PropertyChanged always null - silverlight-4.0

Question

This is my MainPage.xaml :-
<UserControl x:Class="SilverlightPlainWCF.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk" xmlns:my="clr-namespace:SilverlightPlainWCF.CustomersServiceRef" Loaded="UserControl_Loaded">
<UserControl.Resources>
<CollectionViewSource x:Key="customerViewSource" d:DesignSource="{d:DesignInstance my:Customer, CreateList=True}" />
</UserControl.Resources>
<Grid x:Name="LayoutRoot" Background="White">
<sdk:DataGrid AutoGenerateColumns="False" Height="426" HorizontalAlignment="Left" ItemsSource="{Binding}" Margin="12,12,0,0" Name="customerDataGrid" RowDetailsVisibilityMode="VisibleWhenSelected" VerticalAlignment="Top" Width="776">
<sdk:DataGrid.Columns>
<sdk:DataGridTextColumn x:Name="addressColumn" Binding="{Binding Path=Address}" Header="Address" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="cityColumn" Binding="{Binding Path=City}" Header="City" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="companyNameColumn" Binding="{Binding Path=CompanyName}" Header="Company Name" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="contactNameColumn" Binding="{Binding Path=ContactName}" Header="Contact Name" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="contactTitleColumn" Binding="{Binding Path=ContactTitle}" Header="Contact Title" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="countryColumn" Binding="{Binding Path=Country}" Header="Country" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="customerIDColumn" Binding="{Binding Path=CustomerID}" Header="Customer ID" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="faxColumn" Binding="{Binding Path=Fax}" Header="Fax" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="phoneColumn" Binding="{Binding Path=Phone}" Header="Phone" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="postalCodeColumn" Binding="{Binding Path=PostalCode}" Header="Postal Code" Width="SizeToHeader" />
<sdk:DataGridTextColumn x:Name="regionColumn" Binding="{Binding Path=Region}" Header="Region" Width="SizeToHeader" />
</sdk:DataGrid.Columns>
</sdk:DataGrid>
</Grid>
</UserControl>
This is my MainPage.xaml.cs :-
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using SilverlightPlainWCF.CustomersServiceRef;
using System.Diagnostics;
using System.Collections.ObjectModel;
using System.ComponentModel;
namespace SilverlightPlainWCF
{
public partial class MainPage : UserControl, INotifyPropertyChanged
{
public MainPage()
{
InitializeComponent();
this.DataContext = Customers;
this.Loaded += new RoutedEventHandler(MainPage_Loaded);
}
public ObservableCollection<Customer> customers;
public ObservableCollection<Customer> Customers
{
get { return customers; }
set
{
customers = value;
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs("Customers"));
}
}
}
void MainPage_Loaded(object sender, RoutedEventArgs e)
{
CustomersServiceClient objCustomersServiceClient = new CustomersServiceClient();
objCustomersServiceClient.GetAllCustomersCompleted += (s, res) =>
{
if (res.Error == null)
{
Customers = new ObservableCollection<Customer>(res.Result);
}
else
{
MessageBox.Show(res.Error.Message);
}
};
objCustomersServiceClient.GetAllCustomersAsync();
}
private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
// Do not load your data at design time.
// if (!System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
// {
// //Load your data here and assign the result to the CollectionViewSource.
// System.Windows.Data.CollectionViewSource myCollectionViewSource = (System.Windows.Data.CollectionViewSource)this.Resources["Resource Key for CollectionViewSource"];
// myCollectionViewSource.Source = your data
// }
// Do not load your data at design time.
// if (!System.ComponentModel.DesignerProperties.GetIsInDesignMode(this))
// {
// //Load your data here and assign the result to the CollectionViewSource.
// System.Windows.Data.CollectionViewSource myCollectionViewSource = (System.Windows.Data.CollectionViewSource)this.Resources["Resource Key for CollectionViewSource"];
// myCollectionViewSource.Source = your data
// }
}
public event PropertyChangedEventHandler PropertyChanged;
}
}
If i just move the line :-
this.DataContext = Customers;
from constructor to here :-
if (res.Error == null)
{
Customers = new ObservableCollection<Customer>(res.Result);
this.DataContext = Customers;
}
It works fine and I get all the data. What might be the problem?

Answer 1

The reason that it didn't work when you put it in the constructor is that there is not yet any value in the customers field at that moment.
You will only get the value when MainPage_Loaded is triggered, which will not happen because of the following line in your XAML:
Loaded="UserControl_Loaded"
That will execute UserControl_Loaded and not MainPage_Loaded. What you can do is call MainPage_Loaded from UserControl_Loaded, which probably is not what you intend to do. So in that case you should change your XAML instead to:
Loaded="MainPage_Loaded"
And you can delete UserControl_Loaded altogether since you are not using it anymore.
And as for the assigning of the result to the DataGrid, you can actually do it directly by assigning the result straight to the DataContext instead of going through the Customers property.
But if you insist to assign it to the Customers property and have the DataGrid updated accordingly, then the next easiest solution would be to include the following line somewhere in your Customers set method:
DataContext = value;
If you really, really insist that the DataGrid should update itself when the PropertyChanged event is triggered, without having you to code the DataContext = Customers row, then what you want is data binding. By binding the DataContext property to your Customers property, then the DataGrid will be updated when it receive the PropertyChanged event.
To declare the data binding in XAML, you would need to assign a name to your UserControl tag. Then you would assign the binding to the DataContext, something along this line:
DataContext="{Binding Path=Customers, ElementName=theUserControlName}"
And if I were you, instead of having to implement the INotifyPropertyChanged interface, I would instead use Dependency Properties instead. Converting your example to use Dependency Property, I would have:
public static DependencyProperty CustomersProperty =
DependencyProperty.Register("Customers", typeof(ObservableCollection<Customer>), typeof(MainPage), null);
public ObservableCollection<Customer> Customers
{
get { return (ObservableCollection<Customer>) GetValue(CustomersProperty); }
set { SetValue(CustomersProperty, value); }
}
Just that, the property change notification will be handled by the framework.

Answer 2

I believe the problem is that in the constructor you do not have this line:
Customers = new ObservableCollection<Customer>(res.Result);
before you attempt to set the DataContext to that value.