Question regarding IEnumerable and IEnumerator

Question

I use the following code to enable myClass to use foreach. But I am rather new to programming and have some difficulty in understanding the following code. I described my problems in the comments. I would be grateful for providing some information.

    public class MyClass : IEnumerable<string> 
    {  
    //1) What is IEnumerator for?
        // Whats the difference between IEnumerator and IEnumerable
    public IEnumerator<string> GetEnumerator()     
    {
             yield return "first";         
             yield return "second";     
    }      
    //2) What is it for?  It just calls above method 
   IEnumerator IEnumerable.GetEnumerator()
         {
             return GetEnumerator(); 
         } 
    }
   //3) Lastly what benefits I have from implementing genetic interface 
   //IEnumerable<string> instead of just IEnumerable

Answer 1

What is the difference between IEnumerator and IEnumerable?

Jason's answer is good but I thought I'd just add how I think about this. Imagine you have a sequence:

1, 1, 2, 3, 5, 8, 13, ...

Now imagine you have an arrow pointing at some position of that sequence:

1, 1, 2, 3, 5, 8, 13, ...
         ^

An "arrow" is an object that can do two things. First, it can give you the thing it is pointing at. Second, it can make itself point at the next thing.

IEnumerator is an arrow. It has a property, Current, that gives you the thing it is pointing at. It has a method, MoveNext() that makes itself point at the next thing.

How do you get an arrow in the first place? You need an arrow factory. You ask the factory for an arrow, and it gives you an arrow that points to the first element in the sequence.

IEnumerable is an arrow factory. It has a method, GetEnumerator, that gives you an arrow to the first element of the sequence.

A nice property of this scheme is that you can have multiple arrows pointing to different places in the same sequence.

what are the benefits of implementing generic interface IEnumerable instead of just IEnumerable?

Suppose the sequence is of integers. If you implement IEnumerable then when you say

foreach(int x in mysequence)

what that will actually do is convert the int in the sequence to object, boxing the integer, and then immediately unbox the object back to integer, adding a completely unnecessary memory allocation to every single operation. If the compiler knows that the sequence is of integers then it can skip the unnecessary boxing operation.

Suppose the sequence is of strings. If you implement IEnumerable<string> then you can say:

string first = mysequence.First();

If you don't, then you have to say

string first = (string)mysequence.First();

which is unnecessary and error-prone. Rather than instruct the compiler via a cast that the type is string, you can simply guarantee that the type is string by using the type system.

Answer 2

1) What is IEnumerator for? Whats the difference between IEnumerator and IEnumerable?

IEnumerator is an interface that represents methods that let you enumerate a sequence. The difference between IEnumerator and IEnumerable is that the former represents the contract for objects that let you enumerate a sequence, and the latter represents the contract for objects that are a sequence that can be enumerated over.

public IEnumerator<string> GetEnumerator() {
         yield return "first";         
         yield return "second";     
}      


IEnumerator IEnumerable.GetEnumerator() {
    return GetEnumerator(); 
} 

2) What is it for? It just calls above method.

The former represents an implementation of the method GetEnumerator on the contract IEnumerable<string>. The latter represents an explicit implementation of the method GetEnumerator on the contract IEnumerable. The issue is that both contracts have a method named GetEnumerator but with different return types so that a method can't simultaneously satisfy both contracts (any class implementing IEnumerable<T> must also implement IEnumerable as IEnumerable<T> : IEnumerable). The latter invokes the implementation of IEnumerable<string>.GetEnumerator as that is a sensible implementation that returns an IEnumerator as IEnumerator<string> : IEnumerator.

3) Lastly what benefits I have from implementing generic interface IEnumerable<string> instead of just IEnumerable?

Strong typing. You know that the elements in a sequence IEnumerable<string> are all instances of String whereas you don't know that for IEnumerable and could end up trying to cast an element of the latter to an instance of String when it can not be.

Answer 3

This is the declaration for IEnumerable<T>:

public interface IEnumerable<out T> : IEnumerable
{
    IEnumerator<T> GetEnumerator();
}

You have no choice, you have to implement the non-generic IEnumerable interface as well. You'll get a compile error if you omit the IEnumerable.GetEnumerator() implementation. This is baggage left over from the .NET 1.x days where generics weren't available yet and the transition period for .NET 2.0 where it needed to support interop with .NET 1.x assemblies. We're stuck with it.

Answer 4

1) What is IEnumerator for?

IEnumerator is the real working part with the MoveNext() and Current members.

Whats the difference between IEnumerator and IEnumerable

IEnumerable is the interface for a collection to signal that it has an GetEnumerator().

2) [non-generic GetEnumerator] What is it for? It just calls above method

The non-generic method is just there for backward compatibility. Note that it is moved 'out of sight' as much as possible by use of the explicit implementation. Implement it because you must and then forget about it.

3) Lastly what benefits I have from implementing genetic interface IEnumerable instead of just IEnumerable

When used with foreach the adavantage is small, as foreach will type-cast the looping variable. It will let you use var in the foreach:

foreach (var s in myClassInstance)     // needs `IEnumerable<string>` 
foreach (string s in myClassInstance)  // works with  `IEnumerable` as well 

But with IEnumerable<string> you also have a type-safe interface to other areas, most notably LINQ:

MyClass mc = new MyClass ();
string s = mc.FirstOrDefault();

Answer 5

GetEnumerator has been around since before generics were introduced to the language, so as to not break pre-existing code, the default method calls through to the generic implementation.

With a generic enumeration, the consumer of your class doesn't need to cast each item to string when iterating.

Answer 6

Regarding the 3rd question (why use generics?) here are some answers:

Should we use Generic Collection to improve safety and performance?

In short, use generic collections every time you can.