c# collections & generics c#.net software development version 1.0

C# Collections & Generics

C# .NET Software Development

Version 1.0

Overview

Why Collections? Why Generic Datatypes? Collections C# 1.0 Style (object) Collections C# 2.0 Style (Generic) Collections C# 3.0 Initializers

Copyright 2005-2009 by Dennis A. Fairclough all rights reserved. 2


Collections

System.Collections Contain object references

box and unbox value types as required Arrays, Linked Lists, Trees, Maps, etc.

System.Collections.Generic Contain appropriate data types Reduces problems with up/down casting Reduces boxing & unboxing

Generics (C# Standard)

19.1 Generics Generics use

classes, structs, methods interfaces, and Delegates (method pointers).

to be parameterized by the type of data they store and manipulate.


Generics

C# generics will be immediately familiar to users of generics in Eiffel or Ada, Java or to users of C++ templates, though they do not suffer many of the complications of C++.


Stack Example (double) public class RStack { private double[] _stack; private int _top; public int Top { get { return _top; } set { _top = value; } } public RStack(int ssize) { _stack = new double[ssize]; _top = 0; } public void Push(double val) { _stack[_top++] = val; } public double Pop() { return _stack[--_top]; } }//END class RStack


Stack Example (object ref)public class OStack { private object[] _stack; private int _top; public int Top { get { return _top; } set { _top = value; } } public OStack(int ssize) { _stack = new object[ssize]; _top = 0; } public void Push(object val) { _stack[_top++] = val; } public object Pop() { return _stack[--_top]; } }//END class OStack


Stack Example (generic)unbound type (“class Blueprint”) public class Stack<T> { private T[] _stack; private int _top; public int Top { get { return _top; } set { _top = value; } } public Stack(int ssize) { _stack = new T[ssize]; _top = 0; } public void Push(T val) { _stack[_top++] = val; } public T Pop() { return _stack[--_top]; } }//END class Stack


Generic Classes In General

access_modifier class ident<T,K,..>{

}


generic datatype

Generic Methods

public void Foo<T>(Stack<T> x,T val){}Foo<double>(new Stack<double>(5),5.5);



Collection Interfaces IList : ICollection, IEnumerable

IList<> ICollection : IEnumerable

ICollection<> IDictionary : ICollection, IEnumerable

IDictionary<> IEnumerable

IEnumerable<> IEnumerator

IEnumerator<>


IList : ICollection, IEnumerable

Add Clear Contains IndexOf Insert Remove RemoveAt Item (indexer) IsFixedSize IsReadOnly


ICollection

Count (get only) IsSynchronized SyncRoot CopyTo


Dictionary : ICollection,IEnumerable

Add Clear Contains Remove Item (Indexer) Keys Values IsFixedSize IsReadOnly


IEnumerable

GetEnumerator

Allows use of foreach on a collection


IEnumerator

Current MoveNext Reset (deprecated)


Enumeration (Iterators) Enumeration refers to the process of iterating (stepping)

through a collection Enumeration in C# actually implements a modification of

the Iterator design pattern It’s called enumeration because as full-blown iteration saved

for C# Version 2.0 yield break yield return


The Iterator Design Pattern

Collection class

Nested Iterator class - knows enough about the collection to be able to expose one element of the at a time - Stores its current state

Collection class exposes a way to create and initialize one or more

iterator(s)


Implementing an Enumerator A collection class implements IEnumerable interface Nested Enumerator class implements IEnumerator interface IEnumerable is required if you want to use foreach on your

collection Unlike iterators, enumerators have the restriction that, if the

collection changes, they become invalid You have to do this in your implementation

C# Enumerators also restrict access through the enumerator to read-only

Name Description

Current Gets the current element in the collection.

MoveNext Advances the enumerator to the next element of the collection.

Reset Sets the enumerator to its initial position, which is before the first element in the collection.

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Collections (object ref)


Class DescriptionArrayList Implements the IList interface using an array whose size is

dynamically increased as required.

BitArray Manages a compact array of bit values, which are represented as Booleans, where true indicates that the bit is on (1) and false indicates the bit is off (0).

CaseInsensitiveComparer Compares two objects for equivalence, ignoring the case of strings.

