Doing operations/handling Datetime

Question

The idea is for the user to type in a textbox to horaEntrada and in another to horaSaida where in the case of these two variables they should have the format "HH:mm" hours and minutes only thereafter by the method get and set there would assign the variables I mentioned above, after that I created the method TempoPermanencia in which the intention is to subtract the hours and minutes of these variables in order to be able to display the time that the person was parked on the computer screen

Follows the code:

    private string placa, modelo;
    private DateTime horaEntrada, horaSaida;

    public string Placa
    {
        get { return placa; }
        set { placa = value; }
    }

    public string Modelo
    {
        get { return modelo; }
        set { modelo = value; }
    }

    public DateTime HoraEntrada
    {
        get { return horaEntrada; }
        set { horaEntrada = value; }
    }

    public DateTime HoraSaida
    {
        get { return horaSaida; }
        set { horaSaida = value; }
    }

    public DateTime TempoPermanencia()
    {

    }

Answer 1

I understand what you want.

If you do HoraEntrada - HoraSaida the return will be a TimeSpan

The fields HoraEntrada and HoraSaida may be DateTime, makes sense, because the car enters/leaves in a day and time

You must change your on-call time

public TimeSpan TempoPermanencia()
{
    return HoraSaida - HoraEntrada;
}

Can also be

return HoraSaida.Substract(HoraEntrada);

With the object TimeSpanyou can format with hh:mm, also have TotalDays, TotalHours, etc.

To show off would be something like this:

TempoPermanencia().ToString(@"hh\:mm\:ss")

Answer 2

To calculate the difference just make a subtraction, like this:

public TimeSpan TempoPermanencia() => HoraSaida - HoraEntrada;

This will produce a TimeSpan which is exactly what you need. DateTime, Contrary to what many people think is just a point in time, nothing else, so an input or output moment is a point in time. A period, an amount of time spent on something is represented by a TimeSpan. Read his documentation to learn how to use it, take the stored time of the various possible.

The ideal would be not to consider only the hour and the minute. Can’t it be that the exit occurs the next day? Is there not the slightest chance of this occurring? Does it make such a difference to disregard seconds? If seconds are not important, wouldn’t it be interesting if they were discarded in the data entry? It would even be possible to do this on the property, but it would have to use clear criteria. Without certain requirements, no code will be produced right.

I made a more "complete" solution that I do not guarantee that does exactly what you want, lacking criteria to solve certain situations, for example, in the test done can give 1 hour and 43 minutes, or 1 hour and 44 minutes, It works, but there is no clear criterion as to whether this is what should happen. I didn’t even test it. Messing with date and time is a lot more complicated than it looks. I cut out the superfluous information on the property, but I could have stored it all and made that cut account only at the time of calculation.

using System;
using static System.Console;
                    
public class Program {
    public static void Main() {
        var objeto = new AlgumaClasse();
        objeto.HoraEntrada = DateTime.Now;
        objeto.HoraSaida = DateTime.Now.AddHours(1).AddMinutes(43).AddSeconds(22);
        WriteLine($"Permaneceu {(objeto.TempoPermanencia().ToString(@"hh\:mm"))}");
    }
}

class AlgumaClasse {
    public string Placa { get; set; }
 
    public string Modelo { get; set; }

    private DateTime horaEntrada;
    public DateTime HoraEntrada {
        get => horaEntrada;
        set {
            var tempo = default(DateTime).Add(value.TimeOfDay); //corta data
            horaEntrada = tempo.AddSeconds(-tempo.Second); //corta segundos
        }
    }

    private DateTime horaSaida;
    public DateTime HoraSaida {
        get=> horaSaida;
        set {
            var tempo = default(DateTime).Add(value.TimeOfDay);
            horaSaida = tempo.AddSeconds(-tempo.Second);
        }
    }
    public TimeSpan TempoPermanencia() => HoraSaida - HoraEntrada;
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.