Whenever you get Object Reference errors (NullReferenceException -
NRE), the best thing to do is to break down your statement on the
basis of nullability:
StudentName = objRow2.Item("Student")
1. Can StudentName be null ? No, it is being assigned to.
2. Can objRow be null? Yes, if no matching row was found. This is the
possible cause of the error. You always need to check this case.
3. Can Item("Student") be null ? It may be possible that the column
student is not present in the retrieved dataset. But in that case, the
likely exception thrown would be an ArgumentException, not an NRE.
So, in this way, you can easily isolate the root cause of the error.
(Cause #2)
On Feb 25, 3:27 pm, Laura <[email protected]> wrote:
> I got rid of the MissingSchemaAction part of the code and left the
> FillSchema.
> Then i set the Primary Key by going:
>
> objAbsentNumberDA.FillSchema(objDS, SchemaType.Source, "NEWB")
> objAbsentNumberDA.Fill(objDS, "NEWB")
> objDS.Tables("NEWB").PrimaryKey = New DataColumn()
> {objDS.Tables("NEWB").Columns("StudentID")}
>
> But then I keep getting the "Object reference not set to an instance
> of an object." error in relation to the following line of code:
> StudentName = objRow2.Item("Student") - from FillNamesNeeded()
>
> I've been messing with this all morning - any ideas as to what's
> wrong???
>
> Thanks
>
> On Feb 25, 7:51 am, Cerebrus <[email protected]> wrote:
>
>
>
> > Some OleDbDataAdapter datasources do not return Primary Key
> > information along with the returned data. Since you are using an mdb
> > (MS Access), it should return the Primary key when you set the
> > MissingSchemaAction or call the FillSchema method explicitly.
> > Therefore, I am unsure as to why this doesn't work, but there may be a
> > couple of things you can look into:
>
> > 1. Try using only one of the two calls - set the MissingSchemaAction
> > OR call the FillSchema method. Not both.
> > 2. Confirm that your Access database has a column configured as the
> > Primary key within the table "NEWB".
> > 3. If nothing works, try setting the Primary key explicitly to the
> > appropriate column in the following way:
>
> > ---
> > Dim newB as DataTable = objDS.Tables("NEWB")
> > newB.PrimaryKey = new DataColumn() { newB.Columns("MyPrimaryColumn") }
> > ---
>
> > On Feb 23, 11:29 pm, Laura <[email protected]> wrote:
>
> > > Hi,
>
> > > I have a database query that I am reading data from into a form. I can
> > > read one column into a listbox without any problems.
> > > Where I get into difficulty is when I try to use the data read into
> > > the listbox to get the related rows.
>
> > > I keep getting the following error:
>
> > > "Missing PrimaryKey Was Unhanled Exception"
> > > "Table doesn't have a primary key"
>
> > > The query has four fields and one of them is a primary key. Is reading
> > > from queries very different to reading from tables??
>
> > > Here is the code I have;
>
> > > Dim objConnection As New OleDb.OleDbConnection( _
> > > "Provider=Microsoft.Jet.OLEDB.4.0;Data Source= FYP.mdb")
> > > Dim objAbsentNumberDA As New OleDb.OleDbDataAdapter("Select * from
> > > NEWB", objConnection)
> > > Dim objAbsentNumberCB As New OleDb.OleDbCommandBuilder
> > > (objAbsentNumberDA)
> > > Dim strNumber As String
> > > Dim StudentName As String
> > > Dim ClassName As String
>
> > > then under the form load;
>
> > > objAbsentNumberDA.MissingSchemaAction = MissingSchemaAction.AddWithKey
> > > objDS.Clear()
> > > objAbsentNumberDA.FillSchema(objDS, SchemaType.Source, "NEWB")
> > > objAbsentNumberDA.Fill(objDS, "NEWB")
>
> > > lstNumberAbsent.Items.Clear()
> > > FillNumberAbsent()
> > > FillNamesNeeded()
>
> > > Private Sub FillNumberAbsent()
> > > Dim i As Integer
> > > For i = 1 To objDS.Tables("NEWB").Rows.Count
> > > strNumber = objDS.Tables("NEWB").Rows(i - 1).Item
> > > ("Absent")
> > > lstNumberAbsent.Items.Add(strNumber)
> > > Next
> > > End Sub
>
> > > Private Sub FillNamesNeeded()
> > > Dim objRow2 As DataRow
> > > objRow2 = objDS.Tables("NEWB").Rows.Find(strNumber)
> > > StudentName = objRow2.Item("Student")
>
> > > FillNumberAbsent() works fine. It's in FillNamesNeeded() that I get
> > > the error at the line: objRow2 = objDS.Tables("NEWB").Rows.Find
> > > (strNumber)
>
> > > Am I doing something wrong, or has it something to do with the fact
> > > that I'm using queries?
>
> > > Thanks- Hide quoted text -
>
> > - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
