Why can't I check if a 'DateTime' is 'Nothing'?

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

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.

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.