IndexOf

public static int IndexOf<T>(this IEnumerable<T> obj, T value)
{
    return obj.IndexOf(value, 0);
}

public static int IndexOf<T>(this IEnumerable<T> obj, T value, int start)
{
    if (start >= obj.Count()) return -1;
    if (start < 0) throw new IndexOutOfRangeException("start must be a non-negative integer");
    for (int i = start; i < obj.Count(); ++i)
    {
        if (value.Equals(obj.ElementAt(i))) return i;
    }
    return -1;
}

var i = new Collection<int>{0,1,2,3,4,5,6,7};

int first = i.IndexOf(0);
int second = i.IndexOf(0,1);