What is the T?

Answer 9

• type parameter
• the standard is using T
• anything can be used to represent type

Question 10

What are “Sorted” collections?

Answer 10

• SortedSet - sorted & unique
• SortedList - sorted & memory efficient
• SortedDictionary - sorted, fast inserts & removals

Question 11

What does Compare typically do?

Answer 11

• comes from C language
• returns an integer
• if 0 then equal
• if less than 0 then first is less than second
• if greater than 0 then first is greater than second
• ex String.Compare(a, b)

Question 12

How can you convert types?

Answer 12

• doesn’t work on all types, mostly primitive types
• var conv = TypeDescriptor.GetConverter(typeof(T));
• convert.ConvertTo(item, typeof(TOutput))

Question 13

How do you return data when building your own Enumerator?

Answer 13

• foreach loop of objects
• yield return result
• basically a lazy return

Question 14

What is included in method signature?

Answer 14

• everything but the return type including any generic type parameters

Question 15

What are extension methods?

Answer 15

• static class and static methods

- first parameter is “this IBuffer buffer”

Question 16

3 basic .NET delegate types often used

Answer 16

• Action… action delegate must always return void
• can take up to 16 params using generic type arguments
• Action print = d => Console.WriteLine(d);
• above is anonymous method with lambda expression
• Func is similar but always has to return a value
• last types parameter is output type
• Func add = (x, y) => x + y;
• Predicate always returns a boolean
• Predicate isLessThanTen = d => d < 10;
• Converter converter = d => DateTime.AddDays(d);
• takes TInput, TOutput

Question 17

Can you constrain generics?

Answer 17

• yes
• you can force to have specific base class, implement an interface, have a constructor, etc
• uses “where” keyword

ex.
IBuffer where T: class, IInterface, new()

• • class (or struct) constraint must come first
• • can also constrain to a specific Class
• • new() constraint, always last
• compiler will check that type has a default constructor

Question 18

How do you set a generic to it’s default value?

Answer 18

T result = default(T);

Question 19

What is “out” keyword for generics?

Answer 19

• it is a generic modifier
• it makes a parameter covariant
• covariance only works with delegates and interfaces
• covariance not allowed on methods taking an entity… if you have an Employee repo you don’t want to be able to save a Person even though Employee is derived from Person
• for readonly methods
• allows things to work with more derived types

Question 20

What is Contravariance?

Answer 20

• opposite of covariance
• allows generic methods to work with objects that are a more derived type
• for write methods

Question 21

What is invariant?

Answer 21

• by default, generics are invariant
• if I have a repository of Employee i have to continue to treat it as an Employee respository, not a Person or Manager in example
• changes with covariance (out keyword) or contravariance (in keyword)

Question 22

Repo Example using Covariance and Contravariance

Answer 22

Person -> Employee -> Manager

public interface IReadOnlyRepo : IDisposable
{
   T FindById(int id);
    IQueryable FindAll();
}

public interface IWriteOnlyRepo : IDisposable
{
   void Add(T newEntity);
   void Delete(T entity);
   int Commit();
}

public interface IRepo : IReadOnlyRepo, IWriteOnlyRep
{
}

public class SqlRepo : IRepo where T: class, IEntity
{
}

Question 23

What is an Unbound Generic?

Answer 23

• List<>
• Dictionary<>
• Stack<>

can only be used in typeof operator

• opposite is a Closed Constructed Type

Question 24

Generics and Reflection example

Answer 24

static method to create a list

private static object CreateColl(Type collType, Type itemType)
{
var closedType = collectionType.MakeGenericType(itemType);
return Activator.CreateInstance(closedType);
}