how to check if the form is opened or not

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In VB.Net I want to check whether the current form is opened or not for access the function with the form

i give the coding

Invoice.Showgrip()

Somtimes if the form is not active the object reference not set to an instance of an object error will occured so how to check whether the form is visible or active.
help me please

Posted 12-Jan-12 22:50pm

devausha

Updated 9-Nov-22 4:37am

Prasad_Kulkarni

v2

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Sergey Alexandrovich Kryukov 13-Jan-12 4:59am

Why? why?!
--SA

3 solutions

Solution 2

'probably need to check if your form is instantiated

If Invoice isnot nothing then
invoice.Showgrip
End If

Posted 12-Jan-12 23:08pm

Spopy

Solution 3

Null object reference has nothing to do with form status. It can be closed, activated — the instance of its class remains valid. I have no idea why would you need to check up if the form is closed. I'm pretty much sure you never need it. Some advice and example:

If you want to show a form again, never close it but hide it instead. Here is how:

C#

public class MyForm : Form {

    public override void OnFormClosing(FormClosingEventArgs eventArgs) {
        base.OnFormClosing(eventArgs);
        if (eventArgs.ClosegReason == System.Windows.Forms.CloseReason.UserClosing) {
            eventArgs.Cancel = true;
            this.Hide();
        }
    }

    //...

} //class MyForm

Also, if you do it right, form operations should not depend on its status. For example, Form.Show shows the form no matter if it was hidden or shown before the call.

See http://msdn.microsoft.com/en-us/library/system.windows.forms.form.aspx[^].

If you still want to know of the form was closed, you can add some Boolean flag as a field of the form class, override OnFormClosed and set this flag to true, which would indicate the form was closed. Again, I think you won't need it and won't find a good use of it; I added this advice only to formally answer your question.

—SA

Posted 12-Jan-12 23:09pm

Sergey Alexandrovich Kryukov

Updated 12-Jan-12 23:13pm

v2

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Wayne Gaylard · Accepted Answer · 2012-01-12T23:07:00

You need to cycle through all forms and check if the one you want is indeed there

VB

For Each form In My.Application.OpenForms
        If (form.name = yourForm.name) Then
            'form is loaded so can do work 
            'if you need to check whether it is actually visible
            If form.Visible Then
                'do work when visible
            End If
        End If
    Next