CaseInsensitiveHashCodeProvider Supplies a hash code for an object, using a hashing algorithm that ignores the case of strings.

CollectionBase Provides the abstract base class for a strongly typed collection.

Comparer Compares two objects for equivalence, where string comparisons are case-sensitive.

DictionaryBase Provides the abstract base class for a strongly typed collection of key/value pairs.

Hashtable Represents a collection of key/value pairs that are organized based on the hash code of the key.

Queue Represents a first-in, first-out collection of objects.ReadOnlyCollectionBase Provides the abstract base class for a strongly typed non-generic

read-only collection.

SortedList Represents a collection of key/value pairs that are sorted by the keys and are accessible by key and by index.

Stack Represents a simple last-in-first-out (LIFO) non-generic collection of objects.

Classes

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Collections (Generic)


DescriptionComparer Provides a base class for implementations of the IComparer generic interface.

Dictionary Represents a collection of keys and values.Dictionary.KeyCollection Represents the collection of keys in a Dictionary. This class cannot be

inherited.Dictionary.ValueCollection Represents the collection of values in a Dictionary. This class cannot be

inherited.EqualityComparer Provides a base class for implementations of the IEqualityComparer generic

interface.KeyNotFoundException The exception that is thrown when the key specified for accessing an element

in a collection does not match any key in the collection.

LinkedList Represents a doubly linked list.LinkedListNode Represents a node in a LinkedList. This class cannot be inherited.

List Represents a strongly typed list of objects that can be accessed by index. Provides methods to search, sort, and manipulate lists.

Queue Represents a first-in, first-out collection of objects.SortedDictionary Represents a collection of key/value pairs that are sorted on the key.

SortedDictionary.KeyCollection Represents the collection of keys in a SortedDictionary. This class cannot be inherited.

SortedDictionary.ValueCollection Represents the collection of values in a SortedDictionary. This class cannot be inherited

SortedList Represents a collection of key/value pairs that are sorted by key based on the associated IComparer implementation.

Stack Represents a variable size last-in-first-out (LIFO) collection of instances of the same arbitrary type.

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C# Collections General use:

ArrayList and List<T> StringCollection

HashTable and Dictionary<K, V> StringDictionary

ListDictionary LinkedList<T> BitArray

Hybrids: SortedList SortedDictionary HybridDictionary

Specific Use: Queue Stack


ArrayList and List<T> Linear array of objects

Stored in contiguous memory Implements:

IList ICollection IEnumerable ICloneable

Resizes Dynamically Index into it like an array

Elements must be added before they can be accessed

(See ArrayList Demo)


Capacity

Capacity – Gets or Sets the number of elements the ArrayList/List<T> can contain

Count – Gets the number of elements the ArrayList/List<T> actually contains

If Count == Capacity and you add 1 or more elements... A new array of twice the size is allocated The old elements are copied into the new location Expensive ! ! !

Set your Capacity upon creation if possible Default capacity is 4 Don’t change it once it is set

It will change for you if it needs to


Adding elements ArrayList.Add(object obj)

Appends the object to the end of the ArrayList ArrayList.AddRange(ICollection c)

Appends a collection of objects to the end of the ArrayList ArrayList.Insert(int index, object obj)

Inserts the object in the position indicated Has to move (copy) all subsequent elements

ArrayList.InsertRange(int index, ICollection c) Inserts the a collection of objects in the position indicated Has to move all subsequent elements


Removing Elements ArrayList.Remove(object obj)

Removes the first occurrence of obj ArrayList.RemoveAt(int index)

Removes the item at the index indicated ArrayList.RemoveRange(int index, int count)

Removes count items beginning at index ArrayList.Clear()

Removes all the items from the ArrayList Be aware that removing from the middle (or the beginning)

of an ArrayList will require moving all subsequent items.


