Optimizing Aggregate for String Concatenation [closed]

Answer 1

The reason I asked if it was a puzzle was because a puzzle is allowed to sacrifice robustness in varying degrees as long as it satisfies the letter of the stated problem. With that in mind, here goes:

Solution 1 (runs instantly, problem doesn't validate):

public static string Aggregate(this IEnumerable l, Func f) {
     return "";
}

Solution 2 (runs about as fast as the problem requires, but ignores the delegate completely):

public static string Aggregate(this IEnumerable l, Func f) {
    StringBuilder sb = new StringBuilder();
    foreach (string item in l)
        sb.Append(", ").Append(item);
    return sb.Remove(0,2).ToString();
}