如何在 VB.NET 中将可为 null 的 DateTime 设置为 null？
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How do I set a nullable DateTime to null in VB.NET?
提问by ProfK
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.
我试图在我的 UI 上设置一个日期范围过滤器，用复选框说明是否应该使用 DateTimePicker 的值，例如
Dim fromDate As DateTime? = If(fromDatePicker.Checked, fromDatePicker.Value, Nothing)
Yet setting fromDateto Nothingdoesn't result in it being set to Nothingbut to '12:00:00 AM', and the following Ifstatement incorrectly executes the filter because startDateis not Nothing.
然而，设置fromDate为Nothing不会导致它被设置为Nothing'12:00:00 AM'，并且以下If语句错误地执行过滤器，因为startDate不是Nothing。
If (Not startDate Is Nothing) Then
list = list.Where(Function(i) i.InvDate.Value >= startDate.Value)
End If
How do I really ensure startDategets a value of Nothing?
我如何真正确保startDate获得 的值Nothing？
回答by Damien_The_Unbeliever
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.
问题在于它首先检查此作业的右侧，并确定它是DateTime(no ?)类型。然后执行任务。
This will work:
这将起作用：
Dim fromDate As DateTime? = If(fromDatePicker.Checked, _
fromDatePicker.Value, _
CType(Nothing, DateTime?))
Because it forces the right-hand side's type to be DateTime?.
因为它强制右侧的类型为DateTime?.
As I said in my comment, Nothingcan be more akin to C#'s default(T)rather than null:
正如我在评论中所说，Nothing可以更类似于 C#default(T)而不是null：
Nothingrepresents 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.
Nothing表示数据类型的默认值。默认值取决于变量是值类型还是引用类型。
回答by danicomas
With VB.NET and EF 6.X to save null is:
使用 VB.NET 和 EF 6.X 来保存 null 是：
Dim nullableData As New Nullable(Of Date)
Dim nullableData As New Nullable(Of Date)
回答by Ulf ?kerstedt
In addition to @Damien_The_Unbeliever's fine answer, using New DateTime?also works:
除了@Damien_The_Unbeliever 的好答案，使用New DateTime?也有效：
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.
您可能会发现它看起来有点反直觉，为什么可能CType(Nothing, DateTime?)更可取。
回答by SSS
Use New Dateas a magic number:
使用New Date作为一个神奇的数字：
Dim fromDate As New Date
If fromDatePicker.Checked Then
fromDate = fromDatePicker.Value
End If
If fromDate <> New Date Then
list = list.Where(Function(i) i.InvDate.Value >= fromDate.Value)
End If
回答by Raj
squery = "insert into tblTest values('" & Me.txtCode.Text & "', @Date)"
Dim cmd = New SqlCommand(squery, con)
cmd.Parameters.Add("@Date", SqlDbType.DateTime)
If txtRequireDate.Text = "" Then
cmd.Parameters("@Date").Value = DBNull.Value
Else
cmd.Parameters("@Date").Value = txtRequireDate.Text
End If
