Make a Visual C# class usable in a foreach statement

This article demonstrates how to use the IEnumerable and the IEnumerator interfaces to create a class that you can use in a foreach statement.

Original product version:   Visual Studio
Original KB number:   322022

IEnumerator interface

IEnumerable and IEnumerator are frequently used together. Although these interfaces are similar (and have similar names), they have different purposes.

The IEnumerator interface provides iterative capability for a collection that is internal to a class. IEnumerator requires that you implement three methods:

  • 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 = -1;
    }
    public object Current
    {
        get { return carlist[position];}
    }
    

IEnumerable interface

The IEnumerable interface provides support for the foreach iteration. IEnumerable requires that you implement the GetEnumerator method.

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

When to use which interface

Initially, you might find it confusing to use these interfaces. The IEnumerator interface provides iteration over a collection-type object in a class. The IEnumerable interface permits enumeration by using a foreach loop. However, the GetEmunerator method of the IEnumerable interface returns an IEnumerator interface. So to implement IEnumerable, you must also implement IEnumerator. If you don't implement IEnumerator, you can't cast the return value from the GetEnumerator method of IEnumerable to the IEnumerator interface.

In summary, the use of IEnumerable requires that the class implement IEnumerator. If you want to provide support for foreach, implement both interfaces.

Step-by-step example

The following example demonstrates how to use these interfaces. In this example, the IEnumerator and IEnumerable interfaces are used in a class named cars. The cars class has an internal array of car objects. Client applications can enumerate through this internal array by using a foreach construct because of the implementation of these two interfaces.

  1. Follow these steps to create a new Console Application project in Visual C#:

    1. Start Microsoft Visual Studio .NET or Visual Studio.
    2. On the File menu, point to New, and then click Project.
    3. Click Visual C# Projects under Project Types, and then click Console Application under Templates.
    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 = -1;
           }
           //IEnumerable
           public object Current
           {
               get { return carlist[position];}
           }
        }
      }
    
  7. Run the project.

The following output appears in the Console window:

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

Best practices

The example in this article is kept as simple as possible to better explain the use of these interfaces. To make the code more robust and to make sure the code uses the current best practice guidelines, modify the code as follows:

  • 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 might also cause this exception. So 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);
      }
    }
}