How To... Change the Data Type for a SELECT-OPTIONS at run time.

Question

Hello,
I am trying to restrict the values available for entry into a SELECT-OPTIONS at run time depending on user input.
The logic is as follows. The user has two input fields. A PARAMETER field which has the type RSDIOBJNM and allows them to choose an InfoObject. And the user has a SELECT-OPTIONS field to allow them to select the Characteristic values for that InfoObject.
I would like the following example to be possible:
The user enters 0MATERIAL into the PARAMETER. When the user clicks on the SELECT-OPTIONS control code will derive a list of possible options the user can enter in the SELECT-OPTIONS. In this case only values found in the master data or at least no values greater than 18 characters.
I have looked at the following function module SELECT_OPTIONS_RESTRICT and this do not appear to be helpful as they only restrict on the signs allowed for the values (unless I misunderstand, it is a complex function module!).
The code I have so far is (thus the user enters a InfoObject into p_char1 and the select options so_char1 should only accept active values of that InfoObject):
declaration of variables for user interface
  DATA c_char(32) TYPE c.
declaration of count variable
  DATA i_count TYPE i.
declaration of user interface
  SELECTION-SCREEN BEGIN OF BLOCK a1 WITH FRAME TITLE text-001.
    PARAMETERS: p_ipack TYPE RSLOGDPID.
  SELECTION-SCREEN END OF BLOCK a1.
  SELECTION-SCREEN BEGIN OF BLOCK b1 WITH FRAME TITLE text-002.
    PARAMETERS: p_char1 TYPE RSDIOBJNM.
    SELECT-OPTIONS: so_char1  for c_char NO INTERVALS.
    PARAMETERS: p_char2 TYPE RSDIOBJNM.
    SELECT-OPTIONS: so_char2  for c_char NO INTERVALS.
    PARAMETERS: p_char3 TYPE RSDIOBJNM.
    SELECT-OPTIONS: so_char3  for c_char NO INTERVALS.
  SELECTION-SCREEN END OF BLOCK b1.
Is what I am trying to do possible???
Thanks for any help. Ross.

Answer 1

You really want to restrict possible values of a select-option based on another field, not change the length of type of the select-option field, right?
Here is what you do:  Code a custom F4 value help for the select-option at event AT SELECTION-SCREEN ON VALUE REQUEST FOR..  The first thing you do here is read the value of the parameter field (p_ipack in your example).  You can use function module DYNP_VALUES_READ.  Based on this value, you can propose values for the select-option fields.  Note that the use can still enter whatever s(he) wishes in to the select-option field without pressing F4. In this case, you will have to code some input validations taking into account the value in the p_ipack field.