c# - Listpicker error SelectedItem must always be set to a valid value

Question

I have a page in a Windows Phone 7 app where the user can edit or delete an Transaction object. The Transaction object is an Linq-to-Sql class that have a relationship with the Account class and the Category class. In the page, I use a ListPicker to let the user select the account and category for the given transaction, like this:

<toolkit:ListPicker Grid.Row="1" FullModeHeader="Choose the Account" FullModeItemTemplate="{StaticResource FullModeItemTemplate}" ExpansionMode="FullScreenOnly" Background="#26000000" Margin="10,0,10,0" Name="Account" SelectedItem="{Binding Account, Mode=TwoWay}" Tap="ListPicker_Tap" />

<toolkit:ListPicker Grid.Row="7" FullModeHeader="Choose the Category" FullModeItemTemplate="{StaticResource FullModeItemTemplate}" ExpansionMode="FullScreenOnly" Background="#26000000" Margin="10,0,10,0" Name="Category" SelectedItem="{Binding Category, Mode=TwoWay}" Tap="ListPicker_Tap" />

The ListPicker_Tap event is a fix for a bug in the Aug/2011 version of the WPF Toolkit for Windows Phone and is simply this:

    private void ListPicker_Tap(object sender, System.Windows.Input.GestureEventArgs e)
    {
        ListPicker lp = (ListPicker)sender; 
        lp.Open();
    }

If the user edit the transaction, everything is fine, but if the user try to delete it, I get an error saying that "SelectedItem must always be set to a valid value".

Here's the code if the user click in the delete button in the appbar in the TransactionPage.xaml.cs:

    private void appBarDelete_Click(object sender, EventArgs e)
    {
        MessageBoxResult result = MessageBox.Show("Are you sure?
", "Confirm", MessageBoxButton.OKCancel);

        if (result == MessageBoxResult.OK)
        {
            App.ViewModel.DeleteTransaction(transaction);
        }

        NavigationService.GoBack();
    }

My ViewModel.DeleteTransaction method:

    public void DeleteTransaction(Transaction transaction)
    {
        AllTransactions.Remove(transaction);
        transactionRepository.Delete(transaction);
    }

My transactionRepository.Delete method:

    public void Delete(Transaction transaction)
    {
        Context.Transactions.DeleteOnSubmit(transaction);
        Context.SubmitChanges();
    }

I receive the error in the Context.SubmitChanges() execution, the debug points to the NotifyPropertyChanged inside the Transaction class, the line where I get the error is this:

    protected virtual void SendPropertyChanged(String propertyName)
    {
        if ((this.PropertyChanged != null))
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }

In the propertyName attribute the value is "Category". It looks like when deleting the object send the propertychanged event of category and accounts, and because the listpicker is in the TwoWay mode, it have some trouble dealing with it. How could I fix it? I need some help.

Answer 1

This error may also be caused by the order of the XAML properties:

This does NOT work (throws the exception because the ItemsSource is null when the SelectedItem is set):

<toolkit:ListPicker DisplayMemberPath="Title" SelectionMode="Single" 
SelectedItem="{Binding SelectedCategory, Mode=TwoWay}"
ItemsSource="{Binding Categories}" />

This works as the itemssource is initialized first:

<toolkit:ListPicker DisplayMemberPath="Title" SelectionMode="Single" 
ItemsSource="{Binding Categories}"
SelectedItem="{Binding SelectedCategory, Mode=TwoWay}" />