c# Custom string.format and indexOf Implementation

public static string FormatString(string format, params object[] vars)
{
    List<int>[] lists = new List<int>[vars.Length];
    int lastIndex = 0;

    for (int i = 0; i < vars.Length; i++)
    {
        lists[i] = new List<int>();
        int index = -1;
        string current = "{" + i + "}";
        do
        {
            index = format.ctIndexOf(current, lastIndex + 1);
            if (index != -1)
                lists[i].Add(index);
            lastIndex = index;
        } while (index != -1);
        lastIndex = 0;
    }

    for (int i = lists.Length - 1; i >= 0; i--)
    {
        string current = "{" + i + "}";
        for (int j = lists[i].Count - 1; i >= 0; i--)
        {
            int removeAt = lists[i][j];
            format = format.Remove(removeAt, current.Length);
            format = format.Insert(removeAt, vars[i].ToString());
        }
    }

    return format;
}

public static int ctIndexOf(this string str, string find, int startIndex = 0)
{
    for (int i = startIndex; i < (str.Length - find.Length) + 1; i++)
    {
        bool isfound = true;
        for (int j = 0; j < find.Length; j++)
        {
            if (str[i + j] != find[j])
            {
                isfound = false;
                break;
            }
        }
        if (isfound)
            return i;
    }
    return -1;
}