Advantages and disadvantages of passing by reference versus passing by value

Visual Basic What are the advantages and disadvantages of passing by reference versus passing by value? Consider the DropDown Style property of the combo box control. When might it be appropriate to use one style versus another style? What are the implications of allowing the user to enter new values not already in the list? What are the programmatic implications of allowing multiple items to be selected? How do these issues affect your requirement as a programmer to validate user input data?