c# - How to test logic which is dependent on current date

Question

I have this method which is dependent on current date. It checks if today is Sun, Mon, Tue or Wed, then it gives 5 days of lead time for arrival of shipped items. If its Thur, Fri or Sat then it gives 6 days of lead time to account for the weekend.

private DateTime GetEstimatedArrivalDate()
{
    DateTime estimatedDate; 
    if (DateTime.Now.DayOfWeek >= DayOfWeek.Thursday)
    {
        estimatedDate = DateTime.Now.Date.AddDays(6);
    }
    else
    {
        estimatedDate = DateTime.Now.Date.AddDays(5);
    }
    return estimatedDate; 
}

The actual estimation logic is more complex. I have simplified it for the purpose of this question. My question is how do I write a unit test for something like this which depends on todays date?

Answer 1

You need to pass the current date in as a parameter:

private DateTime GetEstimatedArrivalDate(DateTime currentDate)
{
    DateTime estimatedDate; 
    if (currentDate.DayOfWeek >= DayOfWeek.Thursday)
    {
        estimatedDate = currentDate.AddDays(6);
    }
    else
    {
        estimatedDate = currentDate.AddDays(5);
    }
    return estimatedDate; 
}

In real code you call it like this:

DateTime estimatedDate = GetEstimatedArrivalDate(DateTime.Now.Date);

Then you can test it as follows:

DateTime actual = GetEstimatedArrivalDate(new DateTime(2010, 2, 10));
DateTime expected = ...;
// etc...

Note that this also fixes a potential bug in your program where the date changes between consecutive calls to DateTime.Now.