Question

Possible Duplicate:
Is it better to call ToList() or ToArray() in LINQ queries?

I have code like this:

void Foobar(string[] arr, Dictionary<string, string[]>)
{
   var t = arr.Intersect(dic.Keys).ToList(); // .or ToArray() ?
   foreach(var item in t)
   {
      ..
   }

   var j = t.Count; // also I need this
}

which method is preferred?

I could go without any but I need to know the size and I don’t want to call Enumerable.Count<T>() – it seems do do more actions then Array<T>.Size or List<T>.Count. Am I right?

Answer 1

Actually, in the current MS implementation of Count(IEnumerable) there’s a shortcut looking if the IEnumerable is an ICollection and calls Count on it. So the performance should be comparable for counting elements.

ToList and ToArray are a bit the same. If the IEnumerable is a ICollection, then the CopyTo method is called instead, which is a bit faster.

So, choose what makes your code the most readable, and benchmark for YOUR use case to have a definite answer.

Update:
I did a naive benchmark.

Starting with an Array: var items = Enumerable.Range(1,1000).ToArray();

• calling ToList() : 25ms / 10000
• calling ToArray() : 23 ms / 10000

Starting with an IEnumerable: var items = Enumerable.Range(1,1000);

• calling ToList() : 168ms / 10000
• calling ToArray() : 171 ms / 10000

So basically you get comparable performance.