nest yields to return IEnumerable> with lazy evaluation

Question

I wrote a LINQ extension method SplitBetween analogous to String.Split.

> new List(){3,4,2,21,3,2,17,16,1}
> .SplitBet         


        

Answer 1

This one won't use List<>, and won't go BOOM.

public static IEnumerable> SplitBetween(this IEnumerable source,
                                                          Func separatorSelector, 
                                                          bool includeSeparators=false) 
{
    if (source == null)
        throw new ArgumentNullException("source");

    return SplitBetweenImpl(source, separatorSelector, includeSeparators);
}

private static IEnumerable SplitBetweenInner(IEnumerator e,
                                                   Func separatorSelector)
{
    var first = true;

    while(first || e.MoveNext())
    {
        if (separatorSelector((T)e.Current))
            yield break;    

        first = false;
        yield return e.Current;
    }
}

private static IEnumerable> SplitBetweenImpl(this IEnumerable source,
                                                               Func separatorSelector, 
                                                               bool includeSeparators) 
{
    using (var e = source.GetEnumerator())
        while(e.MoveNext())
        {
            if (separatorSelector((T)e.Current) && includeSeparators)
                yield return new T[] {(T)e.Current};
            else
                {
                yield return SplitBetweenInner(e, separatorSelector);
                if (separatorSelector((T)e.Current) && includeSeparators)
                    yield return new T[] {(T)e.Current};
                }
        }
}

---

Test:

void Main()
{
    var list = new List(){1, 2, 3, 10, 1};
    foreach(var col in list.Concat(Ext.ThrowingEnumerable())
                           .SplitBetween(x=>x>=10).Take(1))
    {
        Console.WriteLine("------");
        foreach(var i in col)
            Console.WriteLine(i);
    }
}

Output:

------
1
2
3

---

Test2

var list = new List(){1, 2, 3, 10, 1}
foreach(var col in list.Concat(Ext.ThrowingEnumerable())
                       .SplitBetween(x=>x>=10).Take(2))

Output:

------
1
2
3
------
1
*Exception*

Here, the exception is caused because the first element of the ThrowingEnumerable-enumeration would go into the same group as the 1.

---

Test3:

var list = new List(){1, 2, 3, 10, 1, 17};
foreach(var col in list.Concat(Ext.ThrowingEnumerable())
                       .SplitBetween(x=>x>=10, true).Take(4))

Output:

------
1
2
3
------
10
------
1
------
17

No problem here, because the Exception element would go into it's own group, and thus is not iterated over due to Take(4):