ComboBox items with a binded value - wpf

Question

In my WPF page I would like to create a ComboBox with first item empty and the second one binded with a property value, so I did:
<DataGrid x:Name="grdTmplBit" ColumnWidth="*" Style="{StaticResource DataGridStyle}" EnableColumnVirtualization="True" CanUserSortColumns="true" Margin="4,3,2,7" MaxHeight="930" SelectionMode="Extended" SelectionUnit="FullRow" ItemsSource="{Binding ElementName=ObjectsTree, Path=SelectedItem.InfosView, IsAsync=True}" >
<DataGrid.Columns>
<DataGridTemplateColumn Width="110" Header="COMM" Visibility="Visibile">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<ComboBox Width="90" SelectedValue="{Binding Comm.UnitName, UpdateSourceTrigger=PropertyChanged}" Style="{StaticResource workModeStyle}" SelectedValuePath="Content">
<ComboBoxItem Content="" />
<ComboBoxItem Content="{Binding Comm.UnitName}" />
</ComboBox>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
</DataGrid>
The property UnitName is a string. Please consider that the ComboBox is inside a DataGridTemplateColumn. Basically the second item is always empty and if I change the ComboBox with a TextBox the value are shown correctly. Any help?

Answer 1

I think your issue is that you bind the SelectedValue to Comm.UnitName and you also bind the second ComboBoxItem content to this property. This means the ComboBox will act in one of these ways:
(Initial selected value is from Comm.UnitName) > The selected line shows this item> You select the empty item > The Comm.UnitName will get set to the empty string > Result: Both ComboBoxItems are empty (one is set as empty, the other is synchronized with Comm.UnitName, which is empty now, too)
(Initial selected value is the first item, empty) > The selected line shows this item > The Comm.UnitName will get set to the empty string > Result: The same as above.
In order to solve this issue, you could consider one of these options:
Create a dedicated property for the SelectedValue, so the Comm.UnitName does not get overwritten.
Create a dedicated property for the value in the ComboBoxItem.
Use a OneTime mode binding to only synchronize the initial value (this does not work for all cases, especially not if you save the empty value in Comm.UnitName, same issue).
<ComboBoxItem Content="{Binding Comm.UnitName, Mode=OneTime}" />