Code Listings

Chapter 7: Collections

Implementing IEnumerable<T> with an iterator:

public class MyGenCollection : IEnumerable<int>
{
  int[] data = {1, 2, 3};

  public IEnumerator<int> GetEnumerator()
  {
    foreach (int i in data)
      yield return i;
  }

  IEnumerator IEnumerable.GetEnumerator()     // Explicit implementation
  {                                           // keeps it hidden.
    return GetEnumerator();
  }
}

Implementing IEnumerable<T> directly:

class MyIntList : IEnumerable<int>
{
  int[] data = { 1, 2, 3 };

  // The generic enumerator is compatible with both IEnumerable and
  // IEnumerable<T>. We implement the nongeneric GetEnumerator method
  // explicitly to avoid a naming conflict.

  public IEnumerator<int> GetEnumerator() { return new Enumerator(this); }
  IEnumerator IEnumerable.GetEnumerator() { return new Enumerator(this); }

  class Enumerator : IEnumerator<int>
  {
    int currentIndex = -1;
    MyIntList collection;   

    internal Enumerator (MyIntList collection)
    {
      this.collection = collection;
    }

    public int Current { get { return collection.data [currentIndex]; } }
    object IEnumerator.Current { get { return Current; } }

    public bool MoveNext()
    {
      return ++currentIndex < collection.data.Length;
    }

    public void Reset() { currentIndex = -1; }

    // Given we don’t need a Dispose method, it’s good practice to
    // implement it explicitly, so it’s hidden from the public interface.

    void IDisposable.Dispose() {}
  }
}

Using List<T>:

List<string> words = new List<string>();    // New string-typed list

words.Add ("melon");
words.Add ("avocado");
words.AddRange (new string[] { "banana", "plum" } );
words.Insert (0, "lemon");                                  // insert at
words.InsertRange (0, new string[] { "peach", "nashi" });   // start

words.Remove ("melon");
words.RemoveAt (3);                         // Remove the 4th element
words.RemoveRange (0, 2);                   // Remove first 2 elements

// Remove all strings starting in 'n':
words.RemoveAll (delegate (string s) { return s.StartsWith ("n"); });

Console.WriteLine (words [0]);                          // first word
Console.WriteLine (words [words.Count - 1]);            // last word
foreach (string s in words) Console.WriteLine (s);      // all words
List<string> subset = words.GetRange (1, 2);            // 2nd->3rd words

string[] wordsArray = words.ToArray();    // Creates a new typed array
  
// Copy first two elements to the end of an existing array:
string[] existing = new string [1000];
words.CopyTo (0, existing, 998, 2);

List<string> bigWords = words.ConvertAll <string>        // Converts to
 (delegate (string s) { return s.ToUpper(); } );        // uppercase

List<int> lengths = words.ConvertAll <int>
 (delegate (string s) { return s.Length; } );

Using LinkedList<T>:

LinkedList<string> tune = new LinkedList<string>();
tune.AddFirst ("do");                           // do
tune.AddLast ("so");                            // do - so

tune.AddAfter (tune.First, "re");               // do - re - so
tune.AddAfter (tune.First.Next, "mi");          // do - re - mi - so
tune.AddBefore (tune.Last, "fa");               // do - re - mi - fa - so

tune.RemoveFirst();                             // re - mi - fa - so
tune.RemoveLast();                              // re - mi - fa

LinkedListNode<string> miNode = tune.Find ("mi");
tune.Remove (miNode);                           // re - fa
tune.AddFirst (miNode);                         // mi - re - fa

foreach (string s in tune) Console.WriteLine (s);

Using Queue<T>:

Queue<int> q = new Queue<int>();
q.Enqueue (10);
q.Enqueue (20);
int[] data = q.ToArray();        // Exports to an array
Console.WriteLine (q.Count);      // "2"
Console.WriteLine (q.Peek());     // "10"
Console.WriteLine (q.Dequeue());  // "10"
Console.WriteLine (q.Dequeue());  // "20"
Console.WriteLine (q.Dequeue());  // throws an exception (queue empty)

Using Stack<T>:

