Why resharper does not suggest to use linq when interating over a List?

Question

I am using Resharper 2018.1.3 (I updated to 2018.2.0 and I experienced the same issue) with Visual Studio 2017.

When I type this code:

var test = new List<string>();
var strings = new[] { "a", "b", "c" };
foreach (var a in strings)
{
    test.Add(a);
}

Resharper suggests to convert it into LINQ-expression.

But if I use a List :

var test = new List<string>();
var strings = new List<string> { "a", "b", "c" };
foreach (var a in strings)
{
    test.Add(a);
}

Resharper gives me no hint. Why?

Answer 1

While in this particular case the transformation to LINQ looks safe, in general it can have unexpected side effects. System.Collections.Generic.List has a special implementation of GetEnumerator method which returns a struct. LINQ methods works with IEnumerable where GetEnumerator methods return instance of interface IEnumerator . So, converting foreach to LINQ can cause undesired boxing.

In some cases GetEnumerator method used by foreach and implementation of IEnumerable.GetEnumerator can return completely different Enumerator s like we have in IDictionary where GetEnumerator enumerates over DictionaryEntry s while implementation of IEnumerable.GetEnumerator iterates over objects which are actually instances of KeyValuePair in case of System.Collections.Generic.Dictionary .