The C# yield-foreach howto

Author: Kasper B. Graversen, 25/11/08 Keywords: CSharp, Yield, Foreach, IEnumerator, IEnumerable Abstract: This article explains how to use the C# constructs yield-return, yield-break and foreach. We attack the problem from two different implementation angles using the types IEnumerator and IEnumerable.

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The C# yield-foreach howto

• The client is agnostic to the class' internal representation.
• The client is loosely coupled to the class. It is only dependent on a very general interface when traversing the data.
• The class may provide a read-only enumeration to the client for a more thread-safe or functional implementation approach.
• Direct access using an index is not suitable to all data structures, in particular to data structures such as trees with no or slow random access.
• An enumerator may enforce restrictions on access, such as ensuring that elements can not be skipped or that a previously visited element can not be accessed again.

using System;
using System.Collections.Generic;

namespace yield_foreach {
  class Program {
    static void Main (string[] args) {
      var p = new Program();
      foreach ( var s in p.getSomeStuff() ) {
        Console.WriteLine(s);
      }
    }

    public IEnumerable<int> getSomeStuff() {
      for ( int i = 0; i < 5; i++ ) {
        yield return i;
      }
      yield break;
    }
  }
}

The yield trap

using System;
using System.Collections.Generic;

namespace yield_foreach {
  class Program {
    static void Main (string[] args) {
      var p = new Program();
      foreach ( var s in p ) {
        Console.WriteLine(s);
      }
    }

    public IEnumerator<int> GetEnumerator()
    {
      for ( int i = 0; i < 5; i++ ) {
        yield return i;
      }
      yield break;
    }
  }
}

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