IEnumerator.txt
20210316
IEnumerator Interface
Supports a simple iteration over a non-generic collection.

using System;
using System.Collections;

// Simple business object.
public class Person
{
    public Person(string fName, string lName)
    {
        this.firstName = fName;
        this.lastName = lName;
    }

    public string firstName;
    public string lastName;
}

// Collection of Person objects. This class
// implements IEnumerable so that it can be used
// with ForEach syntax.
// 繼承 IEnumerable 後就可以使用於 ForEach 語法.
public class People : IEnumerable
{
    private Person[] _people;
    public People(Person[] pArray)
    {
        _people = new Person[pArray.Length];

        for (int i = 0; i < pArray.Length; i++)
        {
            _people[i] = pArray[i];
        }
    }

// Implementation for the GetEnumerator method.
    IEnumerator IEnumerable.GetEnumerator()
    {
       return (IEnumerator) GetEnumerator();
    }

    public PeopleEnum GetEnumerator()
    {
        return new PeopleEnum(_people);
    }
}

// When you implement IEnumerable, you must also implement IEnumerator.
public class PeopleEnum : IEnumerator
{
    public Person[] _people;

    // Enumerators are positioned before the first element
    // until the first MoveNext() call.
    int position = -1;

    public PeopleEnum(Person[] list)
    {
        _people = list;
    }

    public bool MoveNext()
    {
        position++;
        return (position < _people.Length);
    }

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

    object IEnumerator.Current
    {
        get
        {
            return Current;
        }
    }

    public Person Current
    {
        get
        {
            try
            {
                return _people[position];
            }
            catch (IndexOutOfRangeException)
            {
                throw new InvalidOperationException();
            }
        }
    }
}

class App
{
    static void Main()
    {
        Person[] peopleArray = new Person[3]
        {
            new Person("John", "Smith"),
            new Person("Jim", "Johnson"),
            new Person("Sue", "Rabon"),
        };

        People peopleList = new People(peopleArray);
        foreach (Person p in peopleList)
            Console.WriteLine(p.firstName + " " + p.lastName);
    }
}

/* This code produces output similar to the following:
 *
 * John Smith
 * Jim Johnson
 * Sue Rabon
 *
 */
 
Remarks
IEnumerator is the base interface for all non-generic enumerators. Its generic equivalent is the System.Collections.Generic.IEnumerator<T> interface.

The foreach statement of the C# language (for each in Visual Basic) hides the complexity of the enumerators. Therefore, using foreach is recommended instead of directly manipulating the enumerator.

Enumerators can be used to read the data in the collection, but they cannot be used to modify the underlying collection.

The Reset method is provided for COM interoperability and does not need to be fully implemented; instead, the implementer can throw a NotSupportedException.

Initially, the enumerator is positioned before the first element in the collection. You must call the MoveNext method to advance the enumerator to the first element of the collection before reading the value of Current; otherwise, Current is undefined.

Current returns the same object until either MoveNext or Reset is called. MoveNext sets Current to the next element.

If MoveNext passes the end of the collection, the enumerator is positioned after the last element in the collection and MoveNext returns false. When the enumerator is at this position, subsequent calls to MoveNext also return false. If the last call to MoveNext returned false, Current is undefined.

To set Current to the first element of the collection again, you can call Reset, if it's implemented, followed by MoveNext. If Reset is not implemented, you must create a new enumerator instance to return to the first element of the collection.

If changes are made to the collection, such as adding, modifying, or deleting elements, the behavior of the enumerator is undefined.

The enumerator does not have exclusive access to the collection; therefore, enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.


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20210316 Before

public class SamplesArray2{

   public static void Main()  {

      // Creates and initializes a new three-dimensional Array of type Int32.
      Array myArr = Array.CreateInstance( typeof(Int32), 2, 3, 4 );
      for ( int i = myArr.GetLowerBound(0); i <= myArr.GetUpperBound(0); i++ )
         for ( int j = myArr.GetLowerBound(1); j <= myArr.GetUpperBound(1); j++ )
            for ( int k = myArr.GetLowerBound(2); k <= myArr.GetUpperBound(2); k++ )  {
               myArr.SetValue( (i*100)+(j*10)+k, i, j, k );
            }

      // Displays the properties of the Array.
      Console.WriteLine( "The Array has {0} dimension(s) and a total of {1} elements.", myArr.Rank, myArr.Length );
      Console.WriteLine( "\tLength\tLower\tUpper" );
      for ( int i = 0; i < myArr.Rank; i++ )  {
         Console.Write( "{0}:\t{1}", i, myArr.GetLength(i) );
         Console.WriteLine( "\t{0}\t{1}", myArr.GetLowerBound(i), myArr.GetUpperBound(i) );
      }

      // Displays the contents of the Array.
      Console.WriteLine( "The Array contains the following values:" );
      PrintValues( myArr );
   }


   public static void PrintValues( Array myArr )  {
      System.Collections.IEnumerator myEnumerator = myArr.GetEnumerator();
      int i = 0;
      int cols = myArr.GetLength( myArr.Rank - 1 );
      while ( myEnumerator.MoveNext() )  {
         if ( i < cols )  {
            i++;
         } else  {
            Console.WriteLine();
            i = 1;
         }
         Console.Write( "\t{0}", myEnumerator.Current );
      }
      Console.WriteLine();
   }
}
/* 
This code produces the following output.

The Array has 3 dimension(s) and a total of 24 elements.
    Length    Lower    Upper
0:    2    0    1
1:    3    0    2
2:    4    0    3
The Array contains the following values:
    0    1    2    3
    10    11    12    13
    20    21    22    23
    100    101    102    103
    110    111    112    113
    120    121    122    123
*/