Question

I’m using the new Resharper version 6. In several places in my code it has underlined some text and warned me that there may be a Possible multiple enumeration of IEnumerable.

I understand what this means, and have taken the advice where appropriate, but in some cases I’m not sure it’s actually a big deal.

Like in the following code:

var properties = Context.ObjectStateManager.GetObjectStateEntry(this).GetModifiedProperties();
if (properties.Contains("Property1") || properties.Contains("Property2") || properties.Contains("Property3")) {
    ...
}

It’s underlining each mention of properties on the second line, warning that I am enumerating over this IEnumerable multiple times.

If I add .ToList() to the end of line 1 (turning properties from a IEnumerable<string> to a List<string>), the warnings go away.

But surely, if I convert it to a List, then it will enumerate over the entire IEnumerable to build the List in the first place, and then enumerate over the List as required to find the properties (i.e. 1 full enumeration, and 3 partial enumerations). Whereas in my original code, it is only doing the 3 partial enumerations.

Am I wrong? What is the best method here?

Answer 1

I don’t know exactly what your properties really is here – but if it’s essentially representing an unmaterialized database query, then your if statement will perform three queries.

I suspect it would be better to do:

string[] propertiesToFind = { "Property1", "Property2", "Property3" };
if (properties.Any(x => propertiesToFind.Contains(x))
{
     ...
}

That will logically only iterate over the sequence once – and if there’s a database query involved, it may well be able to just use a SQL “IN” clause to do it all in the database in a single query.