[问题] 在silverlight中用使用DomainDataSource 对datagrid给值并分页,是不是一定要加上SortDescriptor?

没办法,自己初学者,实在是不知道当我没有用SortDescriptors进行排序的时候我就是在翻页的时候就会报错? Why? 真的是不知道,当我把默认排序加上去的时候就没有任何错误,我不知道是哪里错了,麻烦各位大侠帮帮小弟,

我一翻页就报错,但是我加上那个SortDescriptor 就没有错, 真是头大了

代码

   
   

    
    
    
    <
    
    Grid x:Name
    
    =
    
    "
    
    LayoutRoot
    
    "
    
     Background
    
    =
    
    "
    
    White
    
    "
    
    >
    
    
 
    
    <
    
    riaControls:DomainDataSource Name
    
    =
    
    "
    
    domainDataSource1
    
    "
    
     QueryName
    
    =
    
    "
    
    GetProducts
    
    "
    
     AutoLoad
    
    =
    
    "
    
    True
    
    "
    
     LoadSize
    
    =
    
    "
    
    10
    
    "
    
     PageSize
    
    =
    
    "
    
    5
    
    "
    
    >
    
    
 
    
    <
    
    riaControls:DomainDataSource.DomainContext
    
    >
    
    
 
    
    <
    
    ria:DomainService1
    
    />
    
    
 
    
    </
    
    riaControls:DomainDataSource.DomainContext
    
    >
    
    
 
    
    <!--<
    
    riaControls:DomainDataSource.SortDescriptors
    
    >
    
    
 
    
    <
    
    riaControls:SortDescriptor PropertyPath
    
    =
    
    "
    
    ProductID
    
    "
    
    ></
    
    riaControls:SortDescriptor
    
    >
    
    
 
    
    </
    
    riaControls:DomainDataSource.SortDescriptors
    
    >-->
    
    
 
    
    </
    
    riaControls:DomainDataSource
    
    >
    
    
 
 
    
    <
    
    StackPanel
    
    >
    
    
 
    
    <
    
    sdk:DataGrid AutoGenerateColumns
    
    =
    
    "
    
    True
    
    "
    
     Name
    
    =
    
    "
    
    dataGrid1
    
    "
    
     ItemsSource
    
    =
    
    "
    
    {Binding Data,ElementName=domainDataSource1}
    
    "
    
    >
    
    
 
 
    
    </
    
    sdk:DataGrid
    
    >
    
    
 
    
    <
    
    sdk:DataPager Name
    
    =
    
    "
    
    myDataPager
    
    "
    
     Source
    
    =
    
    "
    
    {Binding Data,ElementName=domainDataSource1}
    
    "
    
    />
    
    
 
    
    </
    
    StackPanel
    
    >
    
    
 
    
    </
    
    Grid
    
    >