Can't modify instance of a class from within the class!

  • Thread starter Thread starter pat
  • Start date Start date
P

pat

Group,

I have a class that has properties:

[...]
Private _lastUpdated as String
Property lastUpdated() As String
Get
Return _lastUpdated
End Get
Set(ByVal Value As String)
_lastUpdated = Value
End Set
End Property
[...]

I have a dataset that needs to have its values loaded into these
properties, and the attempt has been:

Sub populate(ByVal ds As DataSet)
' load a class via a dataset with a known structure
Dim row As DataRow = ds.Tables(0).Rows(0)
Me.lastUpdated = row.Item("lastUpdated"))
[...]
End Sub

This is called as you might expect:

Dim classInstance As New className
classInstance.populate(ds)

The program hangs anywhere the assignment to the class variable occurs.
And the problem isn't with e dataset, because ANY assignment attempt
yields the same result:

Me.lastUpdated = "I dunno.."

...or even trying to assign to the Private variable..

Should I try to pass the calling object in the Sub's fields and attempt
to update in a "ByRef" way instead of "ByVal"? This would make sense,
as the class itself is not aware of the instance, or does it?

Ideas?

TIA!

pat
:)
 
You could pass a reference to your instance as another parameter to the
method and use it to modify the property instead of using Me.
 
pat said:
Group,

I have a class that has properties:

[...]
Private _lastUpdated as String
Property lastUpdated() As String
Get
Return _lastUpdated
End Get
Set(ByVal Value As String)
_lastUpdated = Value
End Set
End Property
[...]

I have a dataset that needs to have its values loaded into these
properties, and the attempt has been:

Sub populate(ByVal ds As DataSet)
' load a class via a dataset with a known structure
Dim row As DataRow = ds.Tables(0).Rows(0)
Me.lastUpdated = row.Item("lastUpdated"))
[...]
End Sub

This is called as you might expect:

Dim classInstance As New className
classInstance.populate(ds)

The program hangs anywhere the assignment to the class variable
occurs. And the problem isn't with e dataset, because ANY assignment
attempt yields the same result:

Me.lastUpdated = "I dunno.."

..or even trying to assign to the Private variable..

Should I try to pass the calling object in the Sub's fields and
attempt to update in a "ByRef" way instead of "ByVal"? This would
make sense, as the class itself is not aware of the instance, or
does it?
Click to expand...


I didn't get the problem. What does "the program hangs" mean? Do you get an
exception? Which one? Or wrong behavior?


Armin
 
Armin,

By hang I mean the program execution stops (not exit, not crash) with
load down to 0. I believe I have found the reason why it hangs, but
don't know how to get around it.

I now think that the reason why the program is stopping is because
there is a NULL value in the dataset. No big deal, except that I can't
figure out how to get around the null in the datacolumn through
testing. Each test hangs the program! I'm now going to do a
ds.WriteXmlSchema and see if I can make the datacolumn null tolerant.

Problem is, I can't have .xsd files in my application laying around,
and I can't figure out how to load schema info into a dataset/datatable
from a string.

To Zack's point, I have to solve this null problem before I can move on
to providing this functionality in the class. Right now it is being
done in a simple module (function) until I get this figured out.

Thanks!

pat
:)
 
Perhaps because I'm accessing the dtacolumn members by index, I am
unable to test for null..? According to:
http://msdn.microsoft.com/library/d...con/html/vbtskHandlingNullValuesInDataset.asp

"In a typed dataset, you can control the representation of columns that
contain null values. This behavior is only available when retrieving
column values through their typed accessors. If you retrieve the value
by means of the table's row index or column index, you will not be able
to set the return value."

I guess I'm off to try to figure out what a "typed accessor" would look
like in this scenario... ?

pat
:)
 
It's really weird you don't have any error. Make sure you don"t have a
"catch error and do nothing'" in your code. What if you step throught the
code.

IMO it's soemthing else in your code (are you sure this code is called ?).
 
pat said:
Perhaps because I'm accessing the dtacolumn members by index, I am
unable to test for null..? According to:
http://msdn.microsoft.com/library/d...con/html/vbtskHandlingNullValuesInDataset.asp

"In a typed dataset, you can control the representation of columns
that contain null values. This behavior is only available when
retrieving column values through their typed accessors. If you
retrieve the value by means of the table's row index or column
index, you will not be able to set the return value."

I guess I'm off to try to figure out what a "typed accessor" would
look like in this scenario... ?

pat
:)
Click to expand...


/Do/ you have a typed dataset? As you are using the syntax

row.Item("lastUpdated")

you can check for null values:

If row.Item("lastUpdated") Is DBNull.Value Then

It was not possible if you would use the "typed accessor" in a typed
DataRow:

If row.lastUpdated Is DBNull.Value Then

This threw an exception because the property 'lastUpdated' is of type
String, and a string can not return DBNull. You would have to call the
Is___Null method:

If Row.IsLastUpdatedNull Then

But, as you probably do not use a typed dataset, this doesn't apply.


Anyways, if there is a problem with Null, you should get an exception when
assigning it to a property of type String. Did you really try stepping
through the code to find out that this line

Me.lastUpdated = row.Item("lastUpdated")

makes the program hang? It's not easy to debug it from here. :-)



Armin
 
Armin & Patrice,

Progress. I am using a web service that completely removes all trace of
XML tags for elements that have a NULL value in the column benig
requested/served. Some requests have nulls, some don't. Nice. I'm
trying to figure out a way around the omitted tags on the server side,
but in the mean time I can see how (definitley not why) the .NET app
stopped.

When I tried _anything_ at all in VB referencing a column of _expected_
XML where the tags were simply not there, the code un-gracefully just
sat there. Load to 0% and just halted. This is inside a Try, but if you
think about it, the Try lines never completed execution, so the
exception could not be caught. In fact, I doubt that if I had just that
one line in a Try that it would do anything because program execution
is halted, and no errors/exceptions are thrown.

I don't think there is a way to debug this as mortal users. This is
probably a feature/bug of VS2003.NET, and MS would probably chalk it up
to not having consistent XML in the first place. I suppose I could try
some very computationally costly tests for the existence of XML
elements, but holy cow that is going to being things to a crawl.

Any of this make sense?

Tanks!

pat
:)
 
Back
Top