Enumerating the Members of a Collection

Introduction to System Collections

When studying arrays, we saw that you could use a for loop to visit each member of an array. This was also done with the help of the [] operator. In the previous lesson, we saw that, when creating a collection, you should provide a method that allows you to retrieve a member of the collection. In both cases, you can list the members of an array or a collection through a technique called an enumeration. Enumerating a collection consists of visiting each member of the list, for any reason judged necessary. For example, you can enumerate a collection to display a list of its members. You can enumerate a collection when looking for a member that responds to a certain criterion.

Besides, or instead of a for loop, the .NET Framework provides another and better support for enumeration. In the C# language, you can enumerate a collection using the foreach operator, but the collection must be prepared for it: you cannot just use foreach for any collection. This support is provided through two main interfaces: IEnumerator and IEnumerable. These two interfaces are defined in the System.Collection namespace. Therefore, if you intend to use them, you can include this namespace in your source file.

Practical Learning: Introducing Built-In Collections

1. Start Microsoft Visual C# and create a new Console Application named FlowerShop5
2. To create a new file, on the main menu, click Project -> Add New Item...
3. In the Templates list, click Code File
4. Set the Name to FlowerAccessories and click Add
5. In the empty file, type the following:
    

   public enum FlowerType { Roses = 1, // Why Lilies = 102, // these Daisies = 104, // fancy Carnations = 112, // numbers? LivePlant = 220, // Just Orchids = 234, // for Tulips = 242, // the Iris = 250, // fun Mixed = 462, // of Other = 464 // it } public enum FlowerColor { Red = 1, White, Yellow, Pink, Orange, Blue, Lavender, Mixed } public enum FlowerArrangement { Bouquet = 1, Balloon, Vase, Basket, Bundle, Any }

6. To create a new class, in the Solution Explorer, right-click FlowerShop5 -> Add -> Class...
7. Set the Name to Flower and press Enter
8. Change the file as follows:
    

   using System; namespace FlowerShop5 { public class Flower { private decimal pc; public Flower Next; public FlowerType Type; public FlowerColor Color; public FlowerArrangement Arrangement; public Flower() { Type = FlowerType.Mixed; Color = FlowerColor.Mixed; Arrangement = FlowerArrangement.Vase; pc = 0.00M; } public Flower(FlowerType type, FlowerColor color, FlowerArrangement argn, decimal price) { Type = type; Color = color; Arrangement = argn; pc = price; } public decimal UnitPrice { get { return pc; } set { pc = value; } } } }

9. To create a new class, in the Class View, right-click FlowerShop5 -> Add -> Class...
10. Set the Name to FlowerInventory and click Add
11. Change the class as follows:
     

    using System; namespace FlowerShop5 { public abstract class AFlower { protected int items; public AFlower() { items = 0; } public int Count { get { return items; } } public abstract int Add(Flower obj); public abstract Flower Get(int index); public abstract bool Delete(); } public class FlowerInventory : AFlower { public Flower Head; public Flower Inventory; public FlowerInventory() { Head = null; } public override int Add(Flower NewFlower) { Flower Sample = new Flower(); Sample = NewFlower; Sample.Next = Head; Head = Sample; return items++; } public override Flower Get(int index) { Flower Current = Head; for(int i = Count - 1; i > index && Current != null; i--) Current = Current.Next; return Current; } public override bool Delete() { if (Head == null) { Console.WriteLine("The inventory is empty"); return false; } Flower Current; Current = Head.Next; Head.Next = Current.Next; items--; return true; } } }

12. Save all  

The IEnumerator interface provides the means of identifying the class that holds a sample of the items that will be enumerated. This interface is equipped with one property and two methods. To use the functionalities provided by the IEnumerator interface, you must create a class that implements it. You can start the class as follows: public class Enumerator : IEnumerator { }

public class Enumerator : IEnumerator
{
    private double[] numbers;
}

The role of the enumerator is to act on a collection. For this reason, the class should be prepared to receive an external collection. This can be done by passing it to a constructor of the enumerator. Here is an example:

public class Enumerator : IEnumerator
{
    private double[] numbers;

    public Enumerator(double[] list)
    {
    }
}

The internal collection would be used in the enumerator class. The external collection would be the source of the values of the list that would be the enumerator. For these reasons, you can/should initialize the internal collection with the values of the external list. This can be done as follows:

public class Enumerator : IEnumerator
{
    private double[] numbers;

    public Enumerator(double[] list)
    {
        this.numbers = list;
    }
}

1. To create a new class, in the Class View, right-click FlowerShop5 -> Add -> Class...
2. Set the Name of the class to FlowerIdentifier and click Add
3. Change the file as follows:
    

   using System; using System.Collections; namespace FlowerShop5 { public class FlowerIdentifier : IEnumerator { public FlowerInventory counts; public void Identify(FlowerInventory list) { this.counts = list; } } }

4. Save the file

In the previous lesson, when introducing some techniques of creating a list, we saw that you should have a type of tag, as a field, that allows you to monitor the item that is being currently accessed or used in the list. This is particularly valuable when visiting the members of the collection. The IEnumerator interface provides a property that is used to identify the current member of the list. This property is called Current. Because the current item is meant to be viewed only, the Current property is read-only. Based on the rules of abstract classes, remember that you must implement all members of an interface in the class that is based on it.

