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 Thu

Answer 1

Generally speaking, you'd want to abstract the method of obtaining the current date and time behind an interface, eg:

public interface IDateTimeProvider
{
    DateTime Now { get; }
}

The real service would be:

public class DateTimeProvider: IDateTimeProvider
{
    public DateTime Now
    {
        get
        {
            return DateTime.Now;
        }
    }
}

And a test service would be:

public class TestDateTimeProvider: IDateTimeProvider
{
    private DateTime timeToProvide;
    public TestDateTimeProvider(DateTime timeToProvide)
    {
        this.timeToProvide = timeToProvide;
    }

    public DateTime Now
    {
        get
        {
            return timeToProvide;
        }
    }
}

For services that require the current time, have them take an IDateTimeProvider as a dependency. For the real thing, pass a new DateTimeProvider(); when you're a component, pass in a new TestDateTimeProvider(timeToTestFor).