Variable value not updating via method call

Question

This smells like a scoping issue with my variable however moving it does not seem to help. Here is a very simple example. I create the currentDay variable. I set it's value. I then call another method which should change the value of currentDay, but it never changes. Just Monday morning blindness?

void Main()
{
    SetScheduleTicketsDate();
}

public static void SetScheduleTicketsDate()
{
    DateTime currentDay = DateTime.Now;
    SchedulePatchGroup(currentDay);
    Console.WriteLine(currentDay);
}

private static void SchedulePatchGroup(DateTime currentDay)
{
    currentDay = currentDay.AddDays(10);
}

Also, I believe that DateTime is a value-type, not a reference type. — vbnet3d
– vbnet3d, Commented Mar 26, 2018 at 15:14

BradleyDotNET · Accepted Answer · 2018-03-26 15:23:43Z

5

Assigning a parameter will not propagate to the caller unless you use ref or out.

Usually that's a code smell; your method should probably just return the updated object anyways.

public static void SetScheduleTicketsDate()
{
    DateTime currentDay = DateTime.Now;
    currentDay = SchedulePatchGroup(currentDay);
    Console.WriteLine(currentDay);
}

private static DateTime SchedulePatchGroup(DateTime currentDay)
{
    return currentDay.AddDays(10);
}

Just using currentDay.AddDays(10) won't do anything either since DateTime is a value type; AddDays does not mutate the current instance and instead returns a new one.

edited Mar 26, 2018 at 15:23

answered Mar 26, 2018 at 15:14

BradleyDotNET

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2 Comments

Vidmantas Blazevicius Over a year ago

Perhaps would be worth noting in your answer that DateTime is not a reference type (which is maybe where the confusion is).

Connie DeCinko CS Over a year ago

Yeah, I would have expected it to mutate the current value like other variables. This makes sense. My SchedulePatchGroup does its thing, coming up with a new currentDay and then returns it rather than directly setting it.

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