GetEnumerator() in the middle of a string

Question

Suppose I have a very long string, and would like to get an enumerator (for LINQ goodness) that begins in the middle of the string, at a given index. That is,

var longString = GetLongString();
var index = 99999;

// seems unnecessarily expensive to allocate a new string
var enumerator = longString.Substring(9999).GetEnumerator();

// Use LINQ.
enumerator.TakeWhile( /* ... */ );

is there a better (ie: cheaper/faster) way to do this?

Answer 1

Consider Enumerable.Skip:

longString.Skip(9999).TakeWhile( /* ... */ );

However, keep in mind that as Michael Liu wrote, Skip works in iterative way (strictly speaking it depends on implementation, but at least this is what you usually have, see e.g. .NET Framework 4.6 Reference Sources). If it causes measurable¹ slowness in your case, look at Michael's extension method.

---

1. D.Knuth: "premature optimization is the root of all evil "

Answer 2

Using longString.Skip(n), as in AlexD's answer, will make n calls to the underlying enumerator's MoveNext method to skip past all the characters you don't care about. For large values of n, you can improve performance by writing a custom iterator method:

public static IEnumerable<char> EnumerableSubstring(this string s, int startIndex)
{
    for (; startIndex < s.Length; startIndex++)
        yield return s[startIndex];
}

Sample usage:

longString.EnumerableSubstring(9999).TakeWhile( /* ... */ )