Question

am I doing this wrong?

                DateTime previousDay = new DateTime(year, month, day);
                string test1 = previousDay.ToString();
                previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
                string test2 = previousDay.ToString();
                previousDay.AddDays(-1);
                string test3 = previousDay.ToString();

For some reason probably obvious to most people, the date doesn’t change from test1 to test3.
Help please!

Answer 1

DateTime is immutable – when you call Subtract it doesn’t change the target of the call, it returns a new value:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
string test2 = previousDay.ToString();
previousDay = previousDay.Subtract(new TimeSpan(1, 0, 0, 0));
string test3 = previousDay.ToString();

Although I’d use the - operator, and TimeSpan.FromDays:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay - TimeSpan.FromDays(1);
string test2 = previousDay.ToString();
previousDay = previousDay - TimeSpan.FromDays(1);
string test3 = previousDay.ToString();

Or better yet, use AddDays:

DateTime previousDay = new DateTime(year, month, day);
string test1 = previousDay.ToString();
previousDay = previousDay.AddDays(-1);
string test2 = previousDay.ToString();
previousDay = previousDay.AddDays(-1);
string test3 = previousDay.ToString();

Or even better yet, use Noda Time 🙂

LocalDate date = new LocalDate(year, month, day);
string test1 = previousDay.ToString();
date = date.PlusDays(-1);
string test2 = previousDay.ToString();
date = date.PlusDays(-1);
string test3 = previousDay.ToString();

Or:

LocalDate date = new LocalDate(year, month, day);
string test1 = previousDay.ToString();
date = date + Period.FromDays(-1);
string test2 = previousDay.ToString();
date = date + Period.FromDays(-1);
string test3 = previousDay.ToString();