WPF ComboBox SelectedValue not updating from binding source.

Question

Here's my binding source object:

Public Class MyListObject

    Private _mylist As New ObservableCollection(Of String)
    Private _selectedName As String

    Public Sub New(ByVal nameList As List(Of String), ByVal defaultName As String)

     For Each name In nameList
      _mylist.Add(name)
     Next

     _selectedName = defaultName

    End Sub

    Public ReadOnly Property MyList() As ObservableCollection(Of String)
     Get
      Return _mylist
     End Get
    End Property

    Public ReadOnly Property SelectedName() As String
     Get
      Return _selectedName
     End Get
    End Property

End Class

Here is my XAML:

<Window x:Class="Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300"
    xmlns:local="clr-namespace:WpfApplication1"
     >

    <Window.Resources>
     <ObjectDataProvider x:Key="MyListObject" ObjectInstance="" />
    </Window.Resources>

     <Grid>

     <ComboBox Height="23"
         Margin="24,91,53,0"
         Name="ComboBox1"
         VerticalAlignment="Top"
         SelectedValue="{Binding Path=SelectedName, Source={StaticResource MyListObject}, Mode=OneWay}"
         ItemsSource="{Binding Path=MyList, Source={StaticResource MyListObject}, Mode=OneWay}"
         />

     <Button Height="23"
       HorizontalAlignment="Left"
       Margin="47,0,0,87"
       Name="btn_List1"
       VerticalAlignment="Bottom"
       Width="75">List 1</Button>

     <Button Height="23"
       Margin="0,0,75,87"
       Name="btn_List2"
       VerticalAlignment="Bottom"
       HorizontalAlignment="Right"
       Width="75">List 2</Button>
    </Grid>
</Window>

Here's the code-behind:

Class Window1

    Private obj1 As MyListObject
    Private obj2 As MyListObject
    Private odp As ObjectDataProvider

    Public Sub New()

     InitializeComponent()

     Dim namelist1 As New List(Of String)
     namelist1.Add("Joe")
     namelist1.Add("Steve")
     obj1 = New MyListObject(namelist1, "Steve")
.
     Dim namelist2 As New List(Of String)
     namelist2.Add("Bob")
     namelist2.Add("Tim")
     obj2 = New MyListObject(namelist2, "Tim")

     odp = DirectCast(Me.FindResource("MyListObject"), ObjectDataProvider)
     odp.ObjectInstance = obj1

    End Sub

    Private Sub btn_List1_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles btn_List1.Click

     odp.ObjectInstance = obj1

    End Sub

    Private Sub btn_List2_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles btn_List2.Click

     odp.ObjectInstance = obj2

    End Sub
End Class

When the Window first loads, the bindings hook up fine. The ComboBox contains the names "Joe" and "Steve" and "Steve" is selected by default. However, when I click a button to switch the ObjectInstance to obj2, the ComboBox ItemsSource gets populated correctly in the dropdown, but the SelectedValue is set to Nothing instead of being equal to obj2.SelectedName.

Thanks in advance!

Answer 1

We had a similar issue last week. It has to do with how SelectedValue updates its internals. What we found was if you set selectedvalue it would not see the change we had to instead set selecteditem which would properly update every thing. My conclusion is that SelectedValue is designed for get operations and not set. But this may just be a bug in the current version of 3.5sp1 .net

Answer 2

I have exactly the same problem but SelectedItem doesn't solve it for me, any other ideas?

Edit:

SelectedItem does work, on the proviso that the combobox contains more than one item. If only one item is contained then SelectedItem still does not work!

Answer 3

Ran into something similar, finally I just subscribed to the SelectionChanged event for the drop down and set my data property with it. Silly and wish it was not needed, but it worked.

Answer 4

Is it reasonable to set the SelectedValuePath="Content" in the combobox's xaml, and then use SelectedValue as the binding?

It appears that you have a list of strings and want the binding to just do string matching against the actual item content in the combobox, so if you tell it which property to use for the SelectedValue it should work; at least, that worked for me when I ran across this problem.

It seems like Content would be a sensible default for SelectedValue but perhaps it isn't?

Answer 5

The Binding Mode needs to be OneWayToSource or TwoWay since the source is what you want updated. Mode OneWay is Source to Target and therefore makes the Source ReadOnly which results in never updating the Source.

Answer 6

You know... I've been fighting with this issue for hours today, and you know what I found out? It was a DataType issue! The list that was populating the ComboBox was Int64, and I was trying to store the value in an Int32 field! No errors were being thrown, it just wasn't storing the values!

Answer 7

To stir up a 2 year old conversation:

Another possibility, if you're wanting to use strings, is to bind it to the Text property of the combobox.

<ComboBox Text="{Binding Test}">
     <ComboBoxItem Content="A" />
     <ComboBoxItem Content="B" />
     <ComboBoxItem Content="C" />
</ComboBox>

That's bound to something like:

public class TestCode
{
    private string _test;
    public string Test 
    { 
      get { return _test; }
      set
      {
         _test = value;
         NotifyPropertyChanged(() => Test); // NotifyPropertyChanged("Test"); if not using Caliburn
      }
    }
}

The above code is Two-Way so if you set Test="B"; in code then the combobox will show 'B', and then if you select 'A' from the drop down then the bound property will reflect the change.