You may know already that SignedXml class in .NET doesn't support the XML Advanced Electronic Signatures (XAdES) standard (more info here: Which standards does SignedXml support?)
Fortunatelly my colleagues in Microsoft France developed a XAdES library for .NET (Microsoft.Xades.dll) that they published on this French site: Kit de démarrage « Signature avancée (XAdES) pour Microsoft .NET Framework v3.5 (et ultérieur)
Disclaimer: I don't speak French, so I don't understand the license for this library and cannot tell if any restrictions come with it.
I hope this helps.
Alex (Alejandro Campos Magencio)