Define custom enumerators and use foreach to loop through : IEnumerator : Data Structure C# Examples

    
//  Code  revised  from  
//A  Programmer's  Introduction  to  C#  2.0,  Third  Edition

using  System;
using  System.Collections;

//  Note:  This  class  is  not  thread-safe
public  class  IntList:  IEnumerable
{
        int[]  values  =  new  int[10];
        int  allocated  =  10;
        int  count  =  0;
        int  revision  =  0;
        
        public  void  Add(int  value)
        {
                //  reallocate  if  necessary
                if  (count  +  1  ==  allocated)
                {
                        int[]  newValues  =  new  int[allocated  *  2];
                        for  (int  index  =  0;  index  <  count;  index++)
                        {
                                newValues[index]  =  values[index];
                        }
                        allocated  *=  2;
                }                
                values[count]  =  value;
                count++;
                revision++;
        }
        
        public  int  Count
        {
                get
                {
                        return(count);
                }
        }
        
        void  CheckIndex(int  index)
        {
                if  (index  >=  count)
                throw  new  ArgumentOutOfRangeException("Index  value  out  of  range");
        }
        
        public  int  this[int  index]
        {
                get
                {
                        CheckIndex(index);
                        return(values[index]);
                }
                set
                {
                        CheckIndex(index);
                        values[index]  =  value;
                        revision++;
                }
        }
        
        public  IEnumerator  GetEnumerator()
        {
                return(new  IntListEnumerator(this));
        }
        
        internal  int  Revision
        {
                get
                {
                        return(revision);
                }
        }
}

class  IntListEnumerator:  IEnumerator
{
        IntList        intList;
        int  revision;
        int  index;
        
        internal  IntListEnumerator(IntList  intList)
        {
                this.intList  =  intList;
                Reset();
        }
        
        public  bool  MoveNext()
        {
                index++;
                if  (index  >=  intList.Count)
                return(false);
                else
                return(true);
        }
        
        public  object  Current
        {
                get
                {
                        if  (revision  !=  intList.Revision)
                        throw  new  InvalidOperationException("Collection  modified  while  enumerating.");
                        return(intList[index]);
                }
        }
        
        public  void  Reset()
        {
                index  =  -1;
                revision  =  intList.Revision;
        }
}

class  MainClass
{
        public  static  void  Main()
        {
                IntList  list  =  new  IntList();
                
                list.Add(1);
                list.Add(55);
                list.Add(43);
                
                foreach  (int  value  in  list)
                {
                        Console.WriteLine("Value  =  {0}",  value);
                }
                
                foreach  (int  value  in  list)
                {
                        Console.WriteLine("Value  =  {0}",  value);
                        list.Add(124);
                }
        }
}