using System;

using System.Collections.Generic;   

using System.Linq;

public class Program

{

    public static void Main()

    {

        var arr = new List<(DateTime, DateTime)>()

        {

            (new DateTime(2021, 12, 15),new DateTime(2022, 2, 27)) ,

            (new DateTime(2022, 1, 31),new DateTime(2022, 2, 27)) ,

            (new DateTime(2022, 2, 1),new DateTime(2022, 2, 19)) ,

            (new DateTime(2022, 2, 20),new DateTime(2022, 12, 30)) ,

            (new DateTime(2021, 11, 30),new DateTime(2022, 12, 30)) ,

            (new DateTime(2021, 11, 30),new DateTime(2022, 2, 25)) ,

            (new DateTime(2021, 07, 31),new DateTime(2022, 9, 25)) ,

        };

        foreach(var item in arr){

        DateTime paymentStartDate = item.Item1;

        DateTime asOfDate = item.Item2;

        Console.WriteLine("Start: "+paymentStartDate+"; asOf: "+asOfDate+"; result: "+ForwardToNextMonth(paymentStartDate, asOfDate));

        }

    }

    public static DateTime ForwardToNextMonth(DateTime paymentStartDate, DateTime asOfDate)

    {

        if (paymentStartDate > asOfDate)

        {

            return paymentStartDate;

        }

        int day = paymentStartDate.Day;

        int daysInMonth = DateTime.DaysInMonth(asOfDate.Year, asOfDate.Month);

        var nextDate = new DateTime(asOfDate.Year, asOfDate.Month, day > daysInMonth ? daysInMonth : day);

        while (asOfDate > nextDate)

        {

            nextDate = nextDate.AddMonths(1);

        }

        daysInMonth = DateTime.DaysInMonth(nextDate.Year, nextDate.Month);

        return new DateTime(nextDate.Year, nextDate.Month, day > daysInMonth ? daysInMonth : day);

    }

}