Stack<int> s = new Stack<int>();
s.Push (1);                      //            Stack = 1
s.Push (2);                      //            Stack = 1,2
s.Push (3);                      //            Stack = 1,2,3
Console.WriteLine (s.Count);     // Prints 3
Console.WriteLine (s.Peek());    // Prints 3,  Stack = 1,2,3
Console.WriteLine (s.Pop());     // Prints 3,  Stack = 1,2
Console.WriteLine (s.Pop());     // Prints 2,  Stack = 1
Console.WriteLine (s.Pop());     // Prints 1,  Stack = <empty>
Console.WriteLine (s.Pop());     // throws exception

Using HashSet<T>:

HashSet<char> letters = new HashSet<char> ("the quick brown fox");

Console.WriteLine (letters.Contains ('t'));      // true
Console.WriteLine (letters.Contains ('j'));      // false

foreach (char c in letters) Console.Write (c);   // the quickbrownfx

HashSet<char> letters = new HashSet<char> ("the quick brown fox");
letters.IntersectWith ("aeiou");
foreach (char c in letters) Console.Write (c);     // euio

HashSet<char> letters = new HashSet<char> ("the quick brown fox");
letters.ExceptWith ("aeiou");
foreach (char c in letters) Console.Write (c);     // th qckbrwnfx

HashSet<char> letters = new HashSet<char> ("the quick brown fox");
letters.SymmetricExceptWith ("the lazy brown fox");
foreach (char c in letters) Console.Write (c);     // quicklazy

Using Dictionary<TKey,TValue>:

var d = new Dictionary<string, int>();

d.Add("One", 1);
d["Two"] = 2;     // adds to dictionary because "two" not already present
d["Two"] = 22;    // updates dictionary because "two" is now present
d["Three"] = 3;

Console.WriteLine (d["Two"]);                // Prints "22"
Console.WriteLine (d.ContainsKey ("One"));   // true (fast operation)
Console.WriteLine (d.ContainsValue (3));     // true (slow operation)
int val = 0;
if (!d.TryGetValue ("onE", out val))
  Console.WriteLine ("No val");              // "No val" (case sensitive)

// Three different ways to enumerate the dictionary:

foreach (KeyValuePair<string, int> kv in d)          //  One ; 1
  Console.WriteLine (kv.Key + "; " + kv.Value);      //  Two ; 22
                                                     //  Three ; 3

foreach (string s in d.Keys) Console.Write (s);      // OneTwoThree
Console.WriteLine();
foreach (int i in d.Values) Console.Write (i);       // 1223

Using SortedDictionary<TKey,TValue>:

// MethodInfo is in the System.Reflection namespace

var sorted = new SortedList <string, MethodInfo>();

foreach (MethodInfo m in typeof (object).GetMethods())
  sorted [m.Name] = m;

foreach (string name in sorted.Keys)
  Console.WriteLine (name);

foreach (MethodInfo m in sorted.Values) 
  Console.WriteLine (m.Name + " returns a " + m.ReturnType);

Console.WriteLine (sorted ["GetHashCode"]);      // Int32 GetHashCode()

Console.WriteLine (sorted.Keys  [sorted.Count - 1]);            // ToString
Console.WriteLine (sorted.Values[sorted.Count - 1].IsVirtual);  // True

Extending Collection<T>:

public class Animal
{
  public string Name;
  public int Popularity;
  public Zoo Zoo { get; internal set; }

  public Animal(string name, int popularity)
  {
    Name = name; Popularity = popularity;
  }
}

public class AnimalCollection : Collection <Animal>
{
  Zoo zoo;
  public AnimalCollection (Zoo zoo) { this.zoo = zoo; }

  protected override void InsertItem (int index, Animal item)
  {
    base.InsertItem (index, item);
    item.Zoo = zoo;
  }
  protected override void SetItem (int index, Animal item)
  {
    base.SetItem (index, item);
    item.Zoo = zoo;
  }
  protected override void RemoveItem (int index)
  {
    this [index].Zoo = null;
    base.RemoveItem (index);
  }
  protected override void ClearItems()
  {
    foreach (Animal a in this) a.Zoo = null;
    base.ClearItems();
  }
}

public class Zoo
{
  public readonly AnimalCollection Animals;
  public Zoo() { Animals = new AnimalCollection (this); }
}

