Question

confused with calling a constructor.

Asked by: IzzyTwinkly

Hi,
From the code below, what happens when
MyColors mc = new MyColors(); inside Main method is executed?
It calls MyColors' default constructor, correct?
GetEnumerator() method should be called to make it work, but which line in Main method calls GetEnumerator() method?
Also using foreach (string color in mc) in Main method, I understand mc part.(since mc is enumerable now), but I don't understand how come 'string color in MyColors class' represent 'Color array's elements' in MyColors class?

using System.Collections;

namespace ColorCollectionEnumerator
{
    class ColorEnumerator : IEnumerator
    {
        string[] Colors;
        int position = -1;

        public ColorEnumerator(string[] theColors) // Constructor
        {
            Colors = new string[theColors.Length];
            for (int i = 0; i < theColors.Length; i++)
            {
                Colors[i] = theColors[i];
            }
        }

        public object Current  // Current
        {
            get { return Colors[position]; }
        }

        public bool MoveNext() // MoveNext
        {
            if (position < Colors.Length - 1)
            {
                position++;
                return true;
            }
            else
                return false;
        }

        public void Reset()    // Reset
        {
            position = -1;
        }
    }
    class MyColors: IEnumerable
    {
        string[] Colors = { "Red", "Yellow", "Blue" };

        public IEnumerator GetEnumerator()
        {
            return new ColorEnumerator(Colors);
        }
    }
    class Program
    {
        static void Main()
        {
            MyColors mc = new MyColors();
            foreach (string color in mc)
                Console.WriteLine("{0}", color);
            Console.Read();
        }
    }
}

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Asked On
2009-09-08 at 01:07:39ID24714112
Topics

C# Programming Language

,

Microsoft Visual C#.Net

Participating Experts
2
Points
500
Comments
10

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Answers

 

by: angelIIIPosted on 2009-09-08 at 01:11:25ID: 25279604

>It calls MyColors' default constructor, correct?
yes

>GetEnumerator() method should be called to make it work, but which line in Main method calls GetEnumerator() method?
foreach  is the one that will "call" the getenumarato

 

by: IzzyTwinklyPosted on 2009-09-08 at 01:34:34ID: 25279738

Okay, after MyColors' default contructor is called, what happens next?
And could you please answers for the following questions too?  
When I use foreach (string color in mc) in Main method, I understand mc part.(since mc is enumerable now), but I don't understand how come 'string color in MyColors class'(string color in mc) represents 'Color array's elements' in MyColors class?

I know it might sound redundant to answer some of these quetions, but I am a C# beginner, so please understand that.

thanks,
Izzy

 

by: angelIIIPosted on 2009-09-08 at 01:48:42ID: 25279804

this is the key:

public IEnumerator GetEnumerator()
        {
            return new ColorEnumerator(Colors);  << this line returned the enumerator for the Colors string array
        }

 

by: angelIIIPosted on 2009-09-08 at 01:49:27ID: 25279809

see, your class returns the enumerator of this:
 string[] Colors;

 

by: jimbobmcgeePosted on 2009-09-08 at 05:05:58ID: 25280945

The foreach keywork implicitly calls GetEnumerator for whatever IEnumerable you are trying to iterate.  This means that, when you type:

  foreach (string color in mc)
  {
      DoSomething(color);
  }

the compiler makes it do something like this:

  IEnumerator e = mc.GetEnumerator();
  while (e.MoveNext) {
    DoSomething((string)e.Current);
  }

Your GetEnumerator implementation returns a ColorEnumerator object, which is constructed by passing the string array 'Colors' to it.  The code in ColorEnumerator loops through and yields each string in the array.

HTH.

J.

 

by: IzzyTwinklyPosted on 2009-09-08 at 13:47:15ID: 25285998

Hi

Only area that I am having a trouble with is Main method.  I understnad what's going on after GetEnumerator() method is called.

Here are my questions again.
1. What happens the default construct in MyColors is called from Main method?
2. Now mc is enumerable like an array.  Then what is the items inside mc?
In foreach (string color in mc), the item represented by color is different from the item in Colors array.  The first one is mc's item and the other one is Colors array's item.

 

by: angelIIIPosted on 2009-09-08 at 13:55:11ID: 25286061

you have to notice that your code has 2 classes:

class ColorEnumerator : IEnumerator
=> this one has only 1 goal: make a IEnumator available for a input of string[], which is the input for the constructor:
public ColorEnumerator(string[] theColors) // Constructor

class MyColors: IEnumerable
    {
        // initialize the local string array to a hard-coded list of values
        string[] Colors = { "Red", "Yellow", "Blue" };

        // return the enumerator generated by that other (helper) class:
        public IEnumerator GetEnumerator()
        {
            return new ColorEnumerator(Colors);
        }
    }



so:
>1. What happens the default construct in MyColors is called from Main method?
MyColors class has no non-default contructor.
so, the only thing that "happens" at this line:
>MyColors mc = new MyColors();

is basically this inside the MyColors:
string[] Colors = { "Red", "Yellow", "Blue" };

>2. Now mc is enumerable like an array.  Then what is the items inside mc?
if you read carefully: mc itself is NOT enumerable. it only implements the interface, and transfers the actual implementation of that interface by calling the helper class

 

by: IzzyTwinklyPosted on 2009-09-10 at 00:20:18ID: 25297934

humm...sorry, but I still don't get it.
jimbobmcgee says, "The foreach keywork implicitly calls GetEnumerator for whatever IEnumerable you are trying to iterate."
so if there is a statement like
foreach(int A in B) and if B implements IEnumerable
foreach will always automatically call GetEnuemrator method in B for whatever IEnumerable I am trying to iterate, correct?

 

by: angelIIIPosted on 2009-09-10 at 01:52:27ID: 25298400

yes, correct

 

by: jimbobmcgeePosted on 2009-09-10 at 05:47:19ID: 25299592

>> if B implements IEnumerable
Class B has to implement IEnumerable otherwise it won't work in a foreach statement.  You'll get a compile error if you try to use foreach on something that is not IEnumerable.

J.

20120131-EE-VQP-002

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