Validate Credit Card Expiry Month and Year.

CreditCardExpiryValidator.cs

using System;

public class Program
{
    public static void Main()
    {
        var x = new CardExpiryValidator("10/2021");
        Console.WriteLine(x.IsExpired);
    }
}

public readonly struct CardExpiryValidator
{
    private static readonly int _year = DateTime.Now.Year;

    public readonly bool IsExpired;

    public readonly int Month;

    public readonly int Year;

    public CardExpiryValidator(string cardExpiry)
    {
        ReadOnlySpan<char> chars = cardExpiry;

        Month = default;
        Year = default;
        IsExpired = true;

        int sliceIndex = 0;

        if (chars.Length == 6 && chars.IndexOf('/') == 1) //9/2024
        {
            sliceIndex = 1;
        }

        if (chars.Length == 7 && chars.IndexOf('/') == 2) //09/2024
        {
            sliceIndex = 2;
        }

        Month = chars.Slice(0, sliceIndex).ToInt();
        Year = chars.Slice(sliceIndex + 1).ToInt();

        if (Month is < 1 or > 12 || Year < _year) return;

        IsExpired = IsCardExpired();
    } 

    private bool IsCardExpired()
    {
        var dateExpiry = DateTime.DaysInMonth(Year, Month);
        var cardExpiry = new DateTime(Year, Month, dateExpiry, 23, 59, 59);

        //Any future/current Month and Year is not expired.
        //Expiry Date must be greater than Current Date.
        var notExpired = cardExpiry > DateTime.Now;

        return !notExpired;//Should return False if card is not expired.
    }
}

internal static class IntegerExtensions
{
    internal static int ToInt(this ReadOnlySpan<char> chars)
    {
        var val = 0;

        for (var i = 0; i < chars.Length; i++)
        {
            if (!(chars[i] >= 48 && chars[i] <= 57)) return 0; //Skip if non integer character found.

            val *= 10;
            val += chars[i] - '0';
        }

        return val;
    }
}