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.

Solution 1

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

Solution 2

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.

Solution 3

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.

Solution 4

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 <> Nothing. Always use Is Nothing or IsNot Nothing.

Solution 5

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

View more solutions

Comments

[DCNYAM]

In addition to John Gant's answer below, you can also check if the datetime variable = Nothing (Note the = instead of "is").

[Muleskinner]

Thanks, using Dim boolNotSet As Boolean = d = Nothing seems like the most simple solution right now. Interesting with the Nullable casting never seen that before

[Karthik Ratnam]

@Chris - I think He is using VB

[Chris Haas]

@Karthik Ratnam, yes he is but it amounts to the same thing and @Marc Gravell's answer gets all of the points across.

[tsuo euoy]

@Chris Haas: Not really, because of the Nothing keyword which has no equivalent in C#. This is not a duplicate.

[DavidRR]

@NYSystemsAnalyst: according to Nothing (Visual Basic), using = Nothing or <> Nothing is not good practice: "When checking whether a reference (or nullable value type) variable is null, do not use = Nothing or <> Nothing. Always use Is Nothing or IsNot Nothing."

[Muleskinner]

@DavidRR I believe you mean Not Is Nothing

[DavidRR]

@Muleskinner: I believe that IsNot Nothing and Not Is Nothing are equivalent. From IsNot Operator (Visual Basic): "IsNot is the opposite of the Is operator. The advantage of IsNot is that you can avoid awkward syntax with Not and Is, which can be difficult to read."

[DavidRR]

@Muleskinner: Here is the list of the keywords in the Visual Studio 2012 edition of Visual Basic. Not sure how long the IsNot keyword has been around.

[Muleskinner]

@DavidRR Sorry my mistake, I see it was introduced in Visual Basic 2005

[Maddin]

Be careful with using variable = nothing, this can lead to NullReferenceException

[phn]

Thanks for explaining. It is interesting to note that isNothing(d) does not work but d = Nothing does!