Question

I work on applications developed in C#/.NET with Visual Studio. Very often ReSharper, in the prototypes of my methods, advises me to replace the type of my input parameters with more generic ones. For instance, List<> with IEnumerable<> if I only use the list with a foreach in the body of my method. I can understand why it looks smarter to write that but I’m quite concerned with the performance. I fear that the performance of my apps will decrease if I listen to ReSharper…

Can someone explain to me precisely (more or less) what’s happening behind the scenes (i.e. in the CLR) when I write:

public void myMethod(IEnumerable<string> list)
{
  foreach (string s in list)
  {
    Console.WriteLine(s);
  }
}

static void Main()
{
  List<string> list = new List<string>(new string[] {"a", "b", "c"});
  myMethod(list);
}

and what is the difference with:

public void myMethod(List<string> list)
{
  foreach (string s in list)
  {
    Console.WriteLine(s);
  }
}

static void Main()
{
  List<string> list = new List<string>(new string[] {"a", "b", "c"});
  myMethod(list);
}

Answer 1

You’re worried about performance – but do you have any grounds for that concern? My guess is that you haven’t benchmarked the code at all. Always benchmark before replacing readable, clean code with more performant code.

In this case the call to Console.WriteLine will utterly dominate the performance anyway.

While I suspect there may be a theoretical difference in performance between using List<T> and IEnumerable<T> here, I suspect the number of cases where it’s significant in real world apps is vanishingly small.

It’s not even as if the sequence type is being used for many operations – there’s a single call to GetEnumerator() which is declared to return IEnumerator<T> anyway. As the list gets larger, any difference in performance between the two will get even smaller, because it will only have any impact at all at the very start of the loop.

Ignoring the analysis though, the thing to take out of this is to measure performance before you base coding decisions on it.

As for what happens behind the scenes – you’d have to dig into the deep details of exactly what’s in the metadata in each case. I suspect that in the case of an interface there’s one extra level of redirection, at least in theory – the CLR would have to work out where in the target object’s type the vtable for IEnumerable<T> was, and then call into the appropriate method’s code. In the case of List<T>, the JIT would know the right offset into the vtable to start with, without the extra lookup. This is just based on my somewhat hazy understanding of JITting, thunking, vtables and how they apply to interfaces. It may well be slightly wrong, but more importantly it’s an implementation detail.