Implementing IEnumerator without using 'yield return' in c#

Question

I'm new to C# programming, I was experimenting with iterators concept in C#. Here, I'm trying to display all the terms in a list, for that I'm trying different ways to obtain the results. In the below code, I'm using two classes ListIterator and ImplementList.

In the ListIterator class : I defined a HashSet and it uses IEnumerator to store the values. Here GetEnumerator() method returns the values in the list. GetEnumerator is implemented in the ImplementList class (other class). Finally, the list is displayed in the console.

public class ListIterator
{ 
 public void DisplayList()
 {
 HashSet<int> myhashSet = new HashSet<int> { 30, 4, 27, 35, 96, 34};
 IEnumerator<int> IE = myhashSet.GetEnumerator();
 while (IE.MoveNext())
 {
 int x = IE.Current;
 Console.Write("{0} ", x);
 }
 Console.WriteLine();
 Console.ReadKey();
 }
}

In the ImplementList class : GetEnumerator() is defined and it returns the list using yield return x.

public class ImplementList : IList<int>
 {
 private List<int> Mylist = new List<int>();
 public ImplementList() { }
 public void Add(int item) 
 { 
 Mylist.Add(item); 
 }
 public IEnumerator<int> GetEnumerator()
 {
 foreach (int x in Mylist)
 yield return x;
 }
 }

Now, I want to rewrite the GetEnumerator() without using yield return. And it should return all the values in a list. I tried using for loop as for(int x=0; x<Mylist.Count; x++), but it doesn't return all the values in the list.

Is it possible to get all the values in the list without using yield return in IEnumerator

• giri: I know it is not efficient, but I just want to try it. Is it possible to implement without yield return??
• Robert Harvey: How Yield Return is implemented internally
• Eugene Podskal: If you really want to implement it manually without yield you may want to take a look at this MSDN code example. But in real scenario it would probably be better to just return MylList.GetEnumerator() - much more clear and effective than both the yield and custom enumerator.

Answer 1

MoveNext() is a method that is called over and over and every time it has to move to the next item. This means you can't implement it by simply using a for loop (unless you use yield return, which is pretty much the reason why yield return exists), you need to rewrite it so that each MoveNext() call executes just part of that loop.

Specifically, it would look like this:

class Enumerator : IEnumerator<int>
{
 private int i = -1;
 private ImplementList list;
 public Enumerator(ImplementList list)
 {
 this.list = list;
 }
 public bool MoveNext()
 {
 i++;
 return i < list.myList.Count;
 }
 public int Current { get { return list.myList[i]; } }
 object IEnumerator.Current { get { return Current; } }
 public void Dispose() {}
 public void Reset() { throw new NotSupportedException(); }
}

This way, every call to MoveNext() performs the i++ and i < list.Count parts of the for loop.

• giri: Will that work with my for loop, I mean (for(int x=0; x<Mylist.count; x++))..
• svick: @GIRI You mean when Mylist is ImplementList? No, why would that use IEnumerable or IEnumerator? You're just incrementing an integer.
• giri: So, when a GetEnumerator is invoked. How we are going to return the values?
• svick: @GIRI GetEnumerator() is invoked either when you use foreach or when you manually call it (though this is very rare in normal code). If you want to use your collection in a for loop, you should write an indexer, and possibly also implement something like IList<T> or IReadOnlyList<T>.
• giri: I'm a learner, can you please give a clear explanation. @svick