Silverlight ComboBox SelectionChanged event fires twice - silverlight

Question

I'm using a ComboBox within a DataGrid. I am using this DataGrid for both "Add" and "Edit". When I change the value of ComboBox in code during "Edit", the SelectionChanged Event gets fired twice. 1st time it assigns the proper value, then 2nd time null is assigned to ComboBox which clears the data I had set previously!!
I can't figure out what exactly I'm doing wrong.
Here's the XAML snippet where I bind the ComboBox to model.
<sdk:DataGridTextColumn x:Name="SlNo" Binding="{Binding SlNo}" Header="sl.no" IsReadOnly="True"/>
<sdk:DataGridTemplateColumn Header="Activity Type">
<sdk:DataGridTemplateColumn.CellEditingTemplate>
<DataTemplate>
<ComboBox x:Name="ActivityTypeCombo" IsDropDownOpen="True"
ItemsSource="{Binding AvailableActivityTypes}"
SelectionChanged="ActivityTypeSelectionChanged"
SelectedItem="{Binding SelectedActivityType, Mode=TwoWay}"
SelectedValue="{Binding Path=Description, Mode=TwoWay}">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Description}"/>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
</DataTemplate>
</sdk:DataGridTemplateColumn.CellEditingTemplate>
</sdk:DataGridTemplateColumn>
The Code Snippet where I set the value is:
foreach (var claimDetailViewModel in Claims)
{
claimDetailViewModel.SelectedActivityType =
_autoFillModel.ActivityTypes.SingleOrDefault(at => at.Id == climDetailViewModel.ActivityTypeId);
}
ClaimDetailsGrid.ItemsSource = Claims;

Answer 1

It seems to me that using both SelectedItem and SelectedValue to control selection will not work properly.
When SelectedItem is set this will most likely trigger SelectedValue to be evaluated, and because there is no SelectedValuePath set it will result in SelectedItem=null.
Try removing the SelectedValue binding.