如何在UWP应用程序中以纵向模式设置弹出窗口放置

Question

我正在开发一个UWP应用程序，并实现了一个弹出窗口。弹出窗口应该在Portrait以及Landscape模式下工作。我已经为横向模式实现了弹出窗口，但在portrait模式下，它将出现在屏幕顶部。这是代码

<Popup x:Name="InValidEntry" Opened="InValidEntry_Opened" x:FieldModifier="Public" IsOpen="{x:Bind ViewModel.FuelPlanInfo.IsInvalidPopupOpened, Mode=TwoWay}" IsLightDismissEnabled="False" 
                               HorizontalAlignment="Center" VerticalAlignment="Center" Margin="0,0,0,0" RenderTransformOrigin="0.5,0.5" Height="396" Width="1047"    >
                                <Canvas x:Name="InValidEntryCanvas" Height="396" Width="1047" Background="{ThemeResource InvalidPopUpBrush}">
                                    <StackPanel x:Name="InValidEntrySP" Width="770" Height="144" Orientation="Vertical"  Canvas.Left="46"  Canvas.Top="29" >
                                        <Grid x:Name="ParentGrid">
                                            <Grid.RowDefinitions>
                                                <RowDefinition Height="*" />
                                                <RowDefinition Height="*"/>

                                            </Grid.RowDefinitions>
                                            <TextBlock x:Name="InvalidEntryMessage" Text="Invalid Entry" FontWeight="Bold" Foreground="{ThemeResource TextColorBrush}" FontFamily="Helvetica" FontSize="40" Grid.Row="0" />
                                            <TextBlock x:Name="InvalidEntryName" Text="{Binding FuelPlanInfo.InvalidPopupText}" FontFamily="Helvetica" FontSize="40" Grid.Row="1" Foreground="{ThemeResource TextColorBrush}" Margin="0,20,0,0" />

                                        </Grid>

                                    </StackPanel>

                                    <StackPanel x:Name="OKButtonSP" Canvas.Left="724" Canvas.Top="274">
                                        <Button x:Name="OKButton" Content="OK" Width="270" Height="72" FontSize="40" Style="{ThemeResource EFFBaseButtonStyle}" FontFamily="Helvetica" FontWeight="Bold" Foreground="#FFFFFF" Background="#0078D7" Tapped="Button_Tapped"  />
                                    </StackPanel>
                                </Canvas>
                            </Popup>

我正在管理这样的

Opened="InValidEntry_Opened"

private void InValidEntry_Opened(object sender, object e)
        {
            ApplicationViewOrientation orientation = ApplicationView.GetForCurrentView().Orientation;
            //if (orientation == ApplicationViewOrientation.Landscape)
            //{
                InValidEntry.HorizontalOffset = 0;
                InValidEntry.VerticalOffset = 450 + (FlightPlanScrollViewer.VerticalOffset * 2);
            //}
            //else
            //{ 
            //InValidEntry.VerticalOffset = 0;
            //InValidEntry.HorizontalOffset=400+ (FlightPlanScrollViewer.VerticalOffset * 2);
            //}
        }

我遇到了问题，因为Potrait模式下的展示位置不合适，但在landscape模式下效果很好。

要求是在屏幕的center打开弹出窗口。