Accessing elements Indexer

Get and Set an item at the index indicated Returns an object (you must cast)

ArrayList.GetRange(int index, int count) Returns a new ArrayList containing the elements from index for

count ArrayList.ToArray()

Builds a System.Array out of the array list ArrayList.Clone()

Makes an exact (shallow) copy of the ArrayList

Collection Initializers &Anonymous Types

Collection Initializer List<int> slist = new List<int>{0,1,2,3,4,5}; List<string> clist = new List<string> {

new string(“szero”.ToCharArray()),new string(“stwo”.ToCharArray()),new string(“sthree”.ToCharArray()),new string(“sfour”.ToCharArray()),new string(“sfive”.ToCharArray()),

} Anonymous Collection Type

var temp = new {name="Dennis",number=100};Copyright 2005-2009 by Dennis A. Fairclough all rights reserved. 28


foreach on an ArrayList casts automatically Elements are read-only during the foreach

ArrayList al = new ArrayList(9);

// using Add and AddRangeal.Add(0);al.Add(1);al.Add(2);al.Add(3);al.AddRange(new object[] {4, 5, 6, 7, 8});

foreach(int element in al){ Console.Write(element.ToString() + " ");}

(Code is from ArrayList Demo)


Searching ArrayList.Contains(object item)

Indicates whether the array list contains item Performs a linear search

ArrayList.IndexOf(object item) Returns the index of the first occurrence of item in the ArrayList Returns -1 if the item is not found

ArrayList.IndexOf(object item, int start) Returns the index of the first occurrence of item in the ArrayList after

start Use this to search for all occurrences of an object

ArrayList.BinarySearch(object item) Requires the ArrayList to be sorted If search fails, BinarySearch returns the inverse of the index of the next-

largest item (or if there is no larger element, ArrayList.Count) O ( log (n) )


Manipulating Items ArrayList.Reverse()

Reverses the elements in the ArrayList ArrayList.Sort()

Sorts the ArrayList Requires that all members implement IComparable

ArrayList.Sort(IComparer comparer) Sorts the Arraylist Uses the comparer object instead of IComparable interface (if it

existed)


New in List<T> Stupid name!

AsReadOnly ConvertAll FindAll ForEach TrueForAll


Specialization: StringCollection StringCollection

in System.Collections.Specialized

An ArrayList wrapper specialized for strings In 2.0 we can just use List<string>


Searching and Sorting

Use IComparable and/or IComparer


IComparable Required for binary searching and sorting Applies to items inside the Collection

An item is said to be “Comparable” to another Comparable items inside a collection can be sorted

and searched


IComparer Strategy Design Pattern Plug in a new comparer object for different results Can be applied to objects that are not “Comparable” Can be applied to override the comparable

functionality that may already exist

(See IComparableDemo )


Hashtable and Dictionary<K, V> A collection of Key-Value pairs

Actually stored in a DictionaryEntry object Key cannot be null Value can be

Implements IDictionary ICollection IEnumerable others


Indexing into Hashtables Access into a Hashtable/Dictionary using an indexer:

Hashtable ht = new Hashtable();ht[“hello world”] = new MyObject();

Dictionary<string, MyObject> dict = new Dictionary<string, MyObject>();dict["one"] = new MyObject();


How hashing works...

GetHashCode() is called on the key Keys are grouped into “buckets”

This reduces the number of lookups needed to access Inside a bucket items are stored in a linear array

If the maximum ratio of entries-to-buckets (called the load factor) is reached, more buckets are allocated and the entries are redistributed. (Expensive!)

You may set the load factor when you create the Hashtable Default is 1.0 (an average of 1 item per bucket) Smaller load factor means faster lookups but take more

memory Load factor values can be between .1 and 1.0


Indexing into a Hashtable Assignment:

If the key does not exist, a new Dictionary entry is created If the key exists, the old value is overwritten

Access: If the key does not exist, a KeyNotFoundException is thrown If the key exists, the value item is returned

Hashtable ht = new Hashtable();ht[“hello world”] = new MyObject();


Add, Remove, Clear Add(object key, object value)

Will add a new DictionaryEntry to the hashtable if key does not already exist

If key exists, Add throws an InvalidArgument exception

Remove(object key) Removes the corresponding dictionary entry if it

exists Clear()

Clears all DictionaryEntries from the Hashtable


Looking for items Contains(object key) ContiansKey(object key)

Both functions act the same Includes Contains to satisfy IDictionary returns true if the Hashtable contains a

corresponding DictionaryEntry O( 1 )

ContainsValue(object value) Searches all DictionaryEntries until it finds value or

exhausts the Hashtable O ( n )


