How to Initialize form without stimulating any events?

Waterharbin · August 22nd, 2010, 04:02 AM

Hello.I got a sample below:
I got a form(name is form2) with a combobox(name is comboBox1) and two textboxes(textbox1 and textbox2) on it.I just want "100+200=300". But the code below can not run.

Code:

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        TextBox1.Text = 100
        ComboBox1.Text = 200
    End Sub
    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        TextBox2.Text = CSng(ComboBox1.Text) + CSng(TextBox1.Text)
    End Sub

I know the cause is the event "ComboBox1_SelectedIndexChanged" has been stimulated when the form2 is initialized. Can anybody tell me what will i do?
Thanks for reading, and Thanks more for posting!

Imar · August 22nd, 2010, 06:40 AM

Hi there,

It should work fine when the combo box contains a number of predefined items.

When it's empty, then ComboBox1_SelectedIndexChanged won't fire because there is no change of index (of the selected item).

BTW: you're likely to get more attention to questions like this in the Essentials forum: http://p2p.wrox.com/visual-basic-2008-essentials-358/

Cheers,

Imar

Waterharbin · August 22nd, 2010, 10:35 PM

Thank you very much ,Imar
I think the problem is in textBox1.The code below can run.

Code:

Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ComboBox1.Text = 200
    End Sub
    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        TextBox2.Text = CSng(ComboBox1.Text) + CSng(ComboBox1.Text)
    End Sub

The textBox1 is not initialized when it is quoted in the event ComboBox1_SelectedIndexChanged.But when I add textBox1 to the form2,I just assign 100 to it in the property panel. Really confused!
I get another way ,it works

Code:

Dim Text1 As Single
Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ComboBox1.Text = 200
        Text1 = TextBox1.Text
    End Sub
    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        TextBox2.Text = CSng(ComboBox1.Text) + Text1
    End Sub

But I do not want to define an extra variant(It is Text1 here).Is there any other way ?
Thank you again
BTW: I love your books ,Best I ever read.

Imar · August 23rd, 2010, 05:43 AM

The problem is: I am not sure what you're trying to accomplish. The simple code makes it difficult to see if this is eventually a real app, or that you're just learning VB. I also don't get the problem and don't see why you would need that variable.

Can you provide more details on what it is you're doing and why?

Cheers,

Imar

Waterharbin · August 23rd, 2010, 08:07 AM

Sorry about my poor description.My motherlanguage is not English.
It's a problem of my application.I write my application with vb6.0 before,now it is updated to vb2008 by my VS2008.The problem occured after updating.
I just want to click the ComboBox1 ,choose an item, add it to Textbox1.Text, and show the result in TextBox2.It's just an simple arithmetic Calculation,get the sum of two numbers.What I post here is just a simple sample to show the problem.
I add a form(form2),a comboBox(ComboBox1,Items are added in property panel),two TextBoxes(TextBox1 and TextBox2).Then double click the form2,add the code below:

Code:

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ComboBox1.Text = 200
        TextBox1.Text = 100
    End Sub

Then double click the ComboBox1,add the code below:

Code:

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        TextBox2.Text = CSng(ComboBox1.Text) + CSng(TextBox1.Text)
    End Sub

The press F5, an error occur:"Untreated InvalidCastException:From string "" to type "Single" is invalid"
Please take a try as what I said, It won't take your a minute.And you will see the problem yourself.
BTW:This problem is discriped in MSDN:ms-help://MS.VSCC.v90/dv_commoner/local/redirect.htm?keyword="B530EFF2-3132-48F8-B8BC-D88AF543D321
Thank you very much for your time.Best wishes!

Imar · August 23rd, 2010, 08:18 AM

Quote:

Sorry about my poor description.My motherlanguage is not English.

It's not mine either. ;-)

Anyway, I did try it before, and as I said it worked fine when I added some items to the DropDownList on the design surface. I didn't get an error; but SelectedIndexChanged din't fire at Load without any items.

Why do you have to do this in _Load? Why not add the items at Design Time? Alternatively you could assign them in an earlier event such as Init.

Finally, instead of the Text property, you can access the drop down's Items collection. Feels a bit more natural.

Cheers,

Imar

Waterharbin · August 23rd, 2010, 10:22 PM

Quote:

Originally Posted by Imar

It's not mine either. ;-)

Well,nice to hear that then,haha.
I find the problem.I change the ComboBox1's"DropDownStyle" property to "DropDownList" at the property panel, and then "No Problem anymore"!Haha.The default one is "DropDown".But It does not work when the "DropDownStyle" property is set to "DropDown".I don't know why,my specialized field is not computer science.I am a civil engineer.
I use vb6.0 before,I just start to use vb2008,I am not familiar with vb2008.I find the solution under your guide,Thank you!!!
Is there an _Init event? I don't find it!
I prefer the code rather than the property panel.That's why I use code to assign the contrls.It's just a personal habit.
Appreciate you for your time!!!

Imar · August 24th, 2010, 05:12 PM

Quote:

Is there an _Init event? I don't find it!

No, sorry. Thinking too much in terms of Web Forms.... ;-)

Glad to hear you got it working.

Cheers,

Imar