Explanation why IEnumerable is more efficient than a List

Question

I keep hearing that in .net 3.5 you should use IEnumerable over a List, but I can’t find any reference materials or articles that explain why it’s so much more proficient. Does anyone know of any content that explains this?

The purpose of asking this question is to get a better understanding of what IEnumerable is doing under the hood. If you can provide me with any links I will do the research and post an answer.

Answer 1

IEnumerable<T> is an interface that is implemented by List<T>. I suspect the reason you're hearing that IEnumerable<T> should be used is because it's a less constrictive interface requirement.

For example, consider the following method signature:

void Output(List<Foo> foos)  {      foreach(var foo in foos) { /* do something */ } } 

This method requires that it be passed a concrete implementation of a List. But it's just doing something in-order. It doesn't really need random access or any of the other things that a List<T> or even an IList<T> give it. Instead, the method should accept an IEnumerable<T>:

void Output(IEnumerable<Foo> foos)  {      foreach(var foo in foos) { /* do something */ } } 

Now we're using the most general (least specific) interface that supports the operations that we need. This is a fundamental aspect of OO-design. We've decreased coupling by requiring only what we need, and not a whole lot else besides. We've also created a more flexible method because the foos parameter might be a Queue<T>, a List<T>, anything that implements IEnumerable<T>. We aren't forcing the caller to convert their data structure to a List unnecessarily.

So it isn't that IEnumerable<T> is more efficient than list in a "performance" or "runtime" aspect. It's that IEnumerable<T> is a more efficient design construct because it's a more specific indication of what your design requires. (Though this can lead to runtime gains in specific cases.)