UnitTest a method that is time-dependent

Question

How would you test a method that is time-dependent?

A simple example could be:

public static int GetAge(DateTime birthdate)
{
    // ...
}

[TestMethod]
public void GetAgeTest()
{
    // This is only ok for year 2013 !
    Assert.AreEqual(GetAge(new DateTime(2000, 1, 1)), 13);
}

The problem is that it will work only for the current year (2013 in this case). As soon as a new year begins, I have to rewrite all time-dependent tests...

What is the best approach to Unit Test such a method?

Answer 1

You could override the mechanism by which your GetAge method finds today's date.

If you wrap DateTime in another class you can then replace the actual calls with fake calls that return known values while unit testing.

You can use dependency injection to switch between the real implementation for production and the fake implementation for testing.

Answer 2

Also, don't forget the "Expected" argument is the first one in AreEquals.

[TestMethod]
public void GetAgeTest()
{
    int age = 13;
    Assert.AreEqual(age, GetAge(DateTime.Now.AddYears(age * -1));
}

Would also be better to split the test in Arrange - Act - Assert groups like this :

[TestMethod]
public void GetAgeTest()
{
    // Arrange
    int age = 13;

    // Act
    int result = GetAge(DateTime.Now.AddYears(age * -1);

    // Assert
    Assert.AreEqual(age, result);
}