Can anyone explain IEnumerable and IEnumerator to me? [closed]
for example, when to use it over foreach?
You don't use IEnumerable "over" foreach. Implementing IEnumerable makes using foreach possible.
When you write code like:
foreach (Foo bar in baz)
{
...
}
it's functionally equivalent to writing:
IEnumerator bat = baz.GetEnumerator();
while (bat.MoveNext())
{
bar = (Foo)bat.Current
...
}
By "functionally equivalent," I mean that's actually what the compiler turns the code into. You can't use foreach on baz in this example unless baz implements IEnumerable.
IEnumerable means that baz implements the method
IEnumerator GetEnumerator()
The IEnumerator object that this method returns must implement the methods
bool MoveNext()
and
Object Current()
The first method advances to the next object in the IEnumerable object that created the enumerator, returning false if it's done, and the second returns the current object.
Anything in .Net that you can iterate over implements IEnumerable. If you're building your own class, and it doesn't already inherit from a class that implements IEnumerable, you can make your class usable in foreach statements by implementing IEnumerable (and by creating an enumerator class that its new GetEnumerator method will return).
The IEnumerable and IEnumerator Interfaces
To begin examining the process of implementing existing .NET interfaces, let’s first look at the role of IEnumerable and IEnumerator. Recall that C# supports a keyword named foreach that allows you to iterate over the contents of any array type:
// Iterate over an array of items.
int[] myArrayOfInts = {10, 20, 30, 40};
foreach(int i in myArrayOfInts)
{
Console.WriteLine(i);
}
While it might seem that only array types can make use of this construct, the truth of the matter is any type supporting a method named GetEnumerator() can be evaluated by the foreach construct.To illustrate, follow me!
Suppose we have a Garage class:
// Garage contains a set of Car objects.
public class Garage
{
private Car[] carArray = new Car[4];
// Fill with some Car objects upon startup.
public Garage()
{
carArray[0] = new Car("Rusty", 30);
carArray[1] = new Car("Clunker", 55);
carArray[2] = new Car("Zippy", 30);
carArray[3] = new Car("Fred", 30);
}
}
Ideally, it would be convenient to iterate over the Garage object’s subitems using the foreach construct, just like an array of data values:
// This seems reasonable ...
public class Program
{
static void Main(string[] args)
{
Console.WriteLine("***** Fun with IEnumerable / IEnumerator *****\n");
Garage carLot = new Garage();
// Hand over each car in the collection?
foreach (Car c in carLot)
{
Console.WriteLine("{0} is going {1} MPH",
c.PetName, c.CurrentSpeed);
}
Console.ReadLine();
}
}
Sadly, the compiler informs you that the Garage class does not implement a method named GetEnumerator(). This method is formalized by the IEnumerable interface, which is found lurking within the System.Collections namespace. Classes or structures that support this behavior advertise that they are able to expose contained subitems to the caller (in this example, the foreach keyword itself). Here is the definition of this standard .NET interface:
// This interface informs the caller
// that the object's subitems can be enumerated.
public interface IEnumerable
{
IEnumerator GetEnumerator();
}
As you can see, the GetEnumerator() method returns a reference to yet another interface named System.Collections.IEnumerator. This interface provides the infrastructure to allow the caller to traverse the internal objects contained by the IEnumerable-compatible container:
// This interface allows the caller to
// obtain a container's subitems.
public interface IEnumerator
{
bool MoveNext (); // Advance the internal position of the cursor.
object Current { get;} // Get the current item (read-only property).
void Reset (); // Reset the cursor before the first member.
}
If you want to update the Garage type to support these interfaces, you could take the long road and implement each method manually. While you are certainly free to provide customized versions of GetEnumerator(), MoveNext(), Current, and Reset(), there is a simpler way. As the System.Array type (as well as many other collection classes) already implements IEnumerable and IEnumerator, you can simply delegate the request to the System.Array as follows:
using System.Collections;
...
public class Garage : IEnumerable
{
// System.Array already implements IEnumerator!
private Car[] carArray = new Car[4];
public Garage()
{
carArray[0] = new Car("FeeFee", 200);
carArray[1] = new Car("Clunker", 90);
carArray[2] = new Car("Zippy", 30);
carArray[3] = new Car("Fred", 30);
}
public IEnumerator GetEnumerator()
{
// Return the array object's IEnumerator.
return carArray.GetEnumerator();
}
}
After you have updated your Garage type, you can safely use the type within the C# foreach construct. Furthermore, given that the GetEnumerator() method has been defined publicly, the object user could also interact with the IEnumerator type:
// Manually work with IEnumerator.
IEnumerator i = carLot.GetEnumerator();
i.MoveNext();
Car myCar = (Car)i.Current;
Console.WriteLine("{0} is going {1} MPH", myCar.PetName, myCar.CurrentSpeed);
However, if you prefer to hide the functionality of IEnumerable from the object level, simply make use of explicit interface implementation:
IEnumerator IEnumerable.GetEnumerator()
{
// Return the array object's IEnumerator.
return carArray.GetEnumerator();
}
By doing so, the casual object user will not find the Garage’s GetEnumerator() method, while the foreach construct will obtain the interface in the background when necessary.
Adapted from the Pro C# 5.0 and the .NET 4.5 Framework
Implementing IEnumerable means your class returns an IEnumerator object:
public class People : IEnumerable
{
IEnumerator IEnumerable.GetEnumerator()
{
// return a PeopleEnumerator
}
}
Implementing IEnumerator means your class returns the methods and properties for iteration:
public class PeopleEnumerator : IEnumerator
{
public void Reset()...
public bool MoveNext()...
public object Current...
}
That's the difference anyway.