C++ Programmer's Cookbook

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Implementing IEnumerator and IEnumerable Interfaces using Inner Class in C#(very good)

.NET framework provides IEnumerable and IEnumerator interfaces to implement collection
like 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


posted on 2006-04-11 12:04 梦在天涯 阅读(876) 评论(0)  编辑 收藏 引用 所属分类: C#/.NET


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