Can't reference control in MDI window from modal window

This is wierd,

I have an MDI child form created. Now when I open a modal from this child form to change some properties, I get a system.nullreference exception. Yet in the VS IDE it seems I am referencing the control correctly...

i.e

Dim frmChild as frmChildWindow

frmChild.TextBox1.Text = "Hello"

The editor sees textbox1 as being a control that exists, so why when I try to set its text value does it give ma a NullReferenceException?

Thanks

plz post the full block of offending code, I can't tell anything from this.

Hey Cyberhawke, up and working again eh, cool.

Attached is a basic MDI project.

To see the problem I am having, run the app, then select

File - New Window

Then right click in the new window, and select properties. Now what will happen here is the child window textbox value should be updated with the value set in the properties window.

But thats when it throws an exception.

Thanks main

Simple, you didn't instantiate your reference to the child window

When you open the properties window, you should have a reference to the instance of the child window of the mdi form.


When you create the properties window you need to create it like this

VB Code:

Private Sub MenuItem1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MenuItem1.Click
    Dim frmFormat As New frmProperties
    Me.AddOwnedForm(frmFormat)
    frmFormat.ShowDialog()
End Sub

Then in your properties window you need to set a reference to set a reference to it's owner:

VB Code:

Private frmChildWindow As frmChild  'Form level variable...
Private Sub frmProperties_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    frmChildWindow = Me.Owner
End Sub
 
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    frmChildWindow.TextBox1.Text = Me.TextBox1.Text
End Sub

Hey Cyberhawke, you are the man!

Thanks very much for helping me out.

This was not so simple though. How do you get to know this stuff? I didn't see it mentioned in any MDI tutorials I did!

Thanks again

It came to me in a dream;)

Actually, the first time I looked at MDI stuff was when you asked, I have been working with OOP for so long that figuring out the appropriate properties and methods to use was just a matter of glancing at the available list. I'm just glad I could help :bigyello:

btw, some things to be cautious of...

A lot of this code creates circular references, Form1 has a reference to Form2 and Form2 has a reference to Form1. VB .NET is very good about cleaning up behind itself, but in a production system it is still a good idea to do a little housekeeping when you close an object.

When Form1 creates an instance of Form2 and shows it, and Form2 has it's reference back to Form1, when closing Form2, you should probably write something like this:

VB Code:

Private frmChildWindow As frmChild
Private Sub frmProperties_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    frmChildWindow = Me.Owner
End Sub
Private Sub frmProperties_Closing(ByVal sender As Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles MyBase.Closing
    frmChildWindow = Nothing
End Sub

Thanks, I'll start putting that in now.

One last dumbaas question though,
how do I reference a control that is in the main MDI parent window.

I can't create a reference to the frmMain,
i.e Dim frmParent as frmMain

So how do I reference a control in the parent application window?

I've tried Me.MdiParent and Me.Parent etc. but it doesn't see the controls in the main window.

Thanks

Same concept, you need to create an object reference to your main form in each of the children...

VB Code:

Public Class frmChild
    Inherits System.Windows.Forms.Form
    Private fMain As frmMain
 
    Private Sub frmChild_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        fMain = Me.MdiParent
    End Sub
 
    Private Sub cmdCommit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdCommit.Click
        Me.txtNotes.Text = fMain.Button1.Text
    End Sub
End Class

HI CyberHawke,

Pardon my ignorance but what is the difference between an Owned form and a Child form?

Depends on how the form was created, an owned form is a form that was created as a non-mdi child form, but has the creating form as it's owner, while a child form is created as an mdichild form and has an mdi form as it's mdiparent

And btw, I caught your little comment about the quick responses to spoiledkids posts, and I assure you it had nothing to do with her icon, if it even is a her...

I just have any responses to my posts sent to my yahoo email account and messenger notifies me when I receive a response :cool:

Hi,


To me it looks as if the practical effect and usage is the same. I can't see anything in MSDN which helps on this.

I can't do that. frmMain is the name of my main app window, but I can't declare it...

i.e Private fMain as frmMain

It tells me frmMain is not defined. Yet frmMain.vb is the name of the window.

Thanks

I assure you it isn't, if you create a form as an mdichild using code in the MDI container form and set the child forms mdiparent property to the MDI container form, it doesn't have an owner, it has an mdiparent. If on the other hand you were to create these same forms and not set the mdiparent property but use rather the MDI containers AddOwnedForms method for each of the new forms, they would not be contained in the MDI container form.

Quote:

---

Originally posted by greg_quinn
I can't do that. frmMain is the name of my main app window, but I can't declare it...

i.e Private fMain as frmMain

It tells me frmMain is not defined. Yet frmMain.vb is the name of the window.

Thanks

---

This did work for me, here is the code...