IEnumerator

If you don't have to use IEnumerator, then you can simply use the generic List's IndexOf() method to find the index (zero-based) of a given object:

int index = l.IndexOf(c);

Notice that if the same object is added more than once to the List, then IndexOf() returns the index of the first such object.

sorry but this doesn't solve my problem:
class myclass {
    public myclass  {     
    }  
}
//main
myclass a,b,c,d;
   List l= new ........
l.Add(a); l.Add(b); l.Add(c); l.Add(d);
int index = c.getPositionInList();

Is there a way to retrieve index?

thanks.

You can't stop MoveNext() starting from the beginning (i.e. an index of 0) but you can skip the first 6 iterations using this code:

        for (int i = 0 ;ie.MoveNext(); i++)
       {
         if (i < 6) continue;  // skips elements 0 to 5 inclusive of the List

         if (ie.Current.Name.ToLower() == "marco")
         {
            index = i;
            break;
         }    
       }

but a way to do this, is there?
ie+6 //avoid movenext() for 6 times;

?

As the List.Enumerator class (the underlying IEnumerator for the List class) doesn't expose an 'Index' property, then the best way to do this is to count the elements yourself:

      IEnumerator ie = l.GetEnumerator();
            
       int index = -1;
 
       for (int i = 0 ;ie.MoveNext(); i++)
       {
         if (((myClass)ie.Current).Name.ToLower() == "marco")
         {
            index = i;
            break;
         }    
       }

       Console.WriteLine(index); // 0

 

Incidentally, you can avoid having to cast the IEnumerator to myClass by using the generic interface IEnumerator instead:

       IEnumerator ie = l.GetEnumerator();
            
       int index = -1;
 
       for (int i = 0 ;ie.MoveNext(); i++)
       {
         if (ie.Current.Name.ToLower() == "marco")
         {
            index = i;
            break;
         }    
       }

       Console.WriteLine(index);  // 0

thanks a lot!
And if I wanna do this how to do?
for ( ;ie.MoveNext(); )
       {
                if ((myClass)ie.Current).Name == "marco"
                                  int index = (int) ie;
       }

Is there a way ?

Try this:

using System;
using System.Collections;
using System.Collections.Generic;

class Program
{
     static void Main()
     {
       List l = new List();
       myClass m = new myClass();
       m.Name  = "Marco";     
       l.Add(m);
       m = new myClass();
       m.Name = "Alan";
       l.Add(m);
       IEnumerator ie = l.GetEnumerator();

       for ( ;ie.MoveNext(); )
       {
         Console.WriteLine(((myClass)ie.Current).Name);
       }
 
       Console.ReadKey(); 
     }
}

public class myClass
{
    string name;
 
    public string Name
    {
       get {return name;}
       set {name = value;}
    }
}