Question

Can someone share a simple example of using the foreach keyword with custom objects?

Answer 1

Given the tags, I assume you mean in .NET – and I’ll choose to talk about C#, as that’s what I know about.

The foreach statement (usually) uses IEnumerable and IEnumerator or their generic cousins. A statement of the form:

foreach (Foo element in source) {     // Body } 

where source implements IEnumerable<Foo> is roughly equivalent to:

using (IEnumerator<Foo> iterator = source.GetEnumerator()) {     Foo element;     while (iterator.MoveNext())     {         element = iterator.Current;         // Body     } } 

Note that the IEnumerator<Foo> is disposed at the end, however the statement exits. This is important for iterator blocks.

To implement IEnumerable<T> or IEnumerator<T> yourself, the easiest way is to use an iterator block. Rather than write all the details here, it’s probably best to just refer you to chapter 6 of C# in Depth, which is a free download. The whole of chapter 6 is on iterators. I have another couple of articles on my C# in Depth site, too:

• Iterators, iterator blocks and data pipelines
• Iterator block implementation details

As a quick example though:

public IEnumerable<int> EvenNumbers0To10() {     for (int i=0; i <= 10; i += 2)     {         yield return i;     } }  // Later foreach (int x in EvenNumbers0To10()) {     Console.WriteLine(x); // 0, 2, 4, 6, 8, 10 } 

To implement IEnumerable<T> for a type, you can do something like:

public class Foo : IEnumerable<string> {     public IEnumerator<string> GetEnumerator()     {         yield return 'x';         yield return 'y';     }      // Explicit interface implementation for nongeneric interface     IEnumerator IEnumerable.GetEnumerator()     {         return GetEnumerator(); // Just return the generic version     } }