https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,692

PerfBlog06062017_ReferenceSource_ConcatIterator.cs

// https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,692 
static IEnumerable<TSource> ConcatIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second) {
    foreach (TSource element in first) yield return element;
    foreach (TSource element in second) yield return element;
}