To implement the Current property, you can implement its get accessor to return the item at the current position. This can be done as follows:

public class Enumerator : IEnumerator
{
    private double[] numbers;
    private int cur;

    public Enumerator(double[] list)
    {
        this.numbers = list;
    }

    public Object Current
    {
        get { return numbers[cur]; }
    }
}

1. In the FlowerIdentifier.cs file, implement the IEnumerator.Current property as follows:
    

   using System; using System.Collections; namespace FlowerShop5 { public class FlowerIdentifier : IEnumerator { private int curPosition = -1; public FlowerInventory counts; public void Identify(FlowerInventory list) { this.counts = list; } public Object Current { get { try { return this.counts.Get(this.curPosition); } catch (IndexOutOfRangeException) { Console.WriteLine("The current item must be accessed " + "within the range of available items"); } } } } }

2. Save the file

Although you should be able to identify the current item at any time, when the application starts, before the collection can be enumerated, the tag that is used to monitor the current item should be set to before the beginning of the count. This can be done by setting the tag to -1. Here is an example:

public class Enumerator : IEnumerator
{
    private double[] numbers;
    private int cur;

    public Enumerator(double[] list)
    {
        this.numbers = list;
        cur = -1;
    }

    public Object Current
    {
        get { return numbers[cur]; }
    }
}

While the collection is being, at one moment you may want to reset the tag of the current item to its original position. To support this operation, the IEnumerator is equipped with a method named Reset. Its syntax is:

void Reset();

When implementing this method, simply assign a non-existing value, which is usually -1, to the monitoring tag of the current item. This can be done as follows:

public class Enumerator : IEnumerator
{
    private double[] numbers;
    private int cur;

    public Enumerator(double[] list)
    {
        this.numbers = list;
        cur = -1;
    }

    public Object Current
    {
        get { return numbers[cur]; }
    }

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

When using the implementer of the IEnumerator interface, if you try accessing an item beyond the maximum number of items, the compiler would throw an IndexOutOfRangeException exception. For this reason, when anticipating a bad behavior, you should catch this exception when implementing the Current property.

1. In the FlowerIdentifier.cs file, implement the IEnumerator.Reset() method as follows:
    

   using System; using System.Collections; namespace FlowerShop5 { public class FlowerIdentifier : IEnumerator { private int curPosition = -1; public FlowerInventory counts; public void Identify(FlowerInventory list) { this.counts = list; } public Object Current { get { try { return this.counts.Get(this.curPosition); } catch (IndexOutOfRangeException) { Console.WriteLine("The current item must be accessed " + "within the range of available items"); } } } public void Reset() { this.curPosition = -1; } } }

2. Save the file

In the previous lesson, we saw that, when using the items of a collection, one way you can locate one item from another is to be able to jump from one item to the next. This operation is also very important when enumerating a collection. To support this operation, the IEnumerator is quipped with the MoveNext() method. Its syntax is:

bool MoveNext();

When implementing this method, first increment the tag that monitors the current item of the collection. After incrementing the tag, check whether it is lower than the total number of items. If it is, return true. Otherwise, return false. This can be done as follows:

public class Enumerator : IEnumerator
{
    private double[] numbers;
    private int cur;

    public Enumerator(double[] list)
    {
        this.numbers = list;
        cur = -1;
    }

    public Object Current
    {
        get { return numbers[cur]; }
    }

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

    public bool MoveNext()
    {
        cur++;
        if (cur < numbers.Length)
            return true;
        else
            return false;
    }
}

1. Implement the IEnumerator.MoveNext() method as follows:
    

   using System; using System.Collections; namespace FlowerShop5 { public class FlowerIdentifier : IEnumerator { private int curPosition = -1; public FlowerInventory counts; public void Identify(FlowerInventory list) { this.counts = list; } public Object Current { get { try { return this.counts.Get(this.curPosition); } catch (IndexOutOfRangeException) { Console.WriteLine("The current item must be accessed " + "within the range of available items"); } return null; } } public void Reset() { this.curPosition = -1; } public bool MoveNext() { this.curPosition++; if (this.curPosition < counts.Count) return true; else return false; } } }

2. Save the file