c# - date difference using datepicker in wpf

Question

Im trying to calculate the how many days there are between two dates and display the result in a textblock, i am using wpf. However i get a nullable object must have a value in the first line :S

    private void button20_Click(object sender, RoutedEventArgs e)
    {
        DateTime start = datePicker1.DisplayDateStart.Value.Date;
        DateTime finish = datePicker2.DisplayDateStart.Value.Date;
        TimeSpan difference = start.Subtract(finish);
        textBlock10.Text = Convert.ToString(difference);
    }

Answer 1

As the error message implies, DisplayDateStart is a nullable property, which means it can (and, by default, does) have no value. You have to handle this condition to produce sensible results.

That said, the DisplayDateStart property refers to the earliest date shown in the DatePicker's calendar, not the date the user has picked: for that, you need the SelectedDate property, which is also nullable.

There are a variety of ways you could handle a NULL value: display nothing in the TextBlock, display "N/A" or some other default, etc. Here's an example:

private void button20_Click(object sender, RoutedEventArgs e)
{
    // This block sets the TextBlock to a sensible default if dates haven't been picked
    if(!datePicker1.SelectedDate.HasValue || !datePicker2.SelectedDate.HasValue)
    {
        textBlock10.Text = "Select dates";
        return;
    }

    // Because the nullable SelectedDate properties must have a value to reach this point, 
    // we can safely reference them - otherwise, these statements throw, as you've discovered.
    DateTime start = datePicker1.SelectedDate.Value.Date;
    DateTime finish = datePicker2.SelectedDate.Value.Date;
    TimeSpan difference = finish.Subtract(start);
    textBlock10.Text = difference.TotalDays.ToString();
}