Cannot access a disposed object in Delphi Prism

Question

I am programming under Delphi Prism for Mono. I keep getting this error, "Cannot access a disposed object" after opening already opened and closed window form, even though the window form is declared within a class and instantiated in the constructor as it follows.

Here is the Class in which it is declared:

  MainForm = partial class(System.Windows.Forms.Form)
  private
    method SignalBtn_Click(sender: System.Object; e: System.EventArgs);
    method CommBtn_Click(sender: System.Object; e: System.EventArgs);
    method button1_Click(sender: System.Object; e: System.EventArgs);
    method button2_Click(sender: System.Object; e: System.EventArgs);
    method button4_Click(sender: System.Object; e: System.EventArgs);
    method button5_Click(sender: System.Object; e: System.EventArgs);
    method MainForm_Load(sender: System.Object; e: System.EventArgs); 
    method ShutdownBtn_Click(sender: System.Object; e: System.EventArgs);
    method MySerialData(sender: System.Object; e:SerialDataReceivedEventArgs);
    method LoginBtn_Click(sender: System.Object; e: System.EventArgs);
  protected
    method Dispose(disposing: Boolean); override;
  public
    RX:Array[0..5] of byte;
    TX:Array[0..6] of byte;
    serialPort1:System.IO.Ports.SerialPort;
    thr:Thread;
    stoploop:Boolean;
    mcommand:Byte;
    thechannel:Integer;
    fr : UnitForm;      <<<<<<<<<<<< Here is the form being declared.
    constructor;
    method FillTable;
    method mythread;
  end;

Here is the contructor where it is instantiated or created:

constructor MainForm;
begin
  //
  // Required for Windows Form Designer support
  //
  InitializeComponent();

  mcommand:=$AA;
  thechannel:=$01;
  stoploop:=false;
  thr:=nil;
  fr := new UnitForm;  <<<<<<<<<<<<<<<<<< fr is created as new.
  //
  // TODO: Add any constructor code after InitializeComponent call

  //
end;

Here is how the form is shown or opened within a button event:

method MainForm.UnitBtn_Click(sender: System.Object; e: System.EventArgs);
begin
  fr.Show;  
end;

Being that fr is a window form that is part of the class much like a global variable, why is the window form disposed when you close it after opening it. Isn't it kept in the memory until the whole program is shutdown for easy access?

UPDATE: Within onFormClosing, I hide the form and it seems to work, but when I try to reopen the form again, it raises the same exception as before "Cannot access disposed object."

method UnitForm.UnitForm_FormClosing(sender: System.Object; e: System.Windows.Forms.FormClosingEventArgs);
begin
  hide;
end;

score 1 · Accepted Answer · edited Aug 04 '11 at 21:21

1

When you look into the documentation of System.Windows.Forms.Form.Close() you will notice, that it's documented that the Form frees all it's resources and disposes itself when closed.

What you want is actually Form.Hide() to hide the form from the user.

Edit update: You would call .Close() on the Form only when you would call Free / FreeAndNil in native Delphi. That's sort of the same thing.

For those who needs to actually know how to do it, here is the example code:

method Form2.Form2_FormClosing(sender: System.Object; e: System.Windows.Forms.FormClosingEventArgs);
begin
  e.Cancel:=true; //line prevents form resources being released
  self.hide;      //line tells the form to disappear.
end;

edited Aug 04 '11 at 21:21

ThN

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answered Jul 22 '11 at 16:05

Sebastian P.R. Gingter

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I see. Makes sense, but what if you clicked on the system menu button with x on it. That's in the upper right hand corner of the window. How do you trigger on that? – ThN Jul 22 '11 at 17:58
The OnFormClosing event is where you can react. The FormClosingEventArgs have a possibility to Cancel the event, and you can instead call Hide there. Of course you should only cancel the event when you don't want to actually Close and Dispose the window. – Sebastian P.R. Gingter Jul 23 '11 at 11:46
@Sebastian: FWIW, in Delphi, you can make a form close without it freeing up itself. It is a property setting. – Rudy Velthuis Jul 23 '11 at 23:52
Yeah Rudy, but Windows Forms isn't the Delphi VCL ;-) – Sebastian P.R. Gingter Jul 24 '11 at 08:30
@Sebastian: I know, but I merely wanted to correct the notion you implied that Close() does the same in Delphi. – Rudy Velthuis Jul 24 '11 at 14:14
@Rudy And what is that property setting? Just to remind you I am working with Delphi Prism for Mono. I thought I could pick up easily from programming for Delphi 5 & 7 for many years, but now I am finding out Delphi Prism is almost just like learning a new language. – ThN Jul 25 '11 at 12:56
It's not so much about the language. When you are used to the Delphi VCL then the differences to the .NET Framework are quite big, but the general way your work with Windows.Forms is still the same. You are just learning a new framework. – Sebastian P.R. Gingter Jul 25 '11 at 13:13

Cannot access a disposed object in Delphi Prism

1 Answers1