C# XAML - How to add a combobox to some datagrid ROWS but not others?

Question

I'm using a key/value observable collection to hold data which is represented in the view by a two column datagrid. Some key values contain a list of items and other key values contain a single string value. Using the below code snippet I'm able to display those key values with items in a combobox. However, the key value 'string' rows display no information and the row becomes read-only. I'm trying not to use code behind. What am I doing wrong? Would it be simple to solve with code behind and if so, what's the best approach?

    <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding KeyValuesCollection}" HorizontalAlignment="Left" Margin="10,10,10,10">
        <DataGrid.Columns>
            <DataGridTextColumn Header="Field" Binding="{Binding Description}" Width="320"/>
            <DataGridTemplateColumn Header="Value" Width="330">
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>                  
                        <ComboBox ItemsSource="{Binding Path=ValueItems}" 
                                  Visibility="{Binding ComboVisible, 
                                  Converter={StaticResource BoolToVis}}" 
                                  DisplayMemberPath="ValueName" 
                                  SelectedValuePath="ID"  
                                  SelectedValue="{Binding Id}" 
                        />
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
        </DataGrid.Columns>
    </DataGrid>

Here is the structure of the items:

public class KeyValue : PropertyChangedBase
{
    public string Description { get; set; }

    public List<ValueItem> ValueItems { get; set; }

    public bool ComboVisible = false;
}

ValueItems is a list of id, string.

Answer 1

Try using a DataTrigger with predefined DataTemplate items:

<DataTemplate x:Key="OneItem" DataType="{x:Type ValueItem}" >
    <TextBox Text="{TemplateBinding Id}" />
</DataTemplate>

<DataTemplate x:Key="MultiItems" DataType="{x:Type ValueItem}" >
    <ComboBox ItemsSource="{TemplateBinding ValueItems}" 
                  DisplayMemberPath="ValueName" 
                  SelectedValuePath="ID"  
                  SelectedValue="{TemplateBinding Id}" />
</DataTemplate>

And then use a Content control to place the style accordingly. I haven't tried this but your data items could easily have a "HasMultipleValueItems" boolean flag for an easy binding.

<DataGridTemplateColumn Header="Value" Width="330">
    <DataGridTemplateColumn.CellTemplate>                    
        <DataTemplate>
            <ContentControl Content="{Binding}">
                <ContentControl.Style>
                    <Style TargetType="{x:Type ContentControl}">
                        <Setter Property="ContentTemplate" Value="{StaticResource OneItem}" />
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding HasMultipleValueItems}" Value="True">
                                <Setter Property="ContentTemplate" Value="{StaticResource MultiItems}" />
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </ContentControl.Style>
            </ContentControl>                      
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>