如何修复Delphi中地址的访问冲突

问题描述

我是delphi的新手，正在尝试启动OOP.但是，使用公共属性设置私有字段时出现访问冲突.

I'm new to delphi and am trying to start OOP. However I get an access violation when using public property to set a private field.

type
  User = class;
  TData = class
  private
    CurrUser: User;
    Connection: TFDConnection;
    Query: TFDQuery;

  procedure SetUser(newUser: User);
  procedure SetConnection(newConn: TFDConnection);
  procedure SetQuery(newQry: TFDQuery);
public
  property CUser: User read CurrUser write SetUser;
  property Conn: TFDConnection read Connection write SetConnection;
  property Qry: TFDQuery read Query write SetQuery;

  class procedure Login(uID: integer); static;
  class procedure Logout(uID: integer); static;
  class procedure ExitApp(); static;
end;
implementation

{$R *.fmx}

procedure TData.SetUser(newUser: User);
begin
  CurrUser := newUser;
end;

procedure TData.SetConnection(newConn: TFDConnection);
begin
  Connection := newConn;
end;

procedure TData.SetQuery(newQry: TFDQuery);
begin
  Query := newQry;
end;

我希望能够使用该属性来设置Connection，但是使用该属性write的任何代码都会给我带来访问冲突:TData.Conn.LoginPrompt:= False;TData.Conn.Connected:= True;

I expect to be able to set the Connection using that property however it gives me the access violation with any code that uses the property write:TData.Conn.LoginPrompt := False;TData.Conn.Connected := True;

var
  TData: frmData.TData;
  LoginForm: TLoginForm;
  ErrorCount : integer;

implementation

{$R *.fmx}

procedure TLoginForm.ExitAppButtonClick(Sender: TObject);
begin
  TData.ExitApp;
end;

procedure TLoginForm.LoginButtonClick(Sender: TObject);
var
  companyPath : string;
  nurseID : integer;
begin
  if(UsernameInput.Text = '') or (PasswordInput.Text = '') or (PincodeInput.Text = '') then
  begin
    ShowMessage('Please enter your login details.');
    Exit;
  end;

  try
    TData.Conn := TFDConnection.Create(nil);
    TData.Conn.Params.DriverID := 'MSAcc';
    TData.Conn.Params.Database := 'D:\PulseDB\AlfaPersonnel\Pulse.mdb';
    TData.Conn.LoginPrompt := False;
    TData.Conn.Connected := True;
    if(TData.Conn.Connected <> True) then
    begin
      ShowMessage('Could not connect, try again');
      Exit;
    end
    else //When Connection
    begin
      TData.Qry := TFDQuery.Create(TData.Conn);
      try
        TData.Qry.Connection := TData.Conn;
        TData.Qry.SQL.Text := 'SELECT * FROM NurseLogin WHERE Username=:uname AND Password=:pword AND PinCode=:pin;';
        TData.Qry.Params.ParamByName('uname').AsString := UsernameInput.Text;
        TData.Qry.Params.ParamByName('pword').AsString := PasswordInput.Text;
        TData.Qry.Params.ParamByName('pin').AsString := PincodeInput.Text;
        TData.Qry.Active := True;

        if TData.Qry.RecordCount = 0 then ShowMessage('Details not recognised.')
        else if TData.Qry.RecordCount = 1 then
             begin
               if TData.Qry.FieldByName('IsActive').AsBoolean then //If the user is active
               begin
                 try
                   //Connect to the path
                   companyPath := TData.Qry.FieldByName('CompanyName').AsString;
                   TData.Conn.Params.Database := 'D\PulseDB\' + companyPath + '\Pulse.mdb';
                   TData.Conn.Connected := True;

                   ShowMessage('Connected to ' + companyPath);
                 finally

                 end;
               end;
             end;
      finally
      end;
    end;
  finally
  end;
end;

推荐答案

您永远不会创建 TData 类的实例.在某些时候，您需要编写:

You don't ever create an instance of your TData class. At some point you need to write:

TData := frmData.TData.Create;

实例化实例的方式.完成后还需要销毁它.像这样:

Which is how you instantiate an instance. And you need to destroy it when you are finished also. Like this:

TData.Free;

您没有实例化实例是对访问冲突的解释.

That you did not instantiate an instance is the explanation for your access violation.

其他一些问题:

1. 对类型使用 T 前缀.您的变量应命名为 Data ，而不是 TData .
2. 尽可能不要使用全局变量.

1. Use the T prefix for types. Your variable should be named Data rather than TData.
2. Don't use global variables if at all possible.

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