Symbol-Table Hashtable

An example from a compiler

Symbol-Table lookup:

Hashtable table = new Hashtable(); ...table["if"] = new CToken("if", TokenType.If);table["elseif"] = new CToken("elseif", TokenType.ElseIf);table["else"] = new CToken("else", TokenType.Else);table["endif"] = new CToken("endif", TokenType.EndIf);table["for"] = new CToken("for", TokenType.For);table["foreach"] = new CToken("foreach", TokenType.Foreach);table["in"] = new CToken("in", TokenType.In);

public CToken IdentifyNewToken(string lexeme){ if(table.Contains(lexeme)) return (GlobalTable[lexeme] as CToken); else return new CToken();}


foreach on a Hashtable Remember, the Hashtable contains a collection of

DictionaryEntry objects That being the case, this won’t work:

You can do this:

foreach(MyObject mo in _hashtable){ // work here}

foreach(DictionaryEntry de in _hashtable){ MyObject mo = (MyObject)de.Value; // work here}


foreach on a Hashtable (continued)

Or better yet:

or:

Note: order is not guaranteed

foreach(MyObject mo in hashtable.Values){ // work here}

foreach(MyObject mo in hashtable.Keys){ // work here}


Keys and Values Properties Keys

An ICollection of the keys Values

An ICollection of the values Order is not specified in either, except that it will be

the same between them Items in these ICollections refer to the items in the

Hashtable (they are shallow copies)


Specialization: StringDictionary

StringDictionary in System.Collections.Specialized

Adapter Wrapper for a Hashtable exposing strings as the key and value In 2.0 use Dictionary<string, string>


ListDictionary Singly Linked-list key-value collection

Faster than Hashtable for small collections (10 Items or less)

Add, Remove, Search are all O(n)

Implements IDictionary ICollection IEnumerable


LinkedList<T> Implements a doubly-linked list (finally!) Uses LinkedListNode<T> Fast insertion/removal Slow indexing (no indexer)

Implements: ICollection IEnumerable


LinkedList<T> Members Count First Last AddBefore AddAfter AddFirst AddLast Clear Contains Remove RemoveFirst RemoveLast RemoveBefore RemoveAfter


BitArray

Compact storage for true/false values Stores in 32-bit chunks Bit-wise operations available

and, or, not, xor Set(int index, bool value) SetAll(bool value)

Use an indexer to access individual items Indexer returns a bool

Implements ICollection IEnumerable ICloneable


Hybrid: SortedList A cross between a Hashtable and an ArrayList Stores DictionaryEntries in an array sorted by the key Indexing is not as fast as a hashtable Size is adjusted dynamically to meet capacity demands Indexer uses the key:

mySortedList[myObject] = valueObject;

To access by index, use GetByIndex(int index) and SetByIndex(int index, object value):

mySortedList.SetByIndex(0, valueObject)

Implements: IDictionary ICollection IEnumerable

(See SortedList Demo)


Hybrid: HybridDictionary Uses ListDictionary when small, Hashtable when large

Moves items when the size increases

Implements IDictionary ICollection IEnumerable


Queue and Queue<T> Fifo data structure Dynamically resized to fit capacity requirements

Implements: ICollection IEnumerable ICloneable


Manipulating a Queue Enqueue(object obj)

Places obj at the end of the Queue Dequeue()

Returns and removes the object at the front of the Queue

Throws an InvalidOperationException if the Queue is empty

Peek() Returns the object at the front of the Queue Throws an InvalidOperationException if the Queue is

empty Clear()

Empties the Queue


Stack and Stack<T>

Lifo data structure Dynamically resizes to meet capacity requirements

Implements: ICollection IEnumerable ICloneable


Stack Manipulation Push(object obj)

Puts obj on the top of the Stack Pop()

Returns and removes the item from the top of the Stack

Throws an InvalidOperationException if the Stack is empty

Peek() Returns the top item from the Stack Throws an InvalidOperationException if the Stack is

empty Clear()

Clears all elements from the stack


CollectionBase Provided as an abstract base class for you to create

your own collections Provides the virtual methods:

Count Clear() RemoveAt()

Provides a place to store your variables: InnerList – is an inherited member (protected)

ArrayList

SortedDictionary<K,V>

Demo Code



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