I need the Expand / Collapse for RowDetailsTemplate

Check this ...

Adding a Button to a WPF DataGrid

OR

XAML :

<DataGrid Name="dg1" AutoGenerateColumns="False" SelectionMode="Single" CanUserAddRows="false" CanUserDeleteRows="False" SelectionUnit="FullRow" >    <DataGrid.CellStyle>        <Style TargetType="DataGridCell">            <Setter Property="BorderThickness" Value="0"/>        </Style>    </DataGrid.CellStyle>    <DataGrid.RowHeaderTemplate>        <DataTemplate>            <Expander Expanded="Expander_Expanded" Collapsed="Expander_Collapsed">            </Expander>        </DataTemplate>    </DataGrid.RowHeaderTemplate>    <DataGrid.Columns>        <DataGridTextColumn Header="Name" IsReadOnly="True" Width="100" Binding="{Binding Name}" />        <DataGridTextColumn Header="Title" IsReadOnly="True" Width="100" Binding="{Binding Title}" />        <DataGridTextColumn Header="Job" IsReadOnly="True" Width="100" Binding="{Binding Job}" />    </DataGrid.Columns>    <DataGrid.RowDetailsTemplate>        <DataTemplate>            <TextBlock Text="{Binding Isi, Converter={StaticResource ResourceKey=isiTextConverter}}" Margin="10,5,0,0" />        </DataTemplate>    </DataGrid.RowDetailsTemplate></DataGrid>

MainWindow.xaml.cs

private void Expander_Expanded(object sender, RoutedEventArgs e){    for (var vis = sender as Visual; vis != null; vis = VisualTreeHelper.GetParent(vis) as Visual)    if (vis is DataGridRow)    {        var row = (DataGridRow)vis;        row.DetailsVisibility = row.DetailsVisibility == Visibility.Visible ? Visibility.Collapsed : Visibility.Visible;        break;    }}private void Expander_Collapsed(object sender, RoutedEventArgs e){    for (var vis = sender as Visual; vis != null; vis = VisualTreeHelper.GetParent(vis) as Visual)        if (vis is DataGridRow)        {            var row = (DataGridRow)vis;            row.DetailsVisibility = row.DetailsVisibility == Visibility.Visible ? Visibility.Collapsed : Visibility.Visible;            break;        }}

Output

Include Collapsed and expanded events as below

   <Custom:DataGridTemplateColumn>   <Custom:DataGridTemplateColumn.CellTemplate>            <DataTemplate>                <Expander Collapsed="exp_Collapsed" Expanded="exp_Expanded"/>            </DataTemplate>        </Custom:DataGridTemplateColumn.CellTemplate>    </Custom:DataGridTemplateColumn>

In the code behind

 private void exp_Collapsed(object sender, RoutedEventArgs e)        {            this.dataGrid1.RowDetailsVisibilityMode = DataGridRowDetailsVisibilityMode.Collapsed;        }        private void exp_Expanded(object sender, RoutedEventArgs e)        {            this.dataGrid1.RowDetailsVisibilityMode = DataGridRowDetailsVisibilityMode.VisibleWhenSelected;        }

I have improved the previous answer :

Instead of using DataGrid.RowHeaderTemplate use DataGridTemplateColumn as below:

<DataGridTemplateColumn>   <DataGridTemplateColumn.CellTemplate>          <DataTemplate>                <Expander Expanded="Expander_OnExpanded"     Collapsed="Expander_OnCollapsed">                </Expander>           </DataTemplate>   </DataGridTemplateColumn.CellTemplate></DataGridTemplateColumn>

Benifit is you need not to re position your mouse after clicking expander button.