Why does my code not calculate the age in number of days?

Question

My C# code is :

private int ageInDays()
{
    string[] month = {"January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"};
    int age = 0;

    DateTime birthDay = new DateTime(Convert.ToInt32(yearCombo.Text), Array.IndexOf(month, monthCombo.Text) + 1, Convert.ToInt32(dayCombo.Text));
    MessageBox.Show(birthDay.Date.ToString());
    DateTime tempDay = birthDay;
    DateTime today = DateTime.Today;
    while (tempDay != today)
    {
        age++;
        tempDay.AddDays(1);
    }
    return age;   
}

When I execute it, the program shows no progress at all. It probably hangs or falls into an infinite loop. Is it due to the heavy amount of processing the program has to do? Why am I not getting any output?

If the above method of calculating the age solely in days is flawed, what's a better/simpler way?

Hint: `DateTime.AddDays` returns a new value, it doesn't modify the existing value. — Jon Skeet, Mar 14 '17 at 11:52
If you debug and step through your program, you can see exactly what is happening instead of guessing. — crashmstr, Mar 14 '17 at 11:53
You know you can just subtract one `DateTime` from another and get a `TimeSpan` that includes a `Days` property `(today - birthDay).Days` — juharr, Mar 14 '17 at 11:54
Hint2: DateTime has a subtract method that returns a TimeSpan. TimeSpan has a Days property. — Damien_The_Unbeliever, Mar 14 '17 at 11:54
[How do I calculate someone's age in C#?](http://stackoverflow.com/questions/9/how-do-i-calculate-someones-age-in-c?rq=1) — TheLethalCoder, Mar 14 '17 at 11:57

Tim Schmelter · Accepted Answer · 2017-03-14T11:58:49.773

3

DateTime is an immutable struct. You have to assign what DateTime.AddDays returns to the variable:

tempDay = tempDay.AddDays(1);

You can get the result in an easier and more efficient way with TimeSpan.Days:

int age = (DateTime.Today - birthDay).Days;

edited Mar 14 '17 at 11:58

answered Mar 14 '17 at 11:52

Tim Schmelter

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"The DateTime is a struct, hence it's immutable." That's a non-sequitur. Value types can be mutable, e.g. `System.ValueTuple`. – Jon Skeet Mar 14 '17 at 11:53
@JonSkeet: avoided misunderstandings with other formulation – Tim Schmelter Mar 14 '17 at 11:54
Right - that's more like it. – Jon Skeet Mar 14 '17 at 11:58

Why does my code not calculate the age in number of days?

1 Answers1