Brendan Enrick

Daily Software Development

Foreach, IEnumerable, IEnumerator, and Duck Typing

During my Software Craftsmanship Precompiler session, I heard one of the students say, “all you need is an IEnumerable to use a foreach loop”. This sparked a bit of fun when I asked Steve Smith, my co-presenter, if that was correct. He confirmed that it was, and I disagreed. Being the scientists that we are, we decided to try it and see what happened. I of course knew that duck typing in C# should allow the Foreach loop to compile without anything having the method required by the IEnumerable interface. This means that we just need a GetEnumerator method.

We wrote the code that did this and it compiled!

Duck typing is awesome, because it allows the language to treat my type the way I want it to because it has the right tools to do the job. The term duck typing comes from the idea that if it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

In the foreach loop example, duck typing assumes that what you have here is able to be enumerated, because it has a method to get the enumerator. If we look at the IEnumerable interface, we can see that this is what we are required to implement.

public class MyCollection : IEnumerable
{
public IEnumerator GetEnumerator()
{
throw new NotImplementedException();
}
}

Now that we have our collection written and implementing IEnumerable, we can write a foreach loop that uses our collection.

var myCollection = new MyCollection();
foreach (var thing in myCollection)
{
// Do something with the thing
}

Finally, because of the duck typing in C#, we can remove the interface from the code, but keep the method.

public class MyCollection
{
public IEnumerator GetEnumerator()
{
throw new NotImplementedException();
}
}

It still compiles!

One of the many cool things about C# that I love. It’s a fun language, so go experimenting!