dll function declare 有关问题

dll function declare 问题
Public   Function   string   GetKeys   (ref   string   key1,ref   string   key2)       "Encrypt.dll "
在delphi中明。
  Function   GetKeys   (key1:pchar   ,   key2:pchar)：string       "Encrypt.dll "
为何报错，pb调用函数正常

[解决办法]
Function GetKeys(key1:pchar , key2:pchar)：pchar;stdcall; external "Encrypt.dll ";