asynchronous - WPF Combobox Category Grouping From Async Call -

i can able group data categories combobox while rendering {initialization} setting below code below

    _cboitem = new list<string>(); _cboitem=service.getdata();     collectionview cvs =     collectionview)collectionviewsource.getdefaultview(_cboitem); cvs.groupdescriptions.add(new  propertygroupdescription("category")); 

but if bind combbox async call, grouping not happening.

code below

    bindingoperations.enablecollectionsynchronization(_cbodoctype, _lock); task.factory.startnew(() => getdoctypeasync());      collectionview cv=(collectionview)collectionviewsource.getdefaultview(_cbodoctype);       cv.groupdescriptions.add(new propertygroupdescription("category"));       onpropertychanged("doctype");