Managed to remember to start on time. Also, I managed to convince a friend I go rock climbing with to give Advent of Code a try. Hi Ashy!

This one seems kinda silly. Just take each number as a string, split it in half, and check if the halves match. Easy!

private static bool IsValidId(long id)
{
    string idString = id.ToString();

    // Odd-length strings should always be valid
    if (idString.Length % 2 == 1) return true;

    int halfLength = idString.Length / 2;

    return !idString[..halfLength].Equals(idString[halfLength..]);
}

Hmm. For part 2, just an extension. Maybe just abstract the check a bit, and be sure to check all iterations up to the half size?

private static bool IsValidIdOfSize(long id, int size)
{
    string idString = id.ToString();

    // Odd-length strings should always be valid
    if (idString.Length % size != 0) return true;

    string pattern = idString[..size];

    for (int i = size; i < idString.Length; i += size)
    {
        if (!idString.Substring(i, size).Equals(pattern)) return true;
    }

    return false;
}