[RESOLVED] System.NullReferenceException Error

Hello,
I am getting NullReferenceException error in the Controls("ComboBox" & m).Text = "No" and probably i didn't define ComboBox and i don't know how to do it. Can you please help?

Code:

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Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        Dim m As Integer

        For m = 35 To 77

            Controls("ComboBox" & m).Text = "No"

        Next m

End Sub

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Indexing the Controls collection with a name that doesn't exist will return Nothing, and you can't get a property of Nothing. Why do you think that ComboBoxes will just appear out of thin air? If you want ComboBoxes on your form then add them to your form. That said, if you added them to some other container, e.g. a Panel, then they will be in the Controls collection of that container, not of the form.

Quote:

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Originally Posted by jmcilhinney

Indexing the Controls collection with a name that doesn't exist will return Nothing, and you can't get a property of Nothing. Why do you think that ComboBoxes will just appear out of thin air? If you want ComboBoxes on your form then add them to your form. That said, if you added them to some other container, e.g. a Panel, then they will be in the Controls collection of that container, not of the form.

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Hello jmcilhinney,
You are like The Dooo of programing :) I added the ComboBoxes in TabControl1, does this make a difference? If so how can i put it to my code?

Code:

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TabControl1.TabPages(x).Controls("ComboBox" & m).text

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where x is the index of the tabpage

Quote:

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Originally Posted by Delaney

Code:

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TabControl1.TabPages(x).Controls("ComboBox" & m).text

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where x is the index of the tabpage

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If the TabPage was added to the TabControl in the designer then there will be a field for it, just like other controls. In that case, you would generally refer to the TabPage by that field, e.g.

Code:

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TabPage1.Controls("ComboBox" & m).Text = "No"

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Thank you so much guys, solved the problem:wave:

I have one more question. When i try to use the same method with SelectedIndex = -1, it's saying "SelectedIndex is not a member of Control". How can i combine it with the Control method?

Indexing the Controls collection returns a Control reference, so you can only access members of the Control class via that reference. As is always the case, if the actual object referred to is a more specific type and you want to access members of that type then you need to cast the reference you have as that type. The analogy I always use is a vet who knows that all their patients are animals but not what specific type of animal they will be, so can only do things that apply to all animals until the owner takes their pet out of the box and reveals the specific type of animal it is. If you don't know how to cast in VB then that's what you should research.

Solved it, thanks again :)

Code:

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TabPage1.Controls("ComboBox" & m).Text = Nothing

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The other option would be:

Code:

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DirectCast(TabPage1.Controls("ComboBox" & m),ComboBox).SelectedIndex = -1

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Quote:

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Originally Posted by Shaggy Hiker

The other option would be:

Code:

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DirectCast(TabPage1.Controls("ComboBox" & m),ComboBox).SelectedIndex = -1

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Thank you, working great :)