WPF not load a control - wpf

Question

I write a very easy UserControl
here the Xaml code
<UserControl x:Name="Test1" x:Class="WpfAppXtesting.UserControl1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:WpfAppXtesting"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800" Loaded="Test1_Loaded">
<Grid x:Name="GridRoot" Background="Aqua">
<TextBlock x:Name="status" HorizontalAlignment="Left" Height="137" Margin="100,137,0,0" TextWrapping="Wrap" Text="TextBlock" VerticalAlignment="Top" Width="483" FontSize="48"/>
</Grid>
and here the code behind
/// <summary>
/// Interaction logic for UserControl1.xaml
/// </summary>
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
this.GridRoot.DataContext = this;
}
private void UserControl1_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "Connected":
status.Text = ((App)sender).Connected.ToString() ;
break;
}
}
private void Test1_Loaded(object sender, RoutedEventArgs e)
{
(Application.Current as App).PropertyChanged += UserControl1_PropertyChanged;
}
}
the problem is , when import this control in a Window in same project The design mode gets this error.
NullReferenceException: Object reference not set to an instance of an object.
if I run the project everything was good.
If I commented the line in Loaded method
the control was right shown in design mode.
Any Idea?
thanks

Answer 1

Do not assume that Application.Current is your application at design time. For example, when you are using Expression Blend, Current is Expression Blend. At design time, MainWindow is not your application's main window. Typically operations that cause a user/custome control to fail at design time include the following.
Casting Current to your custom subclass of App.
Casting MainWindow to your custom subclass of Window.
Here are two approaches to writing code for design time. The first approach is to write defensive code by checking the null condition. The second approach is to check whether design mode is active by calling the GetIsInDesignMode method. You can read about GetIsInDesignMode at here.
Solution 1:
private void Test1_Loaded(object sender, RoutedEventArgs e)
{
var app = Application.Current as App;
if( app != null)
{
app.PropertyChanged += UserControl1_PropertyChanged;
}
}
Solution 2 :
private void Test1_Loaded(object sender, RoutedEventArgs e)
{
if (!DesignerProperties.GetIsInDesignMode(this))
{
// Design-mode specific functionality
(Application.Current as App).PropertyChanged += UserControl1_PropertyChanged;
}
}