Why can t I check if a DateTime is Nothing

Question

In VB NET  is there a way to set a DateTime variable to  not set   And why is it possible to set a DateTime to Nothing  but not possible to check if it is Nothing  For example   Dim d As DateTime   Nothing Dim boolNotSet As Boolean   d Is Nothing    The second statement throws this error    Is  operator does not accept operands of type  Date   Operands must be reference or nullable types

User · Answer

In any programming language  be careful when using Nulls   The example above shows another issue   If you use a type of Nullable  that means that the variables instantiated from that type can hold the value System DBNull Value  not that it has changed the interpretation of setting the value to default using    Nothing  or that the Object of the value can now support a null reference   Just a warning    happy coding   You could create a separate class containing a value type   An object created from such a class would be a reference type  which could be assigned Nothing   An example   Public Class DateTimeNullable Private  value As DateTime   properties Public Property Value   As DateTime     Get         Return  value     End Get     Set ByVal value As DateTime           value   value     End Set End Property   constructors Public Sub New       Value   DateTime MinValue End Sub  Public Sub New ByVal dt As DateTime      Value   dt End Sub   overridables Public Overrides Function ToString   As String     Return Value ToString   End Function   End Class   in Main             Dim dtn As DateTimeNullable   Nothing     Dim strTest1 As String    Falied      Dim strTest2 As String    Failed      If dtn Is Nothing Then strTest1    Succeeded       dtn   New DateTimeNullable DateTime Now      If dtn Is Nothing Then strTest2    Succeeded       Console WriteLine  test1     amp  strTest1      Console WriteLine  test2     amp  strTest2      Console WriteLine   ToString        amp  dtn ToString        Console WriteLine   Value ToString        amp  dtn Value ToString         Console ReadKey          Output       test1   Succeeded        test2   Failed         ToString     4 10 2012 11 28 10 AM       Value ToString     4 10 2012 11 28 10 AM   Then you could pick and choose overridables to make it do what you need   Lot of work - but if you really need it  you can do it

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You can also use below just simple to check   If startDate  lt  gt  Nothing Then your logic End If   It will check that the startDate variable of DateTime datatype is null or not

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DateTime is a value type  which means it always has some value    It s like an integer - it can be 0  or 1  or less than zero  but it can never be  nothing     If you want a DateTime that can take the value Nothing  use a Nullable DateTime

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This is one of the biggest sources of confusion with VB Net  IMO   Nothing in VB Net is the equivalent of default T  in C   the default value for the given type    For value types  this is essentially the equivalent of  zero   0 for Integer  False for Boolean  DateTime MinValue for DateTime       For reference types  it is the null value  a reference that refers to  well  nothing     The statement d Is Nothing is therefore equivalent to d Is DateTime MinValue  which obviously does not compile   Solutions  as others have said   Either use DateTime   i e  Nullable Of DateTime    This is my preferred solution  Or use d   DateTime MinValue or equivalently d   Nothing   In the context of the original code  you could use   Dim d As DateTime    Nothing Dim boolNotSet As Boolean   d HasValue   A more comprehensive explanation can be found on Anthony D  Green s blog

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You can check this like below    if varDate     01 01 0001   then           blank date  do something  else          Date is not blank  Do some other thing end if

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A way around this would be to use Object datatype instead   Private  myDate As Object Private Property MyDate As Date     Get         If IsNothing  myDate  Then Return Nothing         Return CDate  myDate      End Get     Set value As Date          If date   Nothing Then              myDate   Nothing             Return         End If          myDate   value      End Set End Property   Then you can set the date to nothing like so   MyDate   Nothing Dim theDate As Date   MyDate If theDate   Nothing Then      date is nothing End If

User · Answer

DateTime is a value type  which is why it can t be null  You can check for it to be equal to DateTime MinValue  or you can use Nullable Of DateTime  instead   VB sometimes  helpfully  makes you think it s doing something it s not  When it lets you set a Date to Nothing  it s really setting it to some other value  maybe MinValue   See this question for an extensive discussion of value types vs  reference types

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Some examples on working with nullable DateTime values    See Nullable Value Types  Visual Basic  for more        An ordinary DateTime declaration  It is  not  nullable  Setting it to    Nothing  actually results in a non-null value    Dim d1 As DateTime   Nothing Console WriteLine String Format  d1     0   n   d1     Output   d1    1 1 0001 12 00 00 AM     Console WriteLine String Format  d1 is Nothing    0   n    d1 Is Nothing          Compilation error on above expression   d1 Is Nothing              Is  operator does not accept operands of type  Date           Operands must be reference or nullable types       Three different but equivalent ways to declare a DateTime   nullable    Dim d2  As DateTime   Nothing Console WriteLine String Format  d2     0    1   n   d2   d2 Is Nothing      Output   d2      True   Dim d3 As DateTime    Nothing Console WriteLine String Format  d3     0    1   n   d3   d3 Is Nothing      Output   d3      True   Dim d4 As Nullable Of DateTime    Nothing Console WriteLine String Format  d4     0    1   n   d4   d4 Is Nothing      Output   d4      True    Also  on how to check whether a variable is null  from Nothing  Visual Basic      When checking whether a reference  or nullable value type  variable is null  do not use   Nothing or  lt  gt  Nothing  Always use Is Nothing or IsNot Nothing

[vb.net] Why can't I check if a 'DateTime' is 'Nothing'?

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