【WPF】ScrollViewer can't scroll?

original xmal:

                    <ScrollViewer Canvas.Right="20" Canvas.Top="10" CanContentScroll="True">
                        <DataGrid  Name="data" Canvas.Right="50" Canvas.Top="0" IsReadOnly="True">
                            <DataGrid.Columns>
                                <DataGridTextColumn Header="CLASS" Binding="{Binding Path=Class}"/>
                                <DataGridTextColumn Header="ENTRY NO." Binding="{Binding Path=EntryNo}"/>
                                <DataGridTextColumn Header="FEE" Binding="{Binding Path=fee}"/>
                            </DataGrid.Columns>
                        </DataGrid>
                    </ScrollViewer>

Just add heigth and width of ScrollViewer:

                    <ScrollViewer Canvas.Right="20" Canvas.Top="10" CanContentScroll="True" Width="auto" Height="600">
                        <DataGrid  Name="data" Canvas.Right="50" Canvas.Top="0" IsReadOnly="True">
                            <DataGrid.Columns>
                                <DataGridTextColumn Header="CLASS" Binding="{Binding Path=Class}"/>
                                <DataGridTextColumn Header="ENTRY NO." Binding="{Binding Path=EntryNo}"/>
                                <DataGridTextColumn Header="FEE" Binding="{Binding Path=fee}"/>
                            </DataGrid.Columns>
                        </DataGrid>
                    </ScrollViewer>