Question

I have a Combobox which defaults to the first item in the list from ItemsSource using

<ComboBox  x:Name="combo" 
           SelectedIndex="0" 
           ItemsSource="{Binding comboBoxSelections, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}" 
           DisplayMemberPath="Key" 
           IsEditable="True" >

However, below doesn’t bind properly to the a DataTable.
When I add binding to SelectedValue it now binds correctly but no longer defaults to the first item anymore;
SelectedIndex="0" isn’t working now

<ComboBox  x:Name="combo" 
           SelectedValue="{Binding Selection, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
           SelectedIndex="0" 
           ItemsSource="{Binding comboBoxSelections, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}" 
           DisplayMemberPath="Key" 
           IsEditable="True" >

Is it possible to initialise the value in XAML only?

I don’t think I can set it from the constructor as combo is not a single Combobox but part of a DataGridComboBoxColumn

<DataGrid.Columns>
    <DataGridComboBoxColumn Header="Department Id" x:Name="comboboxColumn1"
            SelectedValueBinding="{Binding Department Id}" />
        <DataGridTemplateColumn x:Name="DataGridTempCol" Header="Selection">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <ComboBox  
                      x:Name="combo" 
                      SelectedValue="{Binding Selection, Mode=TwoWay,   UpdateSourceTrigger=PropertyChanged}" 
                      ItemsSource="{Binding comboBoxSelections, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type Window}}}"  
                      DisplayMemberPath="Key" 
                      IsEditable="True" >
                    </ComboBox>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
    </DataGrid.Columns>

Answer 1

Set the SelectedValue to the first row in the ctor.