Hi,
Varying rebar sets are special since each rebar in the set gets a "Rebar Number". In order to retrive a specific parameter for a bar position you could use ParameterValueProvider or Rebar.GetParameterValueAtIndex(...) Method
var pVal = rebar.GetParameterValueAtIndex(new RvtDB.ElementId(BuiltInParameter.REBAR_NUMBER), 0) as StringParameterValue; var rebarNumber = pVal.Value;