Add method 
    
    
  
Adds the specified property definition / sort direction pair to the collection.
The property definition.
The sort direction.
    
  Implementation
  void Add(
    PropertyDefinitionBase propertyDefinition, SortDirection sortDirection) {
  if (this.Contains(propertyDefinition)) {
    throw new ServiceLocalException(
        "PropertyAlreadyExistsInOrderByCollection(${propertyDefinition.GetPrintableName()})");
  }
  this._propDefSortOrderPairList.add(
      new MapEntry<PropertyDefinitionBase, SortDirection>(
          propertyDefinition, sortDirection));
}