Read ObjectSid into anchor attribute

Hi,

I'm developing an ECMA 2.2 and I have set the Anchor attribute to be the objectSid. I have to use powershell (from within the C# MA code) to obtain the objectSid.

The trouble I have now is that I'm unable to translate this objectSid into the right format (to be honest, I don't even know what format it is returned in).

When coding the schema, I code the "ObjectSid" attribute as an AttributeType.Binary

public Schema GetSchema(KeyedCollection<string, ConfigParameter> configParameters)
        {
            Microsoft.MetadirectoryServices.SchemaType userType = Microsoft.MetadirectoryServices.SchemaType.Create("user", false);
            userType.Attributes.Add(SchemaAttribute.CreateSingleValuedAttribute("AccountName", AttributeType.String)); // AccountName is the anchor attribute
            userType.Attributes.Add(SchemaAttribute.CreateSingleValuedAttribute("Email", AttributeType.String));
            userType.Attributes.Add(SchemaAttribute.CreateSingleValuedAttribute("SipAddress", AttributeType.String));
            userType.Attributes.Add(SchemaAttribute.CreateSingleValuedAttribute("EmployeeID", AttributeType.String));
            userType.Attributes.Add(SchemaAttribute.CreateSingleValuedAttribute("ObjectSid", AttributeType.Binary));
            Schema schema = Schema.Create();
            schema.Types.Add(userType);
            return schema;
        }

Then I populate the ObjectSid like so:

                    csentry.AttributeChanges.Add(AttributeChange.CreateAttributeAdd("ObjectSid", obj.Members["ObjectSid"].Value));

where obj is a powershell object which contains the result of a powershell command execution which gets the objectSid. 

This doesn't work and I get an error in the Server logs:

The server encountered an unexpected error while performing an operation for a management agent.
 
 "System.InvalidCastException: Unable to cast object of type 'System.Security.Principal.SecurityIdentifier' to type 'System.Byte[]'.
   at Microsoft.MetadirectoryServices.Impl.Ecma2ConversionServices.AddAttributeToDImage(CDImage* pdimage, String attributeName, AttributeModificationType attributeModificationType, IList`1 attributeValueChanges, Int32 escapeReferenceDNValues)
   at Microsoft.MetadirectoryServices.Impl.Ecma2ConversionServices.ConvertToDImage(CSEntryChange csEntryChange, CDImage** ppDImage, Int32 escapeReferenceDNValues)
   at Microsoft.MetadirectoryServices.Impl.ScriptHost.InvokeExtMA_ImportEntry(UInt32 cBatchSize, UInt16* pcszCustomData, UInt32 cFullObject, _OCTET* rgoctFullObject, UInt32* rgomodt, UInt32* pcpcszChangedAttributes, UInt16*** prgpcszChangedAttributes, Int32 fIsDNStyleNone, UInt16** ppszUpdatedCustomData, _OCTET* rgoctCSImage, Int32* rgextec, UInt16** rgpszErrorName, UInt16** rgpszErrorDetail, Int32* pfMoreToImport)"

How should I handle the objectSid conversion here? Totally lost since I thought the objectSid would be returned as a byte[] array but instead it is being returned as a string.

Thanks