CODEWITHSUNDEEP
c#winforms
Kliver Max
Kliver Max

Reputation: 5299

Cannot access a disposed object issue

I have a form that I am showing by a button click. When I click the button the second time, I get a form again. So I want prevent it, that is, I want to show only one form's instance at once. I'd tried to use a singleton:

public partial class MyForm : Form
{
    private static Object _mutex = new Object();
    private static  MyForm _instance = null;
    public static MyForm GetInstance(...params...)
    {
        if (_instance == null)
        {
            lock (_mutex)
            {
                if (_instance == null)
                {
                    _instance = new MyForm(..params..);
                }
            }
        }
        return _instance;
    }
}

And calling it:

private MyForm Form {set;get;}
private void MyButtonClick(object sender, EventArgs e)
{
   Form = MyForm.GetInstance(...params...);
   if (!Form.Visible)
       Form.Show();
}

After the button click, I get a form. After I close it and click the button again, in the result I get an error:

System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'MyForm'.

What am I doing wrong? Maybe I do not need a singleton, and is there another way to do it?

Upvotes: 0

Views: 1030

Answers (3)

Arun
Arun

Reputation: 851

Use Ispostback method .. 

public partial class MyForm : Form
{
  private static Object _mutex = new Object();
    private static  MyForm _instance = null;
    public static MyForm GetInstance(...params...)
    {
        if (_instance == null)
        {
            lock (_mutex)
            {
                if (Ispostback)
                {
                    _instance = new MyForm(..params..);
                }
            }
        }

        return _instance;
    }
}

Upvotes: 0

Steven Borges
Steven Borges

Reputation: 411

Wouldn't it be easier to just set a variable that controls if the form is open or not? I do it that way all the time, and whenever I want to do it on multiple windows in one Form I make an array out of it, for example:

if(!isOpen[0]) {
   isOpen[0]=true;
   Form.Show;
}

For that you make isOpen accessible to the other Forms, then when you close the form in question you set the variable from there back to false with the Closing event.

Upvotes: 1

Me.Name
Me.Name

Reputation: 12544

The form that is closed, is disposed and cannot be reshown. The instance variable has to be reset to null, or the form has to prevent itself from closing and hiding itself.

e.g.

protected override void OnFormClosed(FormClosedEventArgs e)
{
    _instance = null;
    base.OnFormClosed(e);
}

Upvotes: 3

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