MATLAB OAPI to count the number of defined groups in SAP2000

Dear all,

I am trying to count the number of defined groups in SAP2000 using OAPI in Matlab. My current code is:

GroupDef = NET.explicitCast(SapModel.GroupDef,'SAP2000v1.cGroupDef');

n_group = GroupDef.Count();

However, it does NOT work and Matlab responses like below:

"Unable to convert the object of 'SAP2000v1.wGroup' type to object of

'SAP2000v1.cGroupDef' type.

Please make sure that the input object implements an interface type supplied for

conversion."

Could you help me to fix the problem?

Thank you,