update SOME_TABLE
   set ID = 'This is simple string value'
 where ID = 1;

Statement failed, SQLSTATE = 22018
conversion error from string "This is simple string value"

DECLARE EXTERNAL FUNCTION INT_UDF
    INTEGER BY DESCRIPTOR
RETURNS INTEGER BY VALUE
ENTRY_POINT 'IntUdf' MODULE_NAME 'ModuleName.dll';

type
  TDSC = packed record
    dsc_dtype    : byte;
    dsc_scale    : shortint;
    dsc_length   : word;
    dsc_sub_type : smallint;
    dsc_flags    : word;
    dsc_address  : pointer;
  end;

select INT_UDF( 'This is simple string value' )
  from RDB$DATABASE;

dsc_dtype    = 1   // тип CHAR
dsc_scale    = 0
dsc_length   = 27
dsc_sub_type = 52  // CHARACTER SET ID
dsc_flags    = 0
dsc_address  - содержит pAnsiChar 'This is simple string value'

select INT_UDF( CAST( 'This is simple string value' AS VARCHAR(32) ) )
  from RDB$DATABASE;

dsc_dtype    = 2   // тип VARCHAR
dsc_scale    = 0
dsc_length   = 34  // размер CAST 32 + 2
dsc_sub_type = 52  // CHARACTER SET ID
dsc_flags    = 0
dsc_address  - первые 2 байта содержат длину 27
dsc_address  + 2 - содержит pAnsiChar 'This is simple string value'