Linq Take() and Skip() are stated as Functinally complement, while it is not always true #5152
Description
Honestly, I am not sure if it is Documentation issue of code bug.
Documentation states:
The Take and Skip methods are functional complements. Given a sequence coll and an integer n, concatenating the results of coll.Take(n) and coll.Skip(n) yields the same sequence as coll.
dotnet-api-docs/xml/System.Linq/Enumerable.xml
Line 9591 in f33aa30
dotnet-api-docs/xml/System.Linq/Enumerable.xml
Line 11251 in f33aa30
However, that is not always true, Take and Skip sometimes silently throw away elements, and taken.Concat(skipped) is not equal to original:
class TakeSkipEnumerableFail
{
private static readonly string[] array = Enumerable.Range(1, 7).Select(i => $"{i}").ToArray();
private static int position = 0;
private static IEnumerable<string> GetIEnumerable()
{
while (position < array.Length)
{
yield return array[position];
position++;
}
}
static void Main()
{
var sequence = GetIEnumerable();
var before = sequence.Take(3);
var after = sequence.Skip(3);
var concat = before.Concat(after); // should be 1,2,3,4,5,6,7
var result = $"concat: {string.Join(",", concat)}"; // but actually it is 1,2,3,6,7
Console.WriteLine(result);
}
}Most probably it is due to nature of this very IEnumerable (it is true for enumerations that are not repeatable), but it makes sense to mention this in documentation.