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?

Answer 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.