Nullable datatime reference parameter

Question

All,

My method declaration is as follows:

public bool RouteCheck(ref Nullable<DateTime>  TravelTime)
{
  //Method body not relevant to the question
}

The method is called from two places. I've put some abbreviated sections of code together below:

1)

Nullable<DateTime> TravelTime=null;
RouteCheck(ref TravelTime)

2)

public DateTime PlannedArrivalTime
PlannedArrivalTime="01/01/2021 09:00:00"
RouteCheck(ref PlannedArrivalTime)

The second won't compile because "can't convert ref System.DateTime to ref System.DateTime?".

I don't understand why it won't compile. I would appreciate help on two questions:
a) A Nullable DateTime paramer should accept a null value (TravelTime in my case) or a valid date time PlannedArrivalTime in my case)?
b) If I omitted the PlannedArrivalTime="01/01/2021 09:00:00" line in the second example then PlannedArrivalTime would also be null and be an acceptable value for the method parameter?

I wondering if my syntax is wrong or I'm just not understanding the reason for the error?

I think I could solve it by making PlannedArrivalTime Nullable but that seems like a workaround rather than a proper solution.

Thanks

Answer 1

Hello,

Try the following done in this case with .NET 5, C#9

Helper class

public static class DateHelpers
{
    public static bool RouteCheck(ref DateTime? travelTime) => travelTime.HasValue;
    public static DateTime? TryParse(string text) => DateTime.TryParse(text, out var date) ? date : (DateTime?)null;
}

Program.cs

class Program
{
    static void Main(string[] args)
    {

        Nullable<DateTime> TravelTime = null;
        Console.WriteLine(DateHelpers.RouteCheck(ref TravelTime));

        TravelTime = DateTime.Now;
        Console.WriteLine(DateHelpers.RouteCheck(ref TravelTime));

        string someDate = "01/01/2021 09:00:00";

        var test = DateHelpers.TryParse(someDate);
        if (test.HasValue)
        {
            Console.WriteLine(DateHelpers.RouteCheck(ref TravelTime));
        }
        else
        {
            Console.WriteLine("Not a date time");
        }


        someDate = "";

        test = DateHelpers.TryParse(someDate);
        if (test.HasValue)
        {
            Console.WriteLine(DateHelpers.RouteCheck(ref TravelTime));
        }
        else
        {
            Console.WriteLine("Not a date time");
        }
        Console.ReadLine();
    }
}

Edit

In regards to checking the type if there are other types e.g. int use int.TryParse or perhaps a generic check where T is the type.

using System;

namespace YourNamespace
{
    public static class DateHelpers
    {
        public static bool RouteCheck(ref DateTime? travelTime) => travelTime.HasValue;
        public static DateTime? TryParse(string text) => DateTime.TryParse(text, out var value) ? value : (DateTime?)null;
        public static bool Is<T>(this string value)
        {
            if (string.IsNullOrEmpty(value)) return false;
            var conv = System.ComponentModel.TypeDescriptor.GetConverter(typeof(T));

            if (!conv.CanConvertFrom(typeof(string))) return false;

            try
            {
                conv.ConvertFrom(value);
                return true;
            }
            catch
            {
                // ignored
            }
            return false;
        }
    }
}

Answer 2

DateTime is a struct, not a class, therefore it is never null. You can use ‘Nullable<DateTime>’ or ‘DateTime?’ to have a nullable value.

Answer 3

Compare this....
int is not equal Nullable<int>

    private bool f1(int n)
    {
        return true;
    }

    private bool f1(int? n)
    {
        return false;
    }


               int? n = null;
                n = 0;
                int m = 1;
                f1(n);
                f1(m);