Count clicks in listbox

I'm trying to detect if a user has clicked an item in a listbox. The reason for this is that if a user has clicked the listbox, then the datasettable that fills the second listbox must be cleared and filled up by another dataadapter.

I've written some code but is doesnt do the job:

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
If blnLoadListbox1 = True Then

Dim strSQLGroup As String

If Me.ListBox1.SelectedIndex > -1 Then
Me.dataset.Tables("tblGroup").Clear()
End If

Try
strSQLGroup = "SELECT GroupDescription FROM tblGroup where tblGroup.MainGroupID=" & Me.ListBox1.SelectedValue.ToString
Dim DAGroup As OleDbDataAdapter = New OleDbDataAdapter(strSQLGroup, connection)
DAGroup.Fill(dataset, "tblGroup")
Catch ex As Exception
MsgBox(ex.ToString)
End Try

Me.ListBox2.DataSource = dataset.Tables("tblGroup")
Me.ListBox2.DisplayMember = "GroupDescription"

End If

End Sub


The code crashes and I get an error message at the line : Me.dataset.Tables("tblGroup").Clear(). The message is the following:

An unhandled exception of type 'System.NullReferenceException' occurred in TypedDatasets.exe

Additional information: Object reference not set to an instance of an object.


Any ideas???

Thanks

Tom

You haven't commented your code well enough for someone like me to be able to tell alot about what's going on.

I'm seeing me.dataset, which obviously isn't a standard property.

The error sounds like the dataset was never instantiated, only referenced. Therefore, when you are calling Clear(), your code fails because its trying to call a method from an object that doesn't exist yet.

Is your me.Listbox set to the same datatable you are trying to clear? If so, it may fire a second SelectedIndexChanged event. And that will cause you headaches..

post more details plz..

OK, sorry, I'll comment the code:


Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged

If blnLoadListbox1 = True Then ' Here I check if listbox1 is filled

Dim strSQLGroup As String

[COLOR=red]
'In the following code I check if an item is clicked in listbox1, if so then selected index of listbox1 will be different from -1 and then has the table tblGroup that fills listbox2 and is a table of the dataset to be cleared so it's ready to be filled by a new dataadapter.
/[COLOR=red]

If Me.ListBox1.SelectedIndex > -1 Then
Me.dataset.Tables("tblGroup").Clear()
End If

[COLOR=red]
Here I create the sqlstring for the dataadapter that has to fill the datatable tblGroup that is a part of the dataset
[COLOR=red]

Try
strSQLGroup = "SELECT GroupDescription FROM tblGroup where tblGroup.MainGroupID=" & Me.ListBox1.SelectedValue.ToString
Dim DAGroup As OleDbDataAdapter = New OleDbDataAdapter(strSQLGroup, connection)
DAGroup.Fill(dataset, "tblGroup")
Catch ex As Exception
MsgBox(ex.ToString)
End Try

[COLOR=red]
Here I fill the listbox with the contents of dataset.tblGroup
[COLOR=red]

Me.ListBox2.DataSource = dataset.Tables("tblGroup")
Me.ListBox2.DisplayMember = "GroupDescription"

End If

End Sub


Hope I was clear???

Tom

Both listboxes make use of the same dataset but diferent tables in this dataset. Listbox2 uses tblGroup. If listbox1 is clicked then the data in listbox2 must be updated because there exists a relationship between the two tables in the access database.

this will look better

VB Code:

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
 
If blnLoadListbox1 = True Then ' Here I check if listbox1 is filled
 
Dim strSQLGroup As String
 
'In the following code I check if an item is clicked in listbox1, if so then selected index of listbox1 will be different from -1 and then has the table tblGroup that fills listbox2 and is a table of the dataset to be cleared so it's ready to be filled by a new dataadapter.
 
If Me.ListBox1.SelectedIndex > -1 Then 
Me.dataset.Tables("tblGroup").Clear()
End If
 
'Here I create the sqlstring for the dataadapter that has to fill the datatable tblGroup that is a part of the dataset
 
Try
strSQLGroup = "SELECT GroupDescription FROM tblGroup where tblGroup.MainGroupID=" & Me.ListBox1.SelectedValue.ToString
Dim DAGroup As OleDbDataAdapter = New OleDbDataAdapter(strSQLGroup, connection)
DAGroup.Fill(dataset, "tblGroup")
Catch ex As Exception
MsgBox(ex.ToString)
End Try
 
'Here I fill the listbox with the contents of dataset.tblGroup
 
Me.ListBox2.DataSource = dataset.Tables("tblGroup")
Me.ListBox2.DisplayMember = "GroupDescription"
 
End If
 
End Sub

Two things:

You have an uninstantiated object somewhere. Either your TblGroup table, or its parent dataset.

That's my first guess, because the error seems to suggest it.


Second possibility:
Because you said:
"If listbox1 is clicked then the data in listbox2 must be updated because there exists a relationship between the two tables in the access database."


If these relations are reflected in the dataset, and tblgroup has a one-to-many with the other (or any other table) in that dataset, clearing the table would cause some rows in child tables from this table to be left without a linking key, the clear method would fail. I would think this to be a whole seperate error.

Bottom line:
Set a breakpoint at that line. Run the program, when it hits the breakpoint, Open the command window, and type:

VB Code:

? Me.dataset.Tables("tblGroup").Cols.Count

to see if the table is actually structured. You should get a count > 0.

Using the command window, you should be able to get a better understanding of what is happening.

Also, I don't understand why you would want to clear tblGroup. Doesn't this table hold related records to the other table? You may want to create a temporary table that will either .Clone or .Copy the table, and fill new records in there. But then again, I'm not clear on your design goals.

Thanks guys,

@nemaroller

I don't think dataset is an unistantiated object because I defined it as a datamember in the class where the sub makes part of.

there is a relation between tblGroup and another table in the access database, but not in the dataset.

I'll try your suggestion and keep you informed.

Thanks,

Tom

you are all AHoles. that error is simple. unless DataSet is a windows control, declare it as a new object if you have to.

Is your dataset showing up on the form?
^{If not, then the forms designer did not bla bla bla.
Just create a new instance of your dataset.}

@nemaroller

The reason why I want to clear the table is because the dataset doesn't contain the relationship between the two tables.

So, you would create a dataset with an explicit relation between the tables like this:

DataSet1.Relations.Add(relation)

Then I can use the relation between the tables to fill the listboxes so I wouldn't need a separate dataadapter to fill the listboxes.

Is this a better way??

Tom

Quote:

---

Originally posted by Danny J
you are all AHoles. that error is simple. unless DataSet is a windows control, declare it as a new object if you have to.

Is your dataset showing up on the form?
^{If not, then the forms designer did not bla bla bla.
Just create a new instance of your dataset.}

---

Yes, we already covered that potty mouth.