在AT SELECTION-SCREEN ON VALUE-REQUEST事件中，是不能直接取得选择屏幕字段的默认值或输入的值，而是需要通过函数 DYNP_VALUES_READ读取。
例子程序：
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REPORT ZTEST_F4_DEFAULT.
PARAMETERS: p_folder TYPE file_table-filename.
INITIALIZATION.
* 初始化
p_folder = 'C:PKF'.
*&---------------------------------------------------------------------*
*& Event AT SELECTION-SCREEN
*&---------------------------------------------------------------------*
AT SELECTION-SCREEN ON VALUE-REQUEST FOR p_folder.
PERFORM frm_f4_pfolder.
*&---------------------------------------------------------------------*
*& Form FRM_F4_PFOLDER
*&---------------------------------------------------------------------*
* text
*----------------------------------------------------------------------*
* --> p1 text
* <-- p2 text
*----------------------------------------------------------------------*
FORM frm_f4_pfolder .
DATA: l_path TYPE string,
l_sel TYPE string,
lit_dyn TYPE TABLE OF dynpread,
lwa_dyn TYPE dynpread.
IF p_folder IS INITIAL.
lwa_dyn-fieldname = 'P_FOLDER'.
APPEND lwa_dyn TO lit_dyn.
CALL FUNCTION 'DYNP_VALUES_READ'
EXPORTING
dyname = sy-cprog
dynumb = sy-dynnr
TABLES
dynpfields = lit_dyn
EXCEPTIONS
invalid_abapworkarea = 1
invalid_dynprofield = 2
invalid_dynproname = 3
invalid_dynpronummer = 4
invalid_request = 5
no_fielddescription = 6
invalid_parameter = 7
undefind_error = 8
double_conversion = 9
stepl_not_found = 10
OTHERS = 11.
READ TABLE lit_dyn INTO lwa_dyn WITH KEY fieldname = 'P_FOLDER'.
l_path = lwa_dyn-fieldvalue.
ELSE.
l_path = p_folder.
ENDIF.
CALL METHOD cl_gui_frontend_services=>directory_browse
EXPORTING
initial_folder = l_path
CHANGING
selected_folder = l_sel
EXCEPTIONS
cntl_error = 1
error_no_gui = 2
not_supported_by_gui = 3
OTHERS = 4.
IF sy-subrc EQ 0.
p_folder = l_sel.
ENDIF.
ENDFORM.
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debug截图：
发现，虽然P_folder有初始默认值z:/zpkf，但是在VALUE-REQUEST事件中，P_folder为空,
随后通过DYNP_VALUES_READ读出，如下：
以上。