using System;

public class Program

{

    public static void Main()

    {

        var mmyyyy= "12/2020";

        DateTime expiry = DateTime.MinValue;

        DateTime? expiryNullable = null;

        if(DateTime.TryParseExact(mmyyyy, "MM/yyyy", System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out expiry)){

            expiryNullable = expiry;

            var lastDayofMonth = expiry.GetLastDayOfMonth();

            var lastDayofMonthNullable = expiryNullable.GetLastDayOfMonth();

            Console.WriteLine(expiry.ToString("dd/MM/yyyy"));

            Console.WriteLine(lastDayofMonth.ToString("dd/MM/yyyy"));

            Console.WriteLine(lastDayofMonthNullable.Value.ToString("dd/MM/yyyy"));

        }       

    }   

}

​

public static class Extensions {    

    public static DateTime GetLastDayOfMonth(this DateTime dateTime)

    {

        return new DateTime(dateTime.Year, dateTime.Month, DateTime.DaysInMonth(dateTime.Year, dateTime.Month));

    }

    public static DateTime? GetLastDayOfMonth(this DateTime? dateTime)

    {

        if(dateTime.HasValue)

        {

            return dateTime.Value.GetLastDayOfMonth();

        }

​

        return null;

    }

}