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Asked: 2026-05-27T20:12:01+00:00

Thanks for help with explaining to me about IList and IEnumerable. I have marked

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Thanks for help with explaining to me about IList and IEnumerable. I have marked that question as answered. Now I have a request for just a bit more information.

I have code like this:

for (var index = 0; index < Model.Items.Count(); index++)

It was suggested in a previous post that it’s not efficient as the Count() is a method and it gets executed many times. I tried the following:

for (var index = 0; index < Model.Items.Count; index++)

This seems to work.

Can someone please confirm there’s a difference. Both work so I am not 100 % sure.

Would the following be the most efficient? Does the foreach need an IList or and IEnumerable?

foreach(var item in Items)
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1 Answer

  1. Editorial Team

    The Count() method call actually calls the static Enumerable.Count() method. This is optimized for IList implementations to use the property – but this requires a dynamic type check, and still ends up going through to the property – so using the property is certainly more efficient. In .NET 3.5 it only optimizes for ICollection<T>, whereas in .NET 4 it optimizes for the non-generic ICollection as well. See my Edulinq blog post on Count for more details.

    As for the last bit of your question: a foreach loop may or may not be more efficient, but it would be more readable in my view, which is rather more important. I would definitely use foreach unless you really need the index of each entry.

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