SL AutoCompleteBox绑定实体问题

Question

如下代码,输入国家后,竟然调不出国家列表,Populating事件中好像获取不了列表.

xaml如下:
<input:AutoCompleteBox x:Name="CountryNameAutoCompleteBox" Grid.Row="3" Grid.Column="1" Width="150" MinimumPopulateDelay="200" MinimumPrefixLength="1" HorizontalAlignment="Left" IsTextCompletionEnabled="False" FilterMode="Contains" Tag="CountryCodeValue" SelectionChanged="CountryNameAutoCompleteBox_SelectionChanged" ValueMemberBinding="{Binding Name}">
                                  <input:AutoCompleteBox.ItemTemplate>
                                        <DataTemplate>
                                            <Grid Width="Auto">
                                                <Grid.ColumnDefinitions>
                                                    <ColumnDefinition Width="100"></ColumnDefinition>
                                                    <ColumnDefinition></ColumnDefinition>
                                                </Grid.ColumnDefinitions>
                                               
                                                <TextBlock Grid.Column="0" Text="{Binding Name}" HorizontalAlignment="Left" Foreground="Gray"></TextBlock>
                                                <TextBlock Grid.Column="1" Text="{Binding Code}" HorizontalAlignment="Left" Foreground="Gray"></TextBlock>
                                            </Grid>
                                        </DataTemplate>
                                    </input:AutoCompleteBox.ItemTemplate>
                                </input:AutoCompleteBox>

代码如下:

#region 国家选择
        private void CountryNameAutoCompleteBox_Populating(object sender, PopulatingEventArgs e)
        {
            e.Cancel = true;

            AutoCompleteBox acb = (AutoCompleteBox)sender;
            acb.ItemsSource = null;

            HgCountryContext ctxt = new HgCountryContext();

            EntityQuery<HgCountry> query = ctxt.GetHgCountriesQuery().Take(10);
            LoadOperation<HgCountry> loadop = ctxt.Load(query);
            acb.ItemsSource = loadop.Entities;
            acb.PopulateComplete();
        }

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