Read from DataGrid not working - wpf

Question

I have created a simple datagrid to show some values, let the user change them and read back the changed values in the background program. Here is the XAML design file
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Button x:Name="ButAdd" Content="Add Row" HorizontalAlignment="Left" Height="23" Margin="362,34,0,0" VerticalAlignment="Top" Width="77"/>
<TextBox x:Name="TeBoResult" HorizontalAlignment="Left" Height="90" Margin="52,220,0,0" TextWrapping="Wrap" Text="Display the Row 0 colmn 1 changed value here:" VerticalAlignment="Top" Width="322" AcceptsReturn="True" IsManipulationEnabled="True"/>
<Button x:Name="ButRead" Content="Read Row 2" HorizontalAlignment="Left" Height="24" Margin="425,184,0,0" VerticalAlignment="Top" Width="82"/>
<DataGrid x:Name="DaGr" ItemsSource="{Binding}" HorizontalAlignment="Left" Height="133" Margin="10,75,0,0" VerticalAlignment="Top" Width="174" AutoGenerateColumns="False" IsManipulationEnabled="True" EnableColumnVirtualization="True" >
<DataGrid.Columns>
<DataGridTextColumn Header="Text" Binding="{Binding Path=No, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
<DataGridCheckBoxColumn Header="Check box" Binding="{Binding Path=Sel}"/>
<DataGridComboBoxColumn Header="Combo box" Binding.XmlNamespaceManager="{Binding Path=Drop}"/>
</DataGrid.Columns>
</DataGrid>
<ComboBox x:Name="CoBo_IN" HorizontalAlignment="Left" Height="20" Margin="344,100,0,0" VerticalAlignment="Top" Width="84" Visibility="Visible">
<ComboBoxItem Content="Move" HorizontalAlignment="Left" Width="88"/>
<ComboBoxItem Content="Dock" HorizontalAlignment="Left" Width="88" Selected="ComboBoxItem_Selected"/>
</ComboBox>
</Grid>
And the backgroud vb.net code is here
Imports System.Collections.ObjectModel
Imports System.ComponentModel
Imports System.Runtime.CompilerServices
Imports DataGrid.datset
Imports System.Windows
Class MainWindow
Public Property coll As New ObservableCollection(Of bind)()
Private Sub ButAdd_Click(sender As Object, e As RoutedEventArgs) Handles ButAdd.Click
Dim qw As New bind()
qw.No = "Change Me"
qw.Sel = Nothing
qw.Drop = CoBo_IN
coll.Add(qw)
DaGr.ItemsSource = coll
End Sub
Private Sub ButRead_Click(sender As Object, e As RoutedEventArgs) Handles ButRead.Click
Dim val As String
For Each item As bind In DaGr.Items
val = item.No
TeBoResult.Text = TeBoResult.Text & val
Next
End Sub
End Class
Public Class datset : Implements INotifyPropertyChanged
Public Event PropertyChanged As PropertyChangedEventHandler _
Implements INotifyPropertyChanged.PropertyChanged
Private Sub NotifyPropertyChanged(<CallerMemberName()> Optional ByVal propertyName As String = Nothing)
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(propertyName))
End Sub
Public Structure bind
Public Property No As String
Public Property Sel As Boolean
Public Property Drop As ComboBox
End Structure
End Class
So When I click on the Add Row button the row with the default contents gets added in the TextColumn and CheckBoxColumn but the ComboBoxColumn dosent display the combo!!(but when I double click inside this cell the ComboBox appears but It is empty). What could be reason for this behaviour?
Next the user will change the contents inside the TextColumn and it gets changed in the GUI as required.
Next when the user clicks on the botton Read Row, all the contents of the TextColumn are read one after the other and is displayed in the Result text box. The problem is though the GUI has a new text when it is read sequentially the val variable still shows the previously bound values only. I thought the problem is with TwoWay binding but it seems to be something else.
Why does the read on DataGrid dosen't give an updated value?

Answer 1

I doesn't know to much of VB, but I can see some things here are not ok. For instance ItemsSource="{Binding}". Here you must to reference the collection to bind, in this case coll. The code should be something like this:
<Window x:Class="MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"
x:Name="window">
<Grid>
<Button x:Name="ButAdd" Content="Add Row" HorizontalAlignment="Left" Height="23" Margin="362,34,0,0" VerticalAlignment="Top" Width="77"/>
<TextBox x:Name="TeBoResult" HorizontalAlignment="Left" Height="90" Margin="52,220,0,0" TextWrapping="Wrap" Text="Display the Row 0 colmn 1 changed value here:" VerticalAlignment="Top" Width="322" AcceptsReturn="True" IsManipulationEnabled="True"/>
<Button x:Name="ButRead" Content="Read Row 2" HorizontalAlignment="Left" Height="24" Margin="425,184,0,0" VerticalAlignment="Top" Width="82"/>
<DataGrid x:Name="DaGr" ItemsSource="{Binding col, ElementName =window}" HorizontalAlignment="Left" Height="133" Margin="10,75,0,0" VerticalAlignment="Top" Width="174" AutoGenerateColumns="False" IsManipulationEnabled="True" EnableColumnVirtualization="True" >
<DataGrid.Columns>
<DataGridTextColumn Header="Text" Binding="{Binding Path=No, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
<DataGridCheckBoxColumn Header="Check box" Binding="{Binding Path=Sel, Mode=TwoWay}"/>
<DataGridComboBoxColumn Header="Combo box" Binding.XmlNamespaceManager="{Binding Path=Drop, Mode=TwoWay"/>
</DataGrid.Columns>
</DataGrid>
<ComboBox x:Name="CoBo_IN" HorizontalAlignment="Left" Height="20" Margin="344,100,0,0" VerticalAlignment="Top" Width="84" Visibility="Visible">
<ComboBoxItem Content="Move" HorizontalAlignment="Left" Width="88"/>
<ComboBoxItem Content="Dock" HorizontalAlignment="Left" Width="88" Selected="ComboBoxItem_Selected"/>
</ComboBox>
</Grid>
Note the items source binding, and the name of the window, also note the bindings should be two way, for update the source too. This new binding references to the col collection. Now in he code behind you shoul not re-set the grid intems source each time you add an item. If the collection is an observable collection then it is added automatically, and now the items should works::
Private Sub ButAdd_Click(sender As Object, e As RoutedEventArgs) Handles ButAdd.Click
Dim qw As New bind()
qw.No = "Change Me"
qw.Sel = Nothing
qw.Drop = CoBo_IN
coll.Add(qw)
End Sub