启动程序时，它显示“对象引用未设置为对象的实例。”
lbl_day为空。
我只想在wpf中更改标签的前景。但是，它在toggle_checked中发生了变化，但在toggle_unchecked中没有发生变化。
<Button Content="امکانات اصلی" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="130" Height="40"/>
<Button Content="امکانات تحصیلی" HorizontalAlignment="Left" Margin="180,10,0,0" VerticalAlignment="Top" Width="130" Height="40"/>
<Button Content="سایر امکانات" HorizontalAlignment="Left" Margin="354,10,0,0" VerticalAlignment="Top" Width="124" Height="40"/>
<Label HorizontalAlignment="Right" x:Name="lbl_Saat" Content="00" Height="27" Margin="0,19,35,0" VerticalAlignment="Top" Width="26" Foreground="Black"/>
<Label x:Name="lbl4" HorizontalAlignment="Right" Content=":" Height="27" Margin="0,18,13,0" VerticalAlignment="Top" Width="16" Foreground="Black"/>
<Label HorizontalAlignment="Right" x:Name="lbl_min" Content="00" Height="27" Margin="0,19,19,0" VerticalAlignment="Top" Width="24" Foreground="Black"/>
<Label x:Name="lbl3" HorizontalAlignment="Right" Content=":" Height="27" Margin="0,18,31,0" VerticalAlignment="Top" Width="16" Foreground="Black"/>
<Label HorizontalAlignment="Right" x:Name="lbl_Sanie" Content="00" Height="27" Margin="0,19,0,0" VerticalAlignment="Top" Width="24" Foreground="Black"/>
<Label HorizontalAlignment="Right" x:Name="lbl_Year" Content="00" Height="27" Margin="0,18,79,0" VerticalAlignment="Top" Width="24" Foreground="Black"/>
<Label x:Name="lbl2" HorizontalAlignment="Right" Content="/" Height="28" Margin="0,18,93,0" VerticalAlignment="Top" Width="16" Foreground="Black"/>
<Label HorizontalAlignment="Right" x:Name="lbl_month" Content="00" Height="27" Margin="0,18,99,0" VerticalAlignment="Top" Width="24" Foreground="Black"/>
<Label x:Name="lbl1" HorizontalAlignment="Right" Content="/" Height="27" Margin="0,18,112,0" VerticalAlignment="Top" Width="16" Foreground="Black"/>
<Label HorizontalAlignment="Right" x:Name="lbl_Day" Content="00" Height="27" Margin="0,18,118,0" VerticalAlignment="Top" Width="24" Foreground="Black " />
private void ToggleButton_Checked(object sender, RoutedEventArgs e)
{
//Halate Rooz
MainGrid.Background = Brushes.White;
lbl_Day.Foreground = Brushes.Black;
lbl_min.Foreground = Brushes.Black;
lbl_month.Foreground = Brushes.Black;
lbl_Saat.Foreground = Brushes.Black;
lbl_Sanie.Foreground = Brushes.Black;
lbl_Year.Foreground = Brushes.Black;
lbl1.Foreground = Brushes.Black;
lbl2.Foreground = Brushes.Black;
lbl3.Foreground = Brushes.Black;
lbl4.Foreground = Brushes.Black;
btn_calendar.Foreground = Brushes.Black;
btn_zaman.Foreground = Brushes.Black;
}