dll问题，如何返回string

//////////dll中是这样声明的：
function ReadDbInfo(section:string;databasetype:integer):string;
var
MyIniFile: TIniFile;
PassWord,UserID,DataSource,Defaultdb,ss:string;
begin

MyIniFile := TIniFile.Create('C:\CJSMS\DBINFO.ini');
if databasetype=1 then
begin
with MyIniFile do
begin
Password:=ReadString(Section, 'PassWord','000000');
userid:=ReadString(Section, 'USERID','000000');
DATASOURCE:=ReadString(Section, 'DATASOURCE','000000');
end;

result:='Provider=MSDAORA.1;Password='+PASSWORD+
';User ID='+USERID+';Data Source='+
DATASOURCE+';Persist Security Info=True';
end
else begin
with MyIniFile do
begin
Password:=ReadString(Section, 'PassWord','000000');
userid:=ReadString(Section, 'USERID','000000');
DATASOURCE:=ReadString(Section, 'DATASOURCE','000000');
DEFAULTDB:= ReadString(Section, 'DATABASE','000000');
end;
result:='Provider=SQLOLEDB.1;Password='+PASSWORD+';Persist Security Info=True;User ID='+USERID+
';Initial Catalog='+DEFAULTDB+';Data Source='+DATASOURCE;
end;
MyIniFile.Free;

end;


//////////////////////unit中的声明及引出

unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls;

type
TForm1 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
Function ReadDBinfo(section:string;databasetype:integer):string;
var
Form1: TForm1;

implementation
function ReadDBinfo; external 'sms' index 4;
{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
begin
memo1.Lines.Add(readdbinfo('Ora_48',1));
end;

end.

调用时出现错误，"Invalid pointer operation"