As a workaround, I'm converting the Envelope geometry to a Polygon. This allows moving and editing the rectangles. However, the rectangles are loosing their right angles when editing them.
private void MyDrawObject_DrawComplete(object sender, ESRI.ArcGIS.Client.DrawEventArgs args)
{
GraphicsLayer graphicsLayer = MyMap.Layers["MyGraphicsLayer"] as GraphicsLayer;
Envelope envelope = (Envelope)args.Geometry;
Polygon rectangle = new Polygon();
PointCollection pointCollection = new PointCollection
{
new MapPoint(envelope.XMin, envelope.YMin),
new MapPoint(envelope.XMin, envelope.YMax),
new MapPoint(envelope.XMax, envelope.YMax),
new MapPoint(envelope.XMax, envelope.YMin),
new MapPoint(envelope.XMin, envelope.YMin)
};
rectangle.Rings.Add(pointCollection);
ESRI.ArcGIS.Client.Graphic graphic = new ESRI.ArcGIS.Client.Graphic()
{
Geometry = rectangle,
Symbol = _activeSymbol,
};
graphicsLayer.Graphics.Add(graphic);
}