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Implementing IEnumerator and IEnumerable Interfaces using Inner Class in C#

By chandrakant upadhyay on Feb 28, 2002
.NET framework provides IEnumerable and IEnumerator interfaces to implement collection like behavior to user defined classes.

.NET framework provides IEnumerable and IEnumerator interfaces to implement collectionlike behavior to user defined classes. A developer can implement these interfaces to provide collection like behavior to their classes. These interfaces are implemented through inner classes.

An inner class ( also known as Nested Type ) is a class which is enclosed inside another class.

class A
{
int i ;
class B // Class B is a inner class or Nested Type
{
}
}

IEnumerator and IEnumerable interfaces are defined in System.Collections namespace as :

public interface IEnumerable
{
IEnumerator GetEnumerator();
//Returns a Enumerator
}
public interface IEnumerator
{
bool MoveNext(); //An enumerator is always positioned before the
//first element of the collection, first call to MoveNext
//moves the enumerator over the first element of the
//collection
object Current { get ; } //Returns current object from the collection
void Reset(); //Resets enumerator to just above the first element of the collection.
}

For more details about above interfaces, refer Microsoft .NET framework documentation.

Above two interfaces must be implemented to provide a collection (similar to ArrayList ) like behavior to user defined classes. There is one more interface called Collection interface which I am excluding from our discussion.

To demonstrate the working of above interfaces and inner class I have created a class called ItemCollection which implements IEnumerable interface.

Within ItemCollection class there is one more class ( inner class or Nested Type ) called ItemIterator which implements IEnumerator interface. ItemCollection class also contains a string array itemId, which provides the basis for iteration.

For implementation details, see the code below and follow these steps :

1. Create a file ItemCollection.cs and save the source in that.

//File ItemCollection.cs
using System;
using System.Collections;
//Class ItemCollection implements IEnumerable interface
class ItemCollection : IEnumerable
{
String[] itemId ;
//Constructor to create and populate itemId String array
public ItemCollection( int noOfItem )
{
itemId =
new String[noOfItem] ;
for(int i = 0; i < itemId.Length; i ++ )
{
itemId[i] = i.ToString();
}
}
//Implementation of method GetEnumerator of IEnumerable interface
public virtual IEnumerator GetEnumerator()
{
return new ItemIterator(this);
}
//Inner class ItemIterator, implements IEnumerator
public class ItemIterator : IEnumerator
{
//Declare a variable of type ItemCollection,
//to keep reference to enclosing class instance
private ItemCollection itemCollection;
//Declare a integer pointer and Set to -1, so that
//first call to MoveNext moves the enumerator over
//the first element of the collection.
private int index = -1 ;
//Pass an instance of enclosing class
public ItemIterator(ItemCollection ic)
{
//Save enclosing class reference
itemCollection = ic ;
}
//After an enumerator is created or after a Reset,
//an enumerator is positioned before the first element
//of the collection, and the first call to MoveNext
//moves the enumerator over the first element of the
//collection.
public bool MoveNext()
{
index++ ;
if( index < itemCollection.itemId.Length )
{
return true ;
}
else
{
index = -1;
return false;
}
}
//Return the current object, in our case Item Id string
//from itemId[] array. Throws InvalidOperationException exception
//if index pointing to wrong position
public object Current
{
get
{
if( index <= -1 )
{
throw new InvalidOperationException() ;
}
return itemCollection.itemId[index];
}
}
//Reset pointer to -1
public void Reset()
{
index = -1;
}
}
public static int Main(String[] args)
{
//Instantiate the collection
ItemCollection itemCol = new ItemCollection(10);
//Iterate the collection with various looping construct
//provided in c#
Console.WriteLine("1. Iteration using foreach loop:");
foreach( String itemIdStr in itemCol)
{
Console.Write(itemIdStr + " " );
}
Console.WriteLine("\n\n2. Iteration using for loop:");
for(IEnumerator ie = itemCol.GetEnumerator() ;
ie.MoveNext();)
{
Console.Write(ie.Current + " " );
}
Console.WriteLine("\n\n3. Iteration using while loop:");
IEnumerator ie1 = itemCol.GetEnumerator();
while(ie1.MoveNext())
{
Console.Write(ie1.Current + " ");
}
return 0;
}
}

2. Compile the above code as

csc ItemCollection.cs

3. Run the file ItemCollection.exe to see following output in console window :

1. Iteration with foreach loop:
    0 1 2 3 4 5 6 7 8 9

2. Iteration with for loop:
    0 1 2 3 4 5 6 7 8 9

3. Iteration with while loop:
    0 1 2 3 4 5 6 7 8 9

Disclaimer: The above program is intended solely for learning purpose. Author will not be held responsible for any failure or damage caused due to any other usage.

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