c# XmlSignature with xades SignedProperties and UnsignedProperties

[Iguana]

Hi All!
I need to make an XmlSignature that contains Object with data that
will be signed and some not be signed - to use Xades:SignedProperties
and Xades:UnsignedProperties.
This is my object struct:

<Object>
<xades:QualifyingProperties Target="#s">
<xades:SignedProperties Id="sp">
<xades:SignedSignatureProperties>
<xades:SigningTime>2007-06-05T12:00:00Z</xades:SigningTime>
</xades:SignedSignatureProperties>
</xades:SignedProperties>
</xades:QualifyingProperties>
</Object>

I want to sign only SignedProperties tag with sub tags.
SignedXml.ComputeSignature crash, when I add reference:

Reference reference = new Reference();
reference.Uri = "#sp";
reference.Id = "r";
signedXml.AddReference(reference);

SignedXml.ComputeSignature works good, only when reference uri has
<object> id.
Is posible to sign not all <Object> tags - I want to all tag
SignedProperties?
Thanks for help!
Iguana