Hi Joe

Thanks for the hint. However, this does not solve my issue (the default value of the constraint is to allow null anyway) - I still can't set a null value through the UI as the XamComboEditor limits me to its items (which have been determined using a binding expression). Is there some built-in functionality that allows me to set a cell/editor value to null, or do I have to apply a hack for now?

Cheers

Philipp 

ps: As for users who found the posting searching for a ValueConstraint sample, here's how I applied it:

<!-- defines the dropdown list to specify unit flags -->
<Style TargetType="{x:Type igEditors:XamComboEditor}"
       x:Key="Grid_UnitList">
  <Setter Property="ItemsProvider">
    <Setter.Value>
      <igEditors:ComboBoxItemsProvider ValuePath="UnitId"
                                       DisplayMemberPath="DefaultName"
                                       ItemsSource="{Binding Path=CurrentProject.Units}" />
    </Setter.Value>
  </Setter>
  <Setter Property="ValueConstraint">
    <Setter.Value>
      <igEditors:ValueConstraint Nullable="True" />
    </Setter.Value>
  </Setter>
  <Setter Property="DropDownResizeMode"
          Value="None" />
</Style>

The only built in way that I know of to do this would be to set IsEditable to true and specify a NullText string, or leave IsEditable false and have a null item in the dropdown list.

Another approach would be to specify a context menu that does it. 

 Hi Joe

Thanks for the suggestions! I went with setting IsEditable to true and a NullText string. The other solution didn't quite work: When having a null item in the ItemsSource, an empty line is displayed in the combobox, but selecting it does not assign the null value - this just closes the combobox without setting the field's value at all. IsEditable = true at least allows me to select and manually delete the text. The NullText property however, doesn't work as well - if the field value is null, the cell just stays empty, but I can live with that. Maybe this is related to my binding that contains a DisplayMemberPath. It's not as nice as it could be, but at least it works :-)

Cheers,

Philipp

ps: This forum really could use a formatting option for code snippets...

<!-- defines the dropdown list to specify unit flags -->
<Style TargetType="{x:Type igEditors:XamComboEditor}"
       x:Key="Grid_UnitList">
  <Setter Property="IsEditable"
          Value="True" />
  <Setter Property="NullText"
          Value="(none)" />
  <Setter Property="ItemsProvider">
    <Setter.Value>
      <igEditors:ComboBoxItemsProvider DisplayMemberPath="DefaultName"
                                       ItemsSource="{Binding Source={StaticResource Data_ProjectController}, Path=CurrentProject.Units}" />
    </Setter.Value>
  </Setter>
  <Setter Property="DropDownResizeMode"
          Value="None" />
</Style>