How to make a Visual C# class usable in a foreach statement

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IEnumerator interface

• The MoveNext method, which increments the collection index by 1 and returns a bool that indicates whether the end of the collection has been reached.
• The Reset method, which resets the collection index to its initial value of -1. This invalidates the enumerator.
• The Current method, which returns the current object at [position].

      public bool MoveNext()
      {
         position++;
         return (position < carlist.Length);
      }

      public void Reset()
      {position = 0;}

      public object Current
      {
         get { return carlist[position];}
      }

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IEnumerable interface

 public IEnumerator GetEnumerator()
      {
         return (IEnumerator)this;
      }

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When to use which interface

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Step by step example

1. Follow these steps to create a new Console Application project in Visual C#:
   1. Start Microsoft Visual Studio .NET or Microsoft Visual Studio 2005.
   2. On the File menu, point to New, and then click Project.
   3. Click Visual C# Projectsunder Project Types, and then click Console Application under Templates.
      
      Note In Visual Studio 2005, click Visual C# under Project Types.
   4. In the Name box, type ConsoleEnum.
2. Rename Class1.cs to host.cs, and then replace the code in host.cs with the following code:

   using System;
   
   namespace ConsoleEnum
   {
     class host
     {
       [STAThread]
       static void Main(string[] args)
       {
         cars C = new cars();
         Console.WriteLine("\nInternal Collection (Unsorted - IEnumerable,Enumerator)\n");
         foreach(car c in C)
         Console.WriteLine(c.Make + "\t\t" + c.Year);
    
         Console.ReadLine();
       }
     }
   }

3. On the Project menu, click Add Class, and then type car in the Name box.
4. Replace the code in car.cs with the following code:

   using System;
   using System.Collections;
   namespace ConsoleEnum
   {
      
     public class car 
     {
       private int year;
       private string make;
           
       public car(string Make,int Year)
       {
         make=Make;
         year=Year;
       }
       public int Year
       {
         get  {return year;}
         set {year=value;}
       }
       public string Make
       {
         get {return make;}
         set {make=value;}
       }
     }//end class
   }//end namespace

5. On the Project menu, click Add Class to add another class to the project, and then type cars in the Name box.
6. Replace the code in cars.cs with the following code:

   using System;
   using System.Collections;
   namespace ConsoleEnum
   {
      public class cars : IEnumerator,IEnumerable
      {
         private car[] carlist;
         int position = -1;
   
         //Create internal array in constructor.
         public cars()
         {
            carlist= new car[6]
         {
            new car("Ford",1992),
            new car("Fiat",1988),
            new car("Buick",1932),
            new car("Ford",1932),
            new car("Dodge",1999),
            new car("Honda",1977)
         };
         }
   
         //IEnumerator and IEnumerable require these methods.
         public IEnumerator GetEnumerator()
         {
            return (IEnumerator)this;
         }
   
         //IEnumerator
         public bool MoveNext()
         {
            position++;
            return (position < carlist.Length);
         }
   
         //IEnumerable
         public void Reset()
         {position = 0;}
   
         //IEnumerable
         public object Current
         {
            get { return carlist[position];}
         }
      }      
   }

7. Run the project. Notice that the following output appears in the Console window:

   Ford            1992
   Fiat            1988
   Buick           1932
   Ford            1932
   Dodge           1999
   Honda           1977

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Best practices

• Implement IEnumerator in a nested class so that you can create multiple enumerators.
• Provide exception handling for the Current method of IEnumerator. If the contents of the collection change, the reset method is called. As a result, the current enumerator is invalidated, and you receive an IndexOutOfRangeException exception. Other circumstances may also cause this exception. Therefore, implement a Try...Catch block to catch this exception and to raise an InvalidOperationException exception.

using System;
using System.Collections;
namespace ConsoleEnum
{
  public class cars : IEnumerable
  {
    private car[] carlist;

    //Create internal array in constructor.
    public cars()
    {
      carlist= new car[6]
    {
      new car("Ford",1992),
      new car("Fiat",1988),
      new car("Buick",1932),
      new car("Ford",1932),
      new car("Dodge",1999),
      new car("Honda",1977)
    };
    }

        //private enumerator class
        private class  MyEnumerator:IEnumerator
        {
          public car[] carlist;
          int position = -1;

        //constructor
        public MyEnumerator(car[] list)
        {
          carlist=list;
        }
        private IEnumerator getEnumerator()
        {
          return (IEnumerator)this;
        }
    
    
        //IEnumerator
        public bool MoveNext()
        {
          position++;
          return (position < carlist.Length);
        }

        //IEnumerator
        public void Reset()
        {position = -1;}

        //IEnumerator
        public object Current
        {
          get 
          { 
            try 
            {
              return carlist[position];
            }
      
            catch (IndexOutOfRangeException)
            {
              throw new InvalidOperationException();
            }
          }
        }
      }  //end nested class
    public IEnumerator GetEnumerator()
    {
      return new MyEnumerator(carlist);
    }
  }
}

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