Question

I have 2 nested DataContexts in following code (DataContext and ItemsSource):

<TabItem Header="Something" Name="myTabItemName" d:DataContext="{Binding myViewModel}">
    <Grid>
        <DataGrid AutoGenerateColumns="False" ItemsSource="{Binding muObservableCollection}" Name="myDataGridName" HeadersVisibility="Column">
            <DataGrid.Columns>
                <DataGridTextColumn Header="Foo 01" Binding="{Binding Foo_01}" />
                <DataGridTextColumn Header="Foo 02" Binding="{Binding Foo_02}" />
                <DataGridTemplateColumn Header="My custom combobox">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <!-- Here I want to bind back to 'myViewModel's' myProperty. I've tried this way: -->
                            <ComboBox ItemsSource="{Binding Path=myProperty, RelativeSource={RelativeSource AncestorType={x:Type TabItem}}}" />
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</TabItem>

I want to assign myViewModel (back) to ComboBox's DataContext. The problem is that ComboBox has ‘overwritten’ DataContext in line:

<DataGrid ItemsSource="{Binding muObservableCollection}" ... >

How can I do it?

Answer 1

<ComboBox ItemsSource="{Binding Path=DataContext.myProperty, RelativeSource={RelativeSource AncestorType={x:Type TabItem}}}" />

Should do the trick. However I would like to point out that what you want does not make sense: you bind your TabItem let’s say to a Order-instance, the DataGrid to its Orderlines, why do you want a combobox from the Order in the DataGrid? Al the columns will display the same value and if you change one row, the values for all rows change.

I have run into this situation myself a couple of times that I needed access to an higher level (due to refactoring in combination with commands), but it is rather rare.