合理应用用户登录界面，用户登录时不必创建其他窗体

/////////////////////  （一）项目文件  test.dpr //////////////////////
program SerialGet;

uses
  Forms,
  UMain in 'UMain.pas' {frmMain},
  ULogin in 'ULogin.pas' {frmLogin},
  UDataModule in 'UDataModule.pas' {DataModule1: TDataModule},

{$R *.res}

begin
  Application.Initialize;

  if CreateMutex then                 //创建句柄，判断此应用程序是否在运行
  begin
    //调用全局函数，创建并显示登陆界面
    if doLogin then                   //登陆成功
    begin
      Application.CreateForm(TfrmMain, frmMain);
      //数据模块文件不须在这儿创建，因为 ULogin.pas 中已创建
      //Application.CreateForm(TDataModule1, DataModule1);
      Application.Run;
    end else                          //登陆不成功
    begin
      try
        DataModule1.free;
        Application.terminate;
      except
      end;
    end;
  end else
  begin
    DestroyMutex;                     //释放句柄
  end;
end.

////////////////  （二）登陆窗体 ULogin.pas  ULogin.dfm //////////////////
unit ULogin;

interface
uses ......
type
  ... ... ...
  private
    function checkPsw:integer;
  public
  end;

var
  frmLogin: TfrmLogin;

  function doLogIn:boolean;          // 全项目公用函数
  function CreateMutex: Boolean;     // 全项目公用函数
  procedure DestroyMutex;            // 全项目公用函数

implementation
uses UDataModule;  //引用数据模块
var Mutex: hWnd;

{$R *.dfm}

function doLogIn:boolean;                 //由项目文件调用此函数
begin
  with TfrmLogin.create(application) do   //创建并显示登陆界面
  begin
    //窗体的ShowModal属性
    if ShowModal = mrok then result := true else result := false;
    free;
  end;
end;

procedure DestroyMutex;
begin
  if Mutex <> 0 then CloseHandle(Mutex);
end;

function CreateMutex: Boolean;
var
  PrevInstHandle: THandle;
  AppTitle: PChar;
begin
  AppTitle := StrAlloc(100);
  StrPCopy(AppTitle, Application.Title);
  Result := True;
  Mutex := Windows.CreateMutex(nil, False, AppTitle);
  if (GetLastError = ERROR_ALREADY_EXISTS) or (Mutex = 0) then begin
    Result := False;
    SetWindowText(Application.Handle, '');
    PrevInstHandle := FindWindow(nil, AppTitle);
    if PrevInstHandle <> 0 then begin
      if IsIconic(PrevInstHandle) then
        ShowWindow(PrevInstHandle, SW_RESTORE)
      else
        BringWindowToTop(PrevInstHandle);
      SetForegroundWindow(PrevInstHandle);
    end;
    if Mutex <> 0 then Mutex := 0;
  end;
  StrDispose(AppTitle);
end;

// -1: 密码不对  1:数据库不对  2:没有此用户  3:合法
function TfrmLogin.checkPsw:integer;
var name,sPsw,SQL,sValue:string;
begin
  Application.CreateForm(TDataModule1, DataModule1);  //此处创建了数据模块
  if not DataModule1.ConnOK then
  begin result := 1;   exit;  end;

  name := lowercase(editName.text);  //文本框
  sPsw := lowercase(editPass.text);  //文本框
  sql := 'select * from maker where name="'+name+'"';
  if openSQL(SQL,DataModule1.dsDataSet) <=0 then
  begin result := 2; exit;  end;

  DataModule1.dsDataSet.First ;
  sValue := lowercase(DataModule1.dsDataSet.fieldbyName('loginPsw').asString);
  if sValue<>sPsw then result := -1 else result := 3;
end;

/////////////////////  （三）数据模块 UDataModule.pas //////////////////////
... ... ... ...
type
  public
    ConnOK:boolean;
  end;
var
  DataModule1: TDataModule1;
  function OpenSQL(s: string;query:TADODataSet):integer;
  function DoSQL(s: string;query:TADOQuery):boolean;
 
implementation

{$R *.dfm}

procedure TDataModule1.DataModuleCreate(Sender: TObject); //连接ADOConnection
var SQL,pwd:string;
begin
  try
    pwd := 'deliSerial';
    SQL := 'Provider=Microsoft.Jet.OLEDB.4.0;Data Source='+
         extractfilepath(paramstr(0))+'SerialInfo.mdb'+
         ';Persist Security Info=False;'  +
         'Jet OLEDB:Database Password="'+pwd+'"';
    ADOConnection1.Connected := false;
    ADOConnection1.ConnectionString := SQL;
    ADOConnection1.Connected := true;
    ConnOK:=true;
  except
    ConnOK:=false;
  end;
end;

function OpenSQL(s: string;query:TADODataSet):integer; //查询SQL
var old_Cursor:TCursor;
begin
  old_Cursor:=screen.cursor;
  screen.cursor:=crSQLWait;
  try
    try
      with query do
      begin
        close; commandtext:=s; open;
        result:=query.recordcount;       //返回结果集记录数
      end;
    except
     result:=0;
    end;
  finally
    screen.cursor:=old_Cursor;
  end;
end;

function DoSQL(s: string;query:TADOQuery):boolean;  //运行 SQL
var old_Cursor:TCursor;
begin
  result:=true;
  old_Cursor:=screen.cursor;
  screen.cursor:=crSQLWait;
  try
    try
      with query do
      begin
        close; SQL.Clear; SQL.Add(s); ExecSQL;
      end;
    except
      result:=false;
    end;
  finally
    screen.cursor:=old_Cursor;
  end;
end;

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