C# linq possible multiple enumeration best practices

Question

I sometimes use LINQ constructs in my C# source. I use VS 2010 with ReSharper. Now I'm getting "Possible multiple enumeration of IEnumerable" warning from ReSharper.

I would like to refactor it according to the best practices. Here's briefly what it does:

IEnumerable<String> codesMatching = from c in codes where conditions select c;
String theCode = null;
if (codesMatching.Any())
{
  theCode = codesMatching.First();
}
if ((theCode == null) || (codesMatching.Count() != 1))
{
  throw new Exception("Matching code either not found or is not unique.");
}
// OK - do something with theCode.

A question: Should I first store the result of the LINQ expression in a List? (I'm pretty sure it won't return more than a couple of rows - say 10 at the most.)

Any hints appreciated.

Thanks Pavel

Answer 1

Since you want to verify if your condition is unique, you can try this (and yes, you must store the result):

var codes = (from c in codes where conditions select c).Take(2).ToArray();
if (codes.Length != 1)
{
  throw new Exception("Matching code either not found or is not unique.");
}

var code = codes[0];