private Point _startPosition;
private bool _isMouseLeftDown;        

public MainPage()
{
    // Required to initialize variables
    InitializeComponent();    
}

private void StackPanel_MouseLeftButtonDown(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
    FrameworkElement element = sender as FrameworkElement;

    this._startPosition = e.GetPosition(null, true);
    this._isMouseLeftDown = true;

    element.CaptureMouse();
}

private void StackPanel_MouseLeftButtonUp(object sender, System.Windows.Input.MouseButtonEventArgs e)
{
    FrameworkElement element = sender as FrameworkElement;

    this._startPosition = new Point(-1, -1);
    this._isMouseLeftDown = false;

    element.ReleaseMouseCapture();
}

private void StackPanel_MouseMove(object sender, System.Windows.Input.MouseEventArgs e)
{
    FrameworkElement element = sender as FrameworkElement;

    if (this._isMouseLeftDown)
    {
        Point current = e.GetPosition(null, true);

        if (Math.Abs(current.X - this._startPosition.X) >= 4 || Math.Abs(current.Y - this._startPosition.Y) >= 4)
        {
            DragDropEventData dataObject = new DragDropEventData();

            string sourceData = "Some string data to store...";
            byte[] unicodeText = Encoding.Unicode.GetBytes(sourceData); 
            byte[] utf8Text = Encoding.UTF8.GetBytes(sourceData);

            string uniCodeFormat = "Unicode";
            string utf8DataFormat = "UTF-8";
            
            dataObject.SetData(sourceData);            
            dataObject.SetData(uniCodeFormat, unicodeText);            
            dataObject.SetData(utf8DataFormat, utf8Text);

            this._isMouseLeftDown = false;
            DragDrop.DoDragDrop(element, dataObject, DragDropEffects.Move);
        }
    }
}