Nothing Issue

[JDYoder]

I have this code...

 

Public Structure clsEmployee
   Dim First As String
   Dim Last As String
End Structure

Dim oEmployee As clsEmployee

 

 

How can I check to see if oEmployee equals nothing? The following two lines are invalid...

 

 

If oEmployee Is Nothing Then
If oEmployee = Nothing Then

 

 

I see if I make it a "Class" instead it works, but I'd like to use a Structure in this case if possible. Anyone know what the deal is? I find it especially odd since this line is valid...

 

oEmployee = Nothing

[VagabondSW]

Well, it is a value type and therefore would have to be SET to Nothing for it to equal Nothing, which would be a wasteful exercise. You could compare one of the structure members for Nothing, but that could be error-prone.

 

If oEmployee.Last Is Nothing Then
 'This works but is not ideal
End If

[PlausiblyDamp]

Value types (Structures) cannot be Nothing (null). Reference types (Classes) can.

Net 2.0 introduces the concept of a Nullable type that may work in this scenario - however as a rule of thumb if there is a need to express it as a null / nothing value then use a class rather than a structure.

[JDYoder]

Too bad, but good to know. Thanks for the info.

[JDYoder]

Is this same reason I can get away with...

 

Dim oEmployee As clsEmployee

 

And start using the var right away. But when I turn it into a class, I having to use this syntax right away instead?...

 

Dim oEmployee As New clsEmployee

[PlausiblyDamp]

That's right.

 

Value types effectively are the data, Reference types are a pointer to the data. That's why reference types need to be initialised before use and value types do not.

[Machaira]

Is this same reason I can get away with...

Yes. :) Value types don't need explicit allocation whereas reference types do.

[IngisKahn]

Yup, just like VB6, except now you can assign methods and such to value types.