Help me!

[lishan200012]

//创建DLL文件
library MyDll;
uses
SysUtils,
Classes;

function TestDll(s:String):String;stdcall;
begin
s:='';
s:=s+'Provider=SQLOLEDB.1;';
s:=s+'Password=sa;';
s:=S+'Persist Security Info=True;';
s:=S+'User ID=sa;';
s:=S+'Initial Catalog=posdb;';
s:=S+'Data Source=Servername;';
end;

exports
TestDll;
------------------------------------------------------------------


//下面是主程序
function TestDll(s:String):String;stdcall;
external 'MyDll';

procedure TForm1.Button1Click(Sender: TObject);
begin
DataModule1.ADOConnection1.Close;
DataModule1.ADOConnection1.ConnectionString:==TestDll(); //这边怎么办？
DataModule1.ADOConnection1.LoginPrompt:=false;
DataModule1.ADOConnection1.Connected:=true;
end;
高手请指教

[aiirii]

library MyDll;
uses
ShareMem, //这行
SysUtils,
Classes;

function TestDll(var s:String):boolean;stdcall;
begin
s:='';
s:=s+'Provider=SQLOLEDB.1;';
s:=s+'Password=sa;';
s:=S+'Persist Security Info=True;';
s:=S+'User ID=sa;';
s:=S+'Initial Catalog=posdb;';
s:=S+'Data Source=Servername;';
result := true;
end;


---------------------------------------------------
uses
ShareMem, //这行

function TestDll(s:String):boolean;stdcall;
external 'MyDll';

procedure TForm1.Button1Click(Sender: TObject);
var s: string;
begin
if not TestDll(s) then exit;
DataModule1.ADOConnection1.Close;
DataModule1.ADOConnection1.ConnectionString:==TestDll(); //这边怎么办？
DataModule1.ADOConnection1.LoginPrompt:=false;
DataModule1.ADOConnection1.Connected:=true;
end;

[柯本]

另外,改
dll的
function TestDll(s:String):String;stdcall;

为
function TestDll(var s:String):String;stdcall;
改
DataModule1.ADOConnection1.ConnectionString:==TestDll(); //这边怎么办？
为
DataModule1.ADOConnection1.ConnectionString:=s; //这边怎么办？

[柯本]

第一个要改成:
function TestDll(var s:String):boolean;stdcall;

[刘慧谦]

function TestDll(s:String):boolean;stdcall;
external 'MyDll';
procedure TForm1.Button1Click(Sender: TObject);
var s: string;
begin
if not TestDll(s) then exit;
DataModule1.ADOConnection1.Close;
DataModule1.ADOConnection1.ConnectionString:=s;
DataModule1.ADOConnection1.LoginPrompt:=false;
DataModule1.ADOConnection1.Connected:=true;
end;

[lishan200012]

请问这个“Project Project1.exe raised exception class EInvalidPointer with message 'EInvalidPointer operation'. Process stopped. use step or Run to continue.
”
怎么解决？