Introduction
In this article we will discuss about theIEnumerable interface. We will discuss how IEnumerable
interface facilitate the use of foreach statement to iterate through a set of data. We will then look how to implement our own collections that implement IEnumerable interface. The use of yield keyword and
Enumerating generic collections will also be discussed in this article.Background
Whenever we work with collection of objects, we might find ourselves in need to iterate the collection. The best way to iterate through a collection is by implementing the Iterator pattern. (refer: Understanding and Implementing the Iterator Pattern in C# and C++[^]). C# provides a very clean construct offoreach statement to iterate in a read only manner over a collection. C# also provides us the possibility of using the same
foreach construct and all the enumeration techniques
on our custom collection objects by implementing the IEnumerable interface. So let us see how we can implement IEnumerable interface with our custom collection classes.Using the code
Enumerating the Collection classes
Before starting the discussion let us see how we can use the Built-in classes and iterate over them. Lets start by looking into theArrayList class that implements IEnumerable and see how we can have read only iteration
over that using foreach statement.// Let us first see how we can enumerate an object implementing IEnumerable
ArrayList list = new ArrayList();
list.Add("1");
list.Add(2);
list.Add("3");
list.Add('4');
foreach (object s in list)
{
Console.WriteLine(s);
}
Enumerating the Generic Collection classes
TheArraylist class is a generalized class that let us
keep a collection. We can also have a generic class in which we can
provide the type along with the data. Iterating over generic collection
classes is also possible because they implement IEnumerable<T> interface. Lets see how we can enumerate over a generic collection. // Let us first see how we can enumerate an object implementing IEnumerable<T>
List<string> listOfStrings = new List<string>();
listOfStrings.Add("one");
listOfStrings.Add("two");
listOfStrings.Add("three");
listOfStrings.Add("four");
foreach (string s in listOfStrings)
{
Console.WriteLine(s);
}
Now our objective is to have our own custom collection class and a generic collection class that should implement the IEnumerable and IEnumerable<T> interface respectively to provide the possibility
of enumerating over them.Understanding the yield keyword
Before jumping into the Implementation of these classes, we need to understand a very important keywordyield which actually facilitate the enumeration over collection. yield statement is used while returning a value from a function.A normal method call like the one shown below will return only the first value no matter how many times it is called.
static int SimpleReturn()
{
return 1;
return 2;
return 3;
}
static void Main(string[] args)
{
// Lets see how simeple return works
Console.WriteLine(SimpleReturn());
Console.WriteLine(SimpleReturn());
Console.WriteLine(SimpleReturn());
Console.WriteLine(SimpleReturn());
}
The reason for this is that the normal return statement does not
preserve the state of the function while returning. i.e. every call to
this function is a new call and it will return the first value only.Where as if I replace the return keyword by yield return then the function will become capable of saving its state while returning the value. i.e. when the function is called second time, it will continue the processing from where is has returned in the previous call.
static IEnumerable<int> YieldReturn()
{
yield return 1;
yield return 2;
yield return 3;
}
static void Main(string[] args)
{
// Lets see how yield return works
foreach (int i in YieldReturn())
{
Console.WriteLine(i);
}
}
When we run the above code it will return 1,2 and then 3. The only catch while using the yield return statement is that the function should return an IEnumerable and should be called from an iteration block
i.e. foreach statement.Implementing IEnumerable in our custom Collection class
Now in our custom collection classes, if we define a function that will iterate over all the elements in the collection and return then using theyield keyword, we will be able to get hold of all the elements in the the collection.So let us define our own
MyArrayList class and implement IEnumerable interface, which will force us to implement the GetEnumerator function. This function will iterate over the collection and do a yield return on all the elements.class MyArrayList : IEnumerable
{
object[] m_Items = null;
int freeIndex = 0;
public MyArrayList()
{
// For the sake of simplicity lets keep them as arrays
// ideally it should be link list
m_Items = new object[100];
}
public void Add(object item)
{
// Let us only worry about adding the item
m_Items[freeIndex] = item;
freeIndex++;
}
// IEnumerable Member
public IEnumerator GetEnumerator()
{
foreach (object o in m_Items)
{
// Lets check for end of list (its bad code since we used arrays)
if(o == null)
{
break;
}
// Return the current element and then on next function call
// resume from next element rather than starting all over again;
yield return o;
}
}
}
This class will now let us enumerate all the elements using a foreach stemement.static void Main(string[] args)
{
//Let us now go ahead and use our custom MyArrayList with IEnumerable implemented
MyArrayList myList = new MyArrayList();
myList.Add("1");
myList.Add(2);
myList.Add("3");
myList.Add('4');
foreach (object s in myList)
{
Console.WriteLine(s);
}
}
Note: This class is neither complete not a very good
implementation. The only purpose of the sample implementation is to
demonstrate the implementation of IEnumerable interface.Implementing IEnumerable<T> in our custom Generic Collection class
Let us now take this approach a little further and define a generic collection class capable of being enumerated. To do this we need to implementIEnumerable<T> interface.class MyList<T> : IEnumerable<T>
{
T[] m_Items = null;
int freeIndex = 0;
public MyList()
{
// For the sake of simplicity lets keep them as arrays
// ideally it should be link list
m_Items = new T[100];
}
public void Add(T item)
{
// Let us only worry about adding the item
m_Items[freeIndex] = item;
freeIndex++;
}
#region IEnumerable<T> Members
public IEnumerator<T> GetEnumerator()
{
foreach (T t in m_Items)
{
// Lets check for end of list (its bad code since we used arrays)
if (t == null) // this wont work is T is not a nullable type
{
break;
}
// Return the current element and then on next function call
// resume from next element rather than starting all over again;
yield return t;
}
}
#endregion
#region IEnumerable Members
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
// Lets call the generic version here
return this.GetEnumerator();
}
#endregion
}
This class will now let us enumrate all the elements using a foreach stemement.
|
static void Main(string[] args)
{
// Let us first see how we can enumerate an custom MyList<t> class implementing IEnumerable<T>
MyList<string> myListOfStrings = new MyList<string>();
myListOfStrings.Add("one");
myListOfStrings.Add("two");
myListOfStrings.Add("three");
myListOfStrings.Add("four");
foreach (string s in myListOfStrings)
{
Console.WriteLine(s);
}
}
</t>
So now we have a collection class and a generic collectio class that implement IEnumerable and
IEnumerable<T> respectively. Althogh These class is neither complete not a very good implementation but
they do serve the purpose of the article i.e. to demonstrate the implementation of IEnumerable interface.Point of interest
What we have tried to do in this article is see how can we implement theIEnumerable and IEnumberable<T>
interface. We have looked into the significance of yield keyword. This
stuff is known to most experienced C# programmers but the beginners
might find it useful. I hope this has been informative.
The
JOIN query operator compares the specified properties/keys of two
collections for equality by using the EQUALS keyword. By default, all
joins queries written by the JOIN keyword are treated as equijoins. 
Basically, GROUP JOIN is like as INNER-EQUIJOIN except that the result sequence is organized into groups.
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