无法删除此对象，因为未在 ObjectStateManager 中找到它。

代码如下

 var af =ListViews.SelectedItem as Customer ;
                if (af != null)
                {
                    context.DeleteObject(af);
                    context.SaveChanges();
}
无法删除此对象，因为未在 ObjectStateManager 中找到它。
  <ListView x:Name="ListViews" Grid.Row="1"  Width="Auto" HorizontalAlignment="Stretch" Height="Auto" VerticalAlignment="Stretch">
            <ListView.View>
                <GridView>
                    <GridViewColumn Header="Name" DisplayMemberBinding="{Binding Path=name}"  />
                    <GridViewColumn Header="Pass" DisplayMemberBinding="{Binding Path=pass}" />
                 
                  
                </GridView>
            </ListView.View>
        </ListView>

z请问怎么解决这个不能删除的原因啊