“空"和“空"有什么区别?和“无"在VB6中? [英] What is the difference between "Null" and "Nothing" in VB6?

问题描述

I have a recordset like this:

Dim rs as Recordset
Set rs as New Recordset

'... a lot of coding ...

if Err.Number <> 0 Then ' oops, something gone wrong!
    If rs.State <> adStateClosed Then rs.Close
    Set rs = Nothing
end if

' I want to evaluate if rs is Nothing, or Null

if rs is Nothing then 
' this doesn't throw errors, and works well :D
end if

if rs is Null then
' this throws an error of "types not compatible"
end if

if rs = Null then
' this throws an error of "types not compatible"
end if

if isNull(rs) then
' never enters here, isNull(rs) evaluates to False
end if

I found out that in VB6 I rarely use "Null" (I used it for evaluating empty recordset schema names), but I use "Nothing" for stuff like images, adodb.connections or recordsets. For strings I have vbNullString. I read it is a pointer to a null string.

Is "Null" like a "unknown variable value" and "Nothing" a true null value?

解决方案

Null is a specific subtype of a Variant. It has no existence outside of the Variant type, and is created to allow a Variant to model a database null value.

Nothing is a value of an Object variable. It essentially is identical to a null pointer, i.e. there is no object.

The following raises an error because "Is" can only be used with Object variables:

if rs is Null then
' this throws an error of "types not compatible"
end if

The following raises an error because an Object variable can never be Null:

if rs = Null then
' this throws an error of "types not compatible"
end if

The following evaluates False because IsNull() takes a Variant argument.

if isNull(rs) then
' never enters here, isNull(rs) evaluates to False
end if

It is equivalent to:

VarType(rs) = vbNull

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