Define custom enumerators and use foreach to loop through : IEnumerator « Data Structure « C# / CSharp Tutorial

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C# / CSharp Tutorial » Data Structure » IEnumerator 
11.44.6.Define custom enumerators and use foreach to loop through
// 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 + == 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);
        }
    }
}
Value = 1
Value = 55
Value = 43
Value = 1

Unhandled Exception: System.InvalidOperationException: Collection modified while enumerating.
   at IntListEnumerator.get_Current()
   at MainClass.Main()
11.44.IEnumerator
11.44.1.A simple example of an iterator.
11.44.2.Iterated values can be dynamically constructed.
11.44.3.Use named iterators
11.44.4.Use the Enumerable pattern
11.44.5.Implement IEnumerable and IEnumerator
11.44.6.Define custom enumerators and use foreach to loop through
11.44.7.Build your own IEnumerator/IEnumerable and use it in foreach loop
11.44.8.Circular Iterator
11.44.9.Iteration Sample
11.44.10.Iterator Workflow
11.44.11.IEnumerator and ArrayList, BitArray, Hashtable and array
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