Can't create an impersonated appoinement in C#

Question

I'm trying to create a program that allows me to add multiple appointments/events to multiple users (without using a shared calendar).

The problem arrives when I try to impersonate someone (I'm admin so I think that I don’t have "rights" problems), I get an error System.NullReferenceException : 'Object reference not set to an instance of an object.' when the appoinment is saved (not when the impersonation is created, I don't understand why)

I don’t know what object I have to instantiate

If I don’t impersonate, the code works. The appointment is created

            try
        {
            ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2007_SP1);
            service.UseDefaultCredentials = true;
            service.Credentials = new WebCredentials(userEmail, userPassword);
            service.Url = new Uri("https://outlook.office365.com/ews/exchange.asmx");
            service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, destEmail);
            //service.ImpersonatedUserId = new ImpersonatedUserId(ConnectingIdType.SmtpAddress, myEmail);
            Appointment appointment = new Appointment(service);
            // Set the properties on the appointment object to create the appointment.
            appointment.Subject = subject;
            appointment.Body = "Test Event";
            appointment.Start = DateTime.Now.AddDays(2);
            appointment.End = appointment.Start.AddHours(1);
            appointment.Location = "Here";
            appointment.ReminderDueBy = DateTime.Now;

            // Save the appointment to your calendar.
            appointment.Save();

            // Verify that the appointment was created by using the appointment's item ID.
            Item item = Item.Bind(service, appointment.Id, new PropertySet(ItemSchema.Subject));
            Console.WriteLine("Appoinement created");

        }
        catch (Exception ex)
        {
            Console.WriteLine("The following error occurred: " + ex.Message);
        }

Error when I try to save the appoinement

-------------------- Update 02.07.2021 Thanks for the remarks.

I removed service.UseDefaultCredentials = true, that's dummy if a set new credentials in the next line.

The Appointment class is the default Microsoft.Exchange.WebServices.Data.Appointment, i didnt change anything there.

IT WAS a security/rights problem. Eventhough I'm admin, admins doesn't have Impersonnation ritghs by default. The thing is that I added to myself the impersonation rights needed, and it worked. Thanks

Answer 1

It worked for a while. It stopped working without reason, i didnt change anything.

So I waited a few hours, tryed again, still didnt work and then I re-added the rights, and it works again.

Thanks you very much for the comments/hints