Question

I have a WPF/MVVM (using MVVM-Light) app setup with a ComboBox that is inside a DataTemplate. The XAML of the ComboBox looks like this:

<ComboBox x:Name="cbTeachers" 
          Grid.Column="1" 
          Style="{StaticResource ComboBox}" 
          ItemsSource="{Binding Teachers}" 
          Grid.Row="3" 
          DisplayMemberPath="Name" 
          SelectedValuePath="Id" 
          IsSynchronizedWithCurrentItem="False" 
          SelectedItem="{Binding Path=SelectedTeacher}">
     <i:Interaction.Triggers>
          <i:EventTrigger EventName="SelectionChanged">
               <i:InvokeCommandAction Command="{Binding TeacherSelectedCommand}"
                                      CommandParameter="{Binding SelectedItem, ElementName=cbTeachers}" />
          </i:EventTrigger>
     </i:Interaction.Triggers>
</ComboBox>

The Teachers property for the ItemsSource is a type called ObservableRangeCollection and is based on the code found here: http://geekswithblogs.net/NewThingsILearned/archive/2008/01/16/have-worker-thread-update-observablecollection-that-is-bound-to-a.aspx, but it’s very similar to a standard ObservableCollection. The SelectedTeacher property is set when another property is set and the code looks very similiar to this:

this.SelectedTeacher = (from t in this.Teachers where t.Id == this.DataItem.Teacher.Id select t).Single();

The problem I am running into, which makes zero sense to me, is SelectedTeacher is getting reset to null once I set it. I can step through the debugger and see SelectedTeacher has a value and when I put a breakpoint on the setter for the property it definitely has the value. But then that property gets hit again with a null value. I checked the call stack and it showed the only preceeding line as being External Code (which makes sense since I only set that property in one place and it only gets hit once, as expected). Expanding the External Code option in the call stack window shows the typical WPF call stack of maybe 40 methods so it’s definitely internal to WPF and not something I am doing to make it reset. In fact, when I remove the SelectedItem=”{Binding SelectedTeacher}” the setter for that property doesn’t get called a second time (thus it retains its value), but of course the ComboBox doesn’t show the selected item either. I tried implementing a SelectedIndex option in my viewmodel but that didn’t work either. The ComboBox just won’t select the item. I can change the selected item in the ComboBox just fine, but the initial setting won’t take.

Any ideas? Based on everything I’ve searched it might be related to me using a DataTemplate, but I have to because that template is part of a parent ContentTemplateSelector implementation.

As a side note, I have multiple properties that bind to controls in this DataTemplate and this is the only one that doesn’t work. The others work perfectly. I have also tried the ComboBox with and without the “IsSynchronizedWithCurrentItem” flag and it made no difference.

Answer 1

have you tried to remove to EventTrigger stuff and just to use

SelectedItem="{Binding Path=SelectedTeacher, Mode=TwoWay}"

with Mode=TwoWay?

its not clear to me what you want to achieve with your EventTrigger?