WPF Data Bind from a instantiated class in project A to project B

[singularity~]

I am having a issue with data binding from a instantiated class in project A to Project B. The data binding "seems" to be working correctly; however, I am definitely getting a different instantiation of ClassA.

This is the ClassA - which is a class in a different Project. ClassA is a console application and has a main.

public class ClassA: INotifyPropertyChanged
{
   private string _partNumber;
   private Guid _guid = Guid.NewGuid ( );

   // Make a singleton of this class so other classes have to request an instance
   private static readonly ClassA _Instance = new ClassA ( );

   //
   //.. Read in xml file "This executes properly - I've checked"
   //

   public static ClassA GetInstance ( )
   {
       return _Instance;
   }

   public string PartNumber
    {
        get
        {

            if ( string.IsNullOrEmpty ( _partNumber ) )
            {
                Console.WriteLine("Got PartNumber: " + _partNumber + "  GUID: " + _guid);
                return "Empty";
            }
            Console.WriteLine("Got PartNumber: " + _partNumber + "  GUID: " + _guid);
            return _partNumber;
        }
        set
        {
            if ( value == null )
            {
                throw new ArgumentNullException ( "value" );
            }
            Console.WriteLine("Set Partnumber: " + value + "  GUID: " + _guid);
            _partNumber = value;
        }
    }

}

This is the MainWindow.xaml.cs which is in a different project from ClassA:

public partial class MainWindow : MetroWindow
    {
        public MainWindow ( )
        {
            InitializeComponent ( );
            DataContext = ClassA.ClassAInstance;
        }

        ...
    }

This is the XAML code that calls the binding

<controls:MetroWindow ... ...

   <TextBlock  Grid.Row="1" Grid.Column="1" 
     Style="{StaticResource LblTextData}"
     Text="{Binding PartNumber, UpdateSourceTrigger=PropertyChanged, Mode=OneWay}"/> 

   ... ...

</controls:MetroWindow>

When I run the program I run Console.WriteLine() to verify the data and GUID's. The data printed from ClassB is null and the GUID's are totally different from ClassA. My question is how can I get the same class instance - rather then a totally different instance?

Here is the Console output:

Set Partnumber: 736543  GUID: 360a6dcc-a719-4ad0-a929-6f3ed09eefb2
Get PartNumber:         GUID: 2405b370-0806-4f41-91c6-0fdd6f894077

I am also getting this error at runtime:
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=PartNumber; DataItem=null; target element is 'TextBlock' (Name=''); target property is 'Text' (type 'String')