public Rect Bounds
    {
        get
        {
            var Top = (_targetPosition + _translationY) - _size / 2.0f;
            return new Rect(callOutStarMargin, Top, _size, _size);
        }
    }

private Rect _boundsRect = new Rect();

    public Rect Bounds
    {
        get
        {
            var Top = (_targetPosition + _translationY) - _size / 2.0f;
            _boundsRect.Y = Top;
            return _boundsRect;
        }
    }

var b1 = foo.Bounds;
var b2 = foo.Bounds;
Assert.AreEqual(b2, b1);

public Rect Bounds
{
    get
    {
        if (bounds == null)
            bounds = GetBounds(...);
        return bounds;
    }
}

public Stream OutputStream
{
    get { return new FileStream(...); }
}

foo.OutputStream.Write(data1, 0, data1.Length);
foo.OutputStream.Write(data2, 0, data2.Length);