Does “foreach” cause repeated Linq execution?

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In general LINQ uses deferred execution. If you use methods like First() and FirstOrDefault() the query is executed immediately. When you do something like;

foreach(string s in MyObjects.Select(x => x.AStringProp))

The results are retrieved in a streaming manner, meaning one by one. Each time the iterator calls MoveNext the projection is applied to the next object. If you were to have a Where it would first apply the filter, then the projection.

If you do something like;

List<string> names = People.Select(x => x.Name).ToList();
foreach (string name in names)

Then I believe this is a wasteful operation. ToList() will force the query to be executed, enumerating the People list and applying the x => x.Name projection. Afterwards you will enumerate the list again. So unless you have a good reason to have the data in a list (rather than IEnumerale) you’re just wasting CPU cycles.

Generally speaking using a LINQ query on the collection you’re enumerating with a foreach will not have worse performance than any other similar and practical options.

Also it’s worth noting that people implementing LINQ providers are encouraged to make the common methods work as they do in the Microsoft provided providers but they’re not required to. If I were to go write a LINQ to HTML or LINQ to My Proprietary Data Format provider there would be no guarantee that it behaves in this manner. Perhaps the nature of the data would make immediate execution the only practical option.

Also, final edit; if you’re interested in this Jon Skeet’s C# In Depth is very informative and a great read. My answer summarizes a few pages of the book (hopefully with reasonable accuracy) but if you want more details on how LINQ works under the covers, it’s a good place to look.

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