Why can’t I check if a ‘DateTime’ is ‘Nothing’?

Question

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