DataTemplateSelector选择判断对象问题

xaml代码
<Window x:Class="test.PowerSupplySystem"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="PowerSupplySystem" Height="314" Width="476"
xmlns:dts="clr-namespace:test">
<Window.Resources>
<DataTemplate x:Key="SwitchDataTemplate">
<StackPanel Margin="3">
<TextBox Text="{Binding Mswitch}"/>
<Image Source="/IMAGE/断路器.bmp"/>
</StackPanel>
</DataTemplate>

<DataTemplate x:Key="cableDataTemplate">
<StackPanel Margin="3">
<TextBox Text="{Binding Mswitch}"/>
<Image Source="/IMAGE/电缆.bmp"/>
</StackPanel>
</DataTemplate>




</Window.Resources>

<Grid>
<DataGrid Name="system" Loaded="Window_Loaded" AutoGenerateColumns="False" >
<DataGrid.Columns >
<DataGridTemplateColumn Width="150" Header="开关" />
<DataGridTextColumn Width="300" Header="电缆" Binding="{Binding Mcable}" />
</DataGrid.Columns>
</DataGrid>
</Grid>
</Window>

C#代码

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using BLL;

namespace test
{
/// <summary>
/// PowerSupplySystem.xaml 的交互逻辑
/// </summary>
public partial class PowerSupplySystem : Window
{
public PowerSupplySystem()
{
InitializeComponent();
}


//启动时加载
private void Window_Loaded(object sender, RoutedEventArgs e)
{
system.ItemsSource = PowerSupplySystemBll.GetAll();

}

}

public class TaskListDataTemplateSelector : DataTemplateSelector
{
public override DataTemplate
SelectTemplate(object item, DependencyObject container)
{
FrameworkElement element = container as FrameworkElement;

if (element != null && item != null && item is System)
{
Task taskitem = item as Task;

if (System.内容!=null)
return
element.FindResource("SwitchDataTemplate") as DataTemplate;
else
return
element.FindResource("cableDataTemplate") as DataTemplate;
}

return null;
}
}
这个项目我想采用模板控制器，通过判断datagrid里面Columns是否有内容，作为依据选择模板。
现在想问的是怎么才能在C#里面调用到这个datagrid的内容？