Extending KeyedCollection<,>:

public class Animal
{
  string name;
  public string Name
  {
    get { return name; }
    set {
      if (Zoo != null) Zoo.NotifyNameChange (this, value);
      name = value;
    }
  }
  public int Popularity;
  public Zoo Zoo { get; internal set; }

  public Animal (string name, int popularity)
  {
    Name = name; Popularity = popularity;
  }
}

public class AnimalCollection : KeyedCollection <string, Animal>
{
  Zoo zoo;
  public AnimalCollection (Zoo zoo) { this.zoo = zoo; }

  internal void NotifyNameChange (Animal a, string newName)
  {
    this.ChangeItemKey (a, newName);
  }

  protected override string GetKeyForItem (Animal item)
  {
    return item.Name;
  }

  // The following methods would be implemented as in the previous example
  protected override void InsertItem (int index, Animal item)...
  protected override void SetItem (int index, Animal item)...
  protected override void RemoveItem (int index)...
  protected override void ClearItems()...
}

public class Zoo
{
  public readonly AnimalCollection Animals;
  public Zoo() { Animals = new AnimalCollection (this); }
}

class Program
{
  static void Main()
  {
    Zoo zoo = new Zoo();
    zoo.Animals.Add (new Animal ("Kangaroo", 10));
    zoo.Animals.Add (new Animal ("Mr Sea Lion", 20));
    Console.WriteLine (zoo.Animals [0].Popularity);               // 10
    Console.WriteLine (zoo.Animals ["Mr Sea Lion"].Popularity);   // 20
    zoo.Animals ["Kangaroo"].Name = "Mr Roo";
    Console.WriteLine (zoo.Animals ["Mr Roo"].Popularity);        // 10
  }
}

Using EqualityComparer:

public class Customer
{
  public string LastName;
  public string FirstName;

  public Customer (string last, string first)
  {
    LastName = last;
    FirstName = first;
  }
}

public class LastFirstEqComparer : EqualityComparer <Customer>
{
  public override bool Equals (Customer x, Customer y)
  {
    return x.LastName == y.LastName && x.FirstName == y.FirstName;
  }

  public override int GetHashCode (Customer obj)
  {
    return (obj.LastName + ";" + obj.FirstName).GetHashCode();
  }
}

static void Main()
{
  Customer c1 = new Customer ("Bloggs", "Joe");
  Customer c2 = new Customer ("Bloggs", "Joe");

  // Because we’ve not overridden object.Equals, normal reference
  // type equality semantics apply:

  Console.WriteLine (c1 == c2);               // false
  Console.WriteLine (c1.Equals (c2));         // false

  Dictionary<Customer, string> d = new Dictionary<Customer, string>();
  d [c1] = "Joe";
  Console.WriteLine (d.ContainsKey (c2));         // false

  // Now with the custom equality comparer:
  
  LastFirstEqComparer eq = new LastFirstEqComparer();
  d = new Dictionary<Customer, string> (eq);
  d [c1] = "Joe";
  Console.WriteLine (d.ContainsKey (c2));         // true
}

Using the IComparer interfaces:

class Wish
{
  public string Name;
  public int Priority;

  public Wish (string name, int priority)
  {
    Name = name;
    Priority = priority;
  }
}

class PriorityComparer : Comparer <Wish>
{
  public override int Compare (Wish x, Wish y)
  {
    if (object.Equals (x, y)) return 0;          // Fail-safe check
    return x.Priority.CompareTo (y.Priority);
  }
}

static void Main()
{
  List<Wish> wishList = new List<Wish>();
  wishList.Add (new Wish ("Peace", 2));
  wishList.Add (new Wish ("Wealth", 3));
  wishList.Add (new Wish ("Love", 2));
  wishList.Add (new Wish ("3 more wishes", 1));

  wishList.Sort (new PriorityComparer());
  foreach (Wish w in wishList) Console.Write (w.Name + " | ");
}

SurnameComparer:

class SurnameComparer : Comparer <string>
{
  StringComparer strCmp;
  
  public SurnameComparer (CultureInfo ci)
  {
    // Create a case-sensitive, culture-sensitive string comparer
    strCmp = StringComparer.Create (ci, false);
  }

  string Normalize (string s)
  {
    s = s.Trim();
    if (s.ToUpper().StartsWith ("MC")) s = "MAC" + s.Substring (2);
    return s;
  }

  public override int Compare (string x, string y)
  {
    // Directly call Compare on our culture-aware StringComparer
    return strCmp.Compare (Normalize (x), Normalize (y));
  }
}