How do I set a nullable DateTime to null in VB.NET?

Question

I am trying to set up a date range filter on my UI, with checkboxes to say whether a DateTimePicker's value should be used, e.g.

Dim fromDate As DateTime? = If(fromDatePicker.Checked, fromDatePicker.Value, Nothing)

Yet setting fromDate to Nothing doesn't result in it being set to Nothing but to '12:00:00 AM', and the following If statement incorrectly executes the filter because startDate is not Nothing.

If (Not startDate Is Nothing) Then
    list = list.Where(Function(i) i.InvDate.Value >= startDate.Value)
End If

How do I really ensure startDate gets a value of Nothing?

Solution 1

The issue is that it's examining the right-hand side of this assignment first, and deciding that it is of type DateTime (no ?). Then performing the assignment.

This will work:

Dim fromDate As DateTime? = If(fromDatePicker.Checked, _
                               fromDatePicker.Value, _
                               CType(Nothing, DateTime?))

Because it forces the right-hand side's type to be DateTime?.

As I said in my comment, Nothing can be more akin to C#'s default(T) rather than null:

Nothing represents the default value of a data type. The default value depends on whether the variable is of a value type or of a reference type.

Solution 2

With VB.NET and EF 6.X to save null is:

Dim nullableData As New Nullable(Of Date)

Solution 3

In addition to @Damien_The_Unbeliever's fine answer, using New DateTime? also works:

Dim fromDate As DateTime? = If(fromDatePicker.Checked, _
                               fromDatePicker.Value, _
                               New DateTime?)

You might find that it looks a bit counter intuitive, why perhaps the CType(Nothing, DateTime?) is preferable.

Comments

[Damien_The_Unbeliever]

What do you think the difference is between New Date and Nothing being assigned to a Date (or DateTime) variable? (hint - there isn't one)

[SSS]

More typing: Dim fromDate As Date = Nothing

[Damien_The_Unbeliever]

Dim fromDate As Date is even less typing and has the same observable effects.

[SSS]

True enough. However, fromDate = New Date is a valid comparison, while fromDate Is Nothing is not. So for consistency I would encourage consistent use of New Date. I also recommend explicitly setting variables to Nothing or New Date or 0 or "" to indicate that the uninitialised value is going to be used as a magic number (and to avoid compiler warnings when you hand them as parameters). It's a style preference I guess - makes no difference at the IL level, as you note.

[Damien_The_Unbeliever]

fromDate = Nothing is a valid comparison, and uses one less character :-)

[ProfK]

@SSS Surely we should be moving away from using legacy types/aliases such as Date?

[ProfK]

As you said in your comment as well, "the two are equivalent for reference types", and Nullable(Of DateTime) is a reference type, but it looks like the cast trick works, thanks.

[Damien_The_Unbeliever]

Nullable(Of DateTime) is a reference type. However, at the point in time when Nothing was being converted, on the right hand side, we have this type information: If(Boolean,DateTime,<Nothing>). Since the return type of If has to match the type of one of its expressions, it picked DateTime. It then converts Nothing to a DateTime (creating a default DateTime), and only after that point does it consider the assignment.

[SSS]

I guess that's a style choice too. As I said before, YMMV.

[Auguste Van Nieuwenhuyzen]

I think this is an interesting point, though not sure it's exactly answering the original question ;). Where I'm setting a nullable date to null in VB.NET I'm using nullableData = New Date?.