Datagrid CheckBox Template GetCellContent Null Exception or always False Value error

# Xaml

         <DataGridTemplateColumn Header="사용여부" Width="70" IsReadOnly="True" >
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <CheckBox x:Name="chks" IsChecked="{Binding Usedchk ,UpdateSourceTrigger=PropertyChanged, Mode=twoWay}"  Padding="3,0,0,0" Grid.Column="1" Height="30" Width ="70" Grid.Row="8" HorizontalAlignment="Center" HorizontalContentAlignment="Left" VerticalAlignment="Top" BorderBrush="#FF852789" BorderThickness="3" Background="#FFF38A21" >

                            <CheckBox.Content>
                                <TextBlock Text="사용"/>
                                <!--"{Binding Item}"/>-->
                            </CheckBox.Content>

                        </CheckBox>

                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>

#CS
// Error1. CheckBox mycheckbox = dataGrid1.Columns[7].GetCellContent(dataGrid1.Items[Row_num])) as CheckBox;

// Error2. bool? chks = ((CheckBox)dataGrid1.Columns[7].GetCellContent(dataGrid1.Items[Row_num])).IsChecked;).IsChecked;)

// Solved

FrameworkElement item = dataGrid1.Columns[7].GetCellContent(dataGrid1.Items[Row_num]);

DataGridTemplateColumn temp = (dataGrid1.Columns[7] as DataGridTemplateColumn);
CheckBox cb = temp.CellTemplate.FindName("chks", item) as CheckBox; //

if (cb.IsChecked == true)
{

}