StatusBar record x of y

Question 1

Something very important to know about databinding to collections is that XAML always databinds to a collection view, and not the collection itself. So, even though your XAML appears to bind to the collection, at runtime, you are really binding to the default collection view.

Here's a cool side-effect: A collectionview already has a CurrentPosition property. I think that things are breaking for you because you are inadvertently intervening with your your colleciton.

Below is a really quick and dirty little program that illustrates a working binding to CurrentPostion and Count, without ever defining a currentposition on the collection (because under the covers, you're really binding to the CollectionView, and it already has a CurrentPosition property that notifies on change.

Run this program, and notice that when you click the increment button, the UI updates appropriately.

Here's the XAML:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel x:Name="ContentPanel">
        <Button x:Name="IncrementButton" Content="Increment" Click="IncrementButton_Click"/>
        <StackPanel Orientation="Horizontal">
            <TextBlock x:Name="CurrentPositionTextBlock" Text="{Binding CurrentPosition}"/>
            <TextBlock Text=" / "/>
            <TextBlock Text="{Binding Count}"/>
        </StackPanel>
    </StackPanel>
</Window>

Here's the code-behind:

using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Windows;
using System.Windows.Data;

namespace WpfApplication1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.Collection = new TestObservableCollection<object>() {new object(), new object(), new object()};
            this.ContentPanel.DataContext = this.Collection;
        }

        public TestObservableCollection<object> Collection { get; set; }

        private void IncrementButton_Click(object sender, RoutedEventArgs e)
        {
            this.Collection.GetDefaultView().MoveCurrentToNext();
        }
    }

    public class TestObservableCollection<T> : ObservableCollection<T>
    {
        public ICollectionView GetDefaultView()
        {
            return CollectionViewSource.GetDefaultView(this);
        }
    }
}

Here's more reading for you:

http://msdn.microsoft.com/en-us/library/ms752347(v=vs.110).aspx

http://msdn.microsoft.com/en-us/library/system.windows.data.collectionviewsource(v=vs.110).aspx

EDIT

If you want a little more proof of what's going on, paste the code below into my button-click handler -- you will see that the actual object type that the textbox is bound to is a ListCollectionView, and not the actual collection:

System.Diagnostics.Debug.WriteLine(this.CurrentPositionTextBlock.GetBindingExpression(TextBlock.TextProperty).ResolvedSource.GetType().FullName);

Question 2

Your Count binding is updating because the base ObservableCollection<T> raises PropertyChanged when items are added and removed.

Your positioning code needs to raise PropertyChanged when you reposition the current record pointer, so that the Binding subsystem knows to requery the property, it's rather why INotifyPropertyChanged exists. I would probably write it like the below, though. Note the use of ObservableCollections<T>'s OnPropertyChanged to raise the right event from the already-implemented INotifyPropertyChanged in your inheritance tree.

        public void MoveFirst() {
            this.GetDefaultView().MoveCurrentToFirst();
            OnPropertyChanged(new PropertyChangedEventArgs("CurrentPosition"));
        }
        public void MovePrevious() {
            this.GetDefaultView().MoveCurrentToPrevious();
            OnPropertyChanged(new PropertyChangedEventArgs("CurrentPosition"));
        }
        public void MoveNext() {
            this.GetDefaultView().MoveCurrentToNext();
            OnPropertyChanged(new PropertyChangedEventArgs("CurrentPosition"));
        }
        public void MoveLast() {
            this.GetDefaultView().MoveCurrentToLast();
            OnPropertyChanged(new PropertyChangedEventArgs("CurrentPosition"));
        }