delphi5中的dll如何使用DataModule

Question

我的代码如下：
library dllado;{ Important note about DLL memory management: ShareMem must be the
  first unit in your library's USES clause AND your project's (select
  Project-View Source) USES clause if your DLL exports any procedures or
  functions that pass strings as parameters or function results. This
  applies to all strings passed to and from your DLL--even those that
  are nested in records and classes. ShareMem is the interface unit to
  the BORLNDMM.DLL shared memory manager, which must be deployed along
  with your DLL. To avoid using BORLNDMM.DLL, pass string information
  using PChar or ShortString parameters. }uses
  SysUtils,
  Classes,
  Forms,
  Windows,
  Activex,
  DB,
  ADODB,
  datamodule in 'datamodule.pas' {DLLDM: TDataModule},
  functions in 'functions.pas',
  varibles in 'varibles.pas';{$R *.RES}var
   DllApplication: TApplication;procedure DLLUnloadProc(Reason: Integer); register;
begin
  try
     if Reason = DLL_PROCESS_DETACH then
     begin
        if Assigned(DLLDM) then
        begin
           DLLDM.Free;
        end;
        CoUnInitialize;
        Application:=DllApplication;
     end;
     if Reason = DLL_PROCESS_ATTACH then
     begin
        DllApplication:=Application;
        CoInitialize(Nil);
        if DLLDM=nil then
        begin
           DLLDM:=TDLLDM.Create(nil);
        end;
     end;
  except
  end;
end;exports
   GetMessage;
begin
   DLLProc := @DLLUnloadProc;
   DLLUnloadProc(DLL_PROCESS_ATTACH);
end.
unit datamodule;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  Db, ADODB;type
  TDLLDM = class(TDataModule)
    conn: TADOConnection;
    ads: TADODataSet;
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  DLLDM: TDLLDM;implementation{$R *.DFM}end.
unit functions;interfaceuses Windows,Forms,DB,ADODB;function ConnectToDataBase(vStr:String):Boolean;
function GetMessage(PApp:TApplication;connectionstring:String):String; export; stdcall;implementationuses DataModule,varibles;function GetMessage(PApp:TApplication;connectionstring:String):String; export; stdcall;
begin
   Application:=PApp;
   if Not ConnectToDataBase(connectionstring) then
   begin
      Result:='can not connect to database!';
   end
   else begin
      with DLLDM.ads do
      begin
         Close;
         CommandText:='select count(*) from tb_yhda';
         OPen;
         Result:=Fields[0].AsString;
         Close;
      end;
   end;
end;function ConnectToDataBase(vStr:String):Boolean;
begin
   try
      if Connstr<>vstr then
      begin
         connstr:=vStr;
         DLLDM.conn.Close;
         DllDM.conn.ConnectionString :=connstr;
         DLLDM.conn.Open;
      end;
      Result:=True;
   except
      Result:=False;
   end;
end;end.
调用GetMessage时结果能出来，但报Invalid Pointer Operation错误，麻烦帮我看看

Answer 1

就是简单的静态调用，代码很简单：
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;function GetMessage(PApp:TApplication;connectionstring:String):String; stdcall; external 'dllado.dll'  implementation{$R *.dfm}procedure TForm1.Button1Click(Sender: TObject);
var
   constr:String;
begin
   constr:='连接串';
   showmessage(getmessage(application,constr));
end;end.