Form opens two times the same form in combination with MdiParent

Question

I have a form(MdiParent) with the following code:

public partial class Main : Form
{
  Form1 frm1 = null; 
  Form2 frm2 = null;
}

With a couple of menus I do the following:

private void toolStripMenuItem_ClickForm2(object sender, EventArgs e)
{
     if(frm2 == null)
     {
       frm2 = new Form2(frm1);
       frm2.MdiParent = this;
     }   
   frm2.Show();
}


private void toolStripMenuItem_ClickForm1(object sender, EventArgs e)
{
     if(frm1 == null)
     {
       frm1 = new Form1();
       frm1.MdiParent = this;
     }
   frm1.Show();
}

If I click menuitemForm2 do some operations(DB access and populate a DatagridView, frm1 is never initialized in frm2) in form2 and then click menuitemForm1 in order to show frm1. It is going to produce two windows of the same form1.

Does anyone have faced this strange behaviour before?

Addendum because of the comments...

frm2 does something like this with frm1:

 Form1 frm ;

 public Form2(Form1 form1)
 {
   frm = form1;
 }

 public void Optionalmethod()
 {
   if(frm == null)
   {
     frm = new Form1();
     frm.MdiParent = this.MdiParent;
   }
   frm.Show();
 }

Optionalmethod() is not executed during the testing.

According to my logic form1 can be initialized in two cases:

 1. Inside form2 
 2. In the Main form

For me does not matter where form1 is initialized, but it is important to have only one instance of it. Thats is the reason because I decided to pass the reference even if it is null

If you don't click menuitemForm2 and just click on menuitemForm1, do you get two forms? If so, sounds like you have the click event wired up twice. — LarsTech
If "frm1" hasn't been initialized and/or shown yet, you're passing null into the constructor of Form2. What should happen then (how will it get a reference to Form1)? Perhaps you should be checking if both are null and instantiating them before showing anything. Also, you're not setting the MdiParent of Form1 in that handler (you're setting Form2's MdiParent...which could be null!). — Idle_Mind
@Idle_Mind Good catch on that second MdiParent. Missed that. The OP probably shouldn't be passing forms around like that since they are probably more interested in the "data" that's being stored there. Welcome to Spaghettiville. — LarsTech
@LarsTech menuitemForm1 is working properly by itself. The problem occurs only when menuitemForm2 was clicked before — Ayorus
Then you would have to show us how you are using Form1 inside of Form2. — LarsTech

Unknown Unknown · Accepted Answer · 2016-08-09T07:43:32

I have been unable to replicate your problem, but I would suggest an alternative in any case. I am not personally in favour of passing around form variables between child forms. There is a much neater way to do it. Replace your code with

private void toolStripMenuItem_ClickForm1(object sender, EventArgs e)
{
    ShowForm1();
}

    public void ShowForm1()
    {
        try
        {
            if (frm1 == null)
            {
                frm1 = new Form1();
                frm1.MdiParent = this;
            }
            else if (frm1.MdiParent == null)
            {
                frm1 = new Form1();
                frm1.MdiParent = this;
            }
            frm1.Show();
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }

Please note the extra else if. If a form has been created and closed, the reference is not null, but its MdiParent property will be null.

Now remove the Form1 parameter in the Form2 constructor (plus the private member) and replace the OptionalMethod code with:

        var parent = (MainForm)this.MdiParent;
        parent.ShowForm1();

This should work fine.

HTH

Jonathan

Form opens two times the same form in combination with MdiParent

1 Answers