XAML:
        <Grid Grid.Column="0" Grid.Row="3" Name="gridDetails" Margin="5 20 5 10 " Background="#FFFFEE"  >
            <Grid.RowDefinitions>
                <RowDefinition Height="auto"/>
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="auto"/>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Label Grid.Column="0" Grid.Row="0" Content="Vorname" Margin="2" />
            <TextBox Name="tFirstname" Grid.Column="1" Grid.Row="0"  Margin="2"
                    Text="{Binding Path=Firstname, UpdateSourceTrigger=PropertyChanged }"
                     />
            <Label Grid.Column="2" Grid.Row="0" Content="Firstname" Margin="2" />
            <TextBox Name="tLastname" Grid.Column="3" Grid.Row="0" Margin="2 0 10 0"
                     Text="{Binding Path=Lastname, UpdateSourceTrigger=PropertyChanged}" 
                      />
        </Grid>


CS:
in loaded:
gridDetails.DataContext = empl_items[0];  

private void datagrid_SelectionChanged(object sender, SelectionChangedEventArgs e) {
   if (datagrid.SelectedIndex >= 0) {
       ModelEmployee empl = (ModelEmployee)datagrid.SelectedItem;
       gridDetails.DataContext = empl;
   }
}