Error "Cast from type 'DataRowView' to type 'String' is not valid"

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G

Guest

I've seen this problem posted several times, but have not yet seen a
solution. I make a connection to a SQL database and use a DataAdapter object
to fill a DataSet object with several tables. A DataRelation object links the
tables by an ID column. I bind the parent table to a combo box and a child
table to a DataGrid control using a DataView object. I want the DataGrid
control to reflect the selection in the combo box. For the combo box,
DisplayMember is a text string while ValueMember is the ID field (integer).
In brief:

Private Sub Form1_Load(....)
' Connect to database, populate Tables in DataSet,
' setup DataViews, and define DataRelations.
Call Populate()

ComboBox1.DataSource = MyDataSet.Tables(0)
ComboBox1.DisplayMember = MyDataSet.Tables(0).Columns(1).ColumnName
ComboBox1.ValueMember = MyDataSet.Tables(0).Columns(0).ColumnName
Me.BindingContext(MyDataSet.Tables(0)).Position = 0

Dim strFilter As String
strFilter = "ID = " & ComboBox1.SelectedValue
MyDataView.RowFilter = strFilter
DataGrid1.DataSource = MyDataView
End Sub

Private Sub ComboBox1_SelectedValueChanged(....)
Dim strFilter As String
If (ComboBox1.SelectedIndex <> -1) Then
strFilter = "ID = " & ComboBox1.SelectedValue
MyDataView.RowFilter = strFilter
End If
End Sub

Strangely, the code works if I comment out the
ComboBox1_SelectedValueChanged sub. I can hard code in Form1_Load to select
the first (index=0), or any other item in the combo box, and the DataGrid
reflects the correct item. But of course I cannot select another item in the
combo box and have the DataGrid reflect this. The error is raised in
CombBox1_SelectedValueChanged on the statement:

strFilter = "ID = " & ComboBox1.SelectedValue

and the error is "Cast from type 'DataRowView' to type 'String' is not
valid. I've tried to use ToString and CStr to no avail. I think the key is
that I assign a DataSource to the combo box, plus the DisplayMember and
ValueMember properites, then attempt to use the SelectedValue property when
the user makes a selection. Any ideas?

Note. My code is based on "Using a Data Set with Tables in a Parent-Child
Relationship" from Chapter 10 (pg. 402-408) of "Programming Microsoft SQL
Server 2000 with Microsoft Visual Basic .NET" by Rick Dobson.

Richard
 
Hi,

Richard Mueller said:
I've seen this problem posted several times, but have not yet seen a
solution. I make a connection to a SQL database and use a DataAdapter
object
to fill a DataSet object with several tables. A DataRelation object links
the
tables by an ID column. I bind the parent table to a combo box and a child
table to a DataGrid control using a DataView object. I want the DataGrid
control to reflect the selection in the combo box. For the combo box,
DisplayMember is a text string while ValueMember is the ID field
(integer).
In brief:

Private Sub Form1_Load(....)
' Connect to database, populate Tables in DataSet,
' setup DataViews, and define DataRelations.
Call Populate()

ComboBox1.DataSource = MyDataSet.Tables(0)
ComboBox1.DisplayMember = MyDataSet.Tables(0).Columns(1).ColumnName
ComboBox1.ValueMember = MyDataSet.Tables(0).Columns(0).ColumnName
Me.BindingContext(MyDataSet.Tables(0)).Position = 0

Dim strFilter As String
strFilter = "ID = " & ComboBox1.SelectedValue
MyDataView.RowFilter = strFilter
DataGrid1.DataSource = MyDataView
End Sub

Private Sub ComboBox1_SelectedValueChanged(....)
Dim strFilter As String
If (ComboBox1.SelectedIndex <> -1) Then
strFilter = "ID = " & ComboBox1.SelectedValue
MyDataView.RowFilter = strFilter
End If
End Sub

Strangely, the code works if I comment out the
ComboBox1_SelectedValueChanged sub. I can hard code in Form1_Load to
select
the first (index=0), or any other item in the combo box, and the DataGrid
reflects the correct item. But of course I cannot select another item in
the
combo box and have the DataGrid reflect this. The error is raised in
CombBox1_SelectedValueChanged on the statement:

strFilter = "ID = " & ComboBox1.SelectedValue

and the error is "Cast from type 'DataRowView' to type 'String' is not
valid. I've tried to use ToString and CStr to no avail. I think the key is
that I assign a DataSource to the combo box, plus the DisplayMember and
ValueMember properites, then attempt to use the SelectedValue property
when
the user makes a selection. Any ideas?
Click to expand...

You're probely getting this error because SelectedValueChanged fires after
setting the ComboBox.DataSource but before you have set the
ComboBox.ValueMember. So at that point SelectedValue returns a DataRowView.
So you could change it to:

Private Sub ComboBox1_SelectedValueChanged(....)
Dim strFilter As String
If (ComboBox1.SelectedIndex <> -1) AndAlso _
(ComboBox1.ValueMember<>"") Then
strFilter = "ID = " & ComboBox1.SelectedValue
MyDataView.RowFilter = strFilter
End If
End Sub

BUT, if you have a DataRelation between the two DataTables, then you don't
need to do that, you can let the child rows be _automatically_ filtered.
All you have to do is bind the DataGrid to the same DataSource as the
ComboBox and set DataGrid.DataMember to the name of the DataRelation between
them:

Private Sub Form1_Load(....)
' Connect to database, populate Tables in DataSet,
' setup DataViews, and define DataRelations.
Call Populate()

ComboBox1.DataSource = MyDataSet.Tables(0)
ComboBox1.DisplayMember = MyDataSet.Tables(0).Columns(1).ColumnName
ComboBox1.ValueMember = MyDataSet.Tables(0).Columns(0).ColumnName

DataGrid1.DataSource = MyDataSet.Tables(0)
DataGrid1.DataMember = "NameOfRelationBetweenTables"
End Sub

This way the DataGrid should be auto filtered when you change ComboBox
selection.


HTH,
Greetings
 
Bart,

Thank you very much. Using the DataRelation worked. I just had to specify
the "RelationName" attribute of my DataRelation:

DataGrid1.DataSource = MyDataSet.Tables(0)
DataGrid1.DataMember = MyDataRelation.RelationName

Richard
 
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