How to use ‘foreach’ in IEnumerator?

Ken H

Hello friend,

Questions are as follows:

private void button1_Click(object sender, EventArgs e){

List list = new List();
for (int i = 1; i <= 10; i++) list.Add(i);
IEnumerable enumer = list;
foreach (int item in enumer)
{
textBox1.Text += item.ToString() + "|";
}
IEnumerator enumer2 = list.GetEnumerator();
while (enumer2.MoveNext())
{
textBox2.Text += enumer2.Current.ToString() + "|";
}
IEnumerator enumer3 = list.GetEnumerator();
foreach (int item in enumer3) // Try use foreach,but compilation errors in here.
{
textBox3.Text += item.ToString() + "|";
}

}

Thanks.

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vulpes11y ago
Accepted answer
You can only use foreach on an object which doesn't implement either IEnumerable or IEnumerable if it has a GetEnumerator() method which returns an 'enumerator'.

An enumerator for this purpose is any object which has a Current property and a MoveNext() method. The object doesn't necessarily have to implement IEnumerator or IEnumerator.

Here's an example of such an object:

using System;

class MyClass
{
public MyClassEnumerator GetEnumerator()
{
return new MyClassEnumerator(this);
}

public MyClass(string[] names)
{
Names = names;
}

public string[] Names {get; set;}
}

class MyClassEnumerator
{
public string Current
{
get{ return mc.Names[index]; }
}

public bool MoveNext()
{
++index;
if(index == mc.Names.Length)
{
return false;
}

return true;
}

public MyClassEnumerator(MyClass mc)
{
this.mc = mc;
index = -1;
}

private MyClass mc;
private int index;
}

class Program
{
static void Main()
{
string[] names = {"Ken", "Michal", "Vulpes"};
MyClass mc = new MyClass(names);

foreach(string name in mc)
{
Console.WriteLine(name);
}

Console.ReadKey();
}
}

The output is:

Ken
Michal
Vulpes
Ken H11y ago
I think I'm clear now. the 'foreach' it will auto bind to GetEnumerator() method,and its return type of the need to implement 'Current' property and MoveNext methods?

Thank for you. :)
Ken H11y ago
Vulpes,
If the object does not inherit IEnumerable can be used 'foreach'?
Does that mean that all objects inherit the IEnumerator interface can only use MoveNext () method?

I am still confused IEnumerable and IEnumerator.
Vulpes11y ago
The reason that code doesn't work is because list.GetEnumerator() returns a struct of type List.Enumerator.

You can't use foreach to iterate through that struct because it's not a collection and so doesn't implement either IEnumerable or the non-generic IEnumerable.

You can use it to iterate through the underlying List using a while statement but, as you've already done that with enumer2, there's no point in doing it again.

FWIW you can also use it to iterate though the list using an 'ordinary' for statement:

IEnumerator enumer3 = list.GetEnumerator();

for(bool b = enumer3.MoveNext(); b ; b = enumer3.MoveNext())
{
textBox3.Text += enumer3.Current.ToString() + "|";
}

Where you might be getting confused here is that the IEnumerable interface has a single member, GetEnumerator(), which returns an IEnumerator.

So we always know that if a collection implements IEnumerable it will expose an IEnumerator which we can use to iterate through the collection.
Michal Habalcik11y ago
well, those are 2 interfaces that have to be implemented to iterate through non-generic collection, so not your case. You should just remove them and work with the list directly as it has both these implemented.
Ken H11y ago
yes,i need to know the relationship between 'IEnumerable' and 'IEnumerator'.
Michal Habalcik11y ago
hi, is the enumerator really necessary?

why not just:

foreach (var item in list)
{
textBox3.Text += item.ToString() + "|";
}