How to unit test an empty enum?

Question

I have an extension method that calculates due date based on date period type. The method looks like this:

public static DateTime CalculateDueDate(this DateTime date, OffsetType offsetType, int offset)
{
    switch (offsetType)
    {
        case OffsetType.Days:
            return date.AddDays(offset);
        case OffsetType.Weeks:
            return date.AddWeeks(offset);
        case OffsetType.Months:
            return date.AddMonths(offset);
        default:
            throw new ArgumentOutOfRangeException("offsetType", offsetType, null);
    }
}

where OffsetType enum has these possible values:

public enum OffsetType
{
    Months = 1,
    Weeks = 2,
    Days = 3
}

How can I make sure that ArgumentOutOfRangeException is thrown when OffsetType enum is not provided (or provided with invalid value)? Do I even need to worry about unit testing that exception if OffsetType parameter is not null?

UPDATE:

I wish I could vote for multiple answers. I decided to use out-of-range value suggested by Lee and dasblinkenlight. Here is my fins unit test:

    [Test]
    public void CalculateDueDate_Throw_Exception_Test()
    {
        var date = DateTime.Now;
        var period = 3;
        var offsetType = (OffsetType) (-1);

        Assert.Throws<ArgumentOutOfRangeException>(() => date.CalculateDueDate(offsetType, period));
    }

Answer 1

You can just cast an out-of-range value to the enum type:

Assert.Throws<ArgumentOutOfRangeException>(() => {
    CalculateDueDate(date, (OffsetType)(-1), offset);
});

Which values you can use depend on the underlying type of the enum.

Answer 2

You can construct an illegal value by explicitly casting an out-of-range int to OffsetType, for example

OffsetType t = (OffsetType)5;

Now you can call CalculateDueDate, pass it the illegal value constructed for the unit test, and assert that an exception of type ArgumentOutOfRangeException is thrown.

Note: Although it is redundant in situations when you perform an exhaustive switch covering all enum cases, in situations when you cover some valid cases with a default branch you should check for invalid values using Enum.IsDefined(typeof(OffsetType), t).