How to loop through items on an other form? - vb.net

Question

So I have a three forms with text boxes, and I'm saving them to db on the last one, one is pretty huge and iterative so I'd like to write a loop. But when I try to loop through them it doesn't seem to grab them from the other form, what am I doing wrong?
For i = 1 To 12
Dim txtbox_bem As TextBox = CType(frm_new_prot_2.Controls("frm_new_prot_2.txtbox_bem1_" & i), TextBox)
Dim txtbox_soll As TextBox = CType(frm_new_prot_2.Controls("txtbox_soll1_" & i), TextBox)
Dim txtbox_messw As TextBox = CType(frm_new_prot_2.Controls("txtbox_messw1_" & i), TextBox)
Dim txtbox_einheit As TextBox = CType(Controls("frm_new_prot_2.txtbox_einheit1_" & i), TextBox)
Dim txtbox_max As TextBox = CType(Controls("frm_new_prot_2.txtbox_max1_" & i), TextBox)
Dim txtbox_min As TextBox = CType(Controls("frm_new_prot_2.txtbox_min1_" & i), TextBox)
Dim txtbox_tol As TextBox = CType(Controls("frm_new_prot_2.txtbox_tol1_" & i), TextBox)
Dim ckbox_ok As CheckBox = CType(Controls("frm_new_prot_2.ckbox_ok1_" & i), CheckBox)
command.CommandText = "INSERT INTO tb_1_" & i & " (Auftraggeber, Protokollnr, 1_" & i & "bem, 1_" & i & "soll, 1_" & i & "messw, 1_" & i & "einheit, 1_" & i & "max, 1_" & i & "min, 1_" & i & "tol, 1_" & i & "ok) VALUES ('" & Auftraggeber & "'," & Protokollnr & ",'" & txtbox_bem.Text & "','" & txtbox_soll.Text & "','" & txtbox_messw.Text & "','" & txtbox_einheit.Text & "','" & txtbox_max.Text & "','" & txtbox_min.Text & "','" & txtbox_tol.Text & "','" & ckbox_ok.Text & "')"
command.CommandType = CommandType.Text
command.ExecuteNonQuery()
MsgBox("success row " & i)
Next
As you see I tried different combinations. The error I'm getting is:
"System.NullReferenceException: 'Object reference not set to an instance of an object.' txtbox_bem was Nothing."

Answer 1

The line ..
Dim txtbox_bem As TextBox = CType(frm_new_prot_2.Controls("txtbox_bem1_.txtbox_bem1_" & i), TextBox)
is searching through the controls of frm_new_prot_2 for a control called "frm_new_prot_2.txtbox_bem1_" & i.
That won't exist. In the controls collection will only contain the textbox with a name of "txtbox_bem1_" & i, so the corrected version should be
Dim txtbox_bem As TextBox = CType(frm_new_prot_2.Controls("txtbox_bem1_" & i), TextBox)
The other lines in your code probably won't work either. They should probably be ..
Dim txtbox_bem As TextBox = CType(frm_new_prot_2.Controls("txtbox_bem1_" & i), TextBox)
Dim txtbox_soll As TextBox = CType(frm_new_prot_2.Controls("txtbox_soll1_" & i), TextBox)
Dim txtbox_messw As TextBox = CType(frm_new_prot_2.Controls("txtbox_messw1_" & i), TextBox)
Dim txtbox_einheit As TextBox = CType(frm_new_prot_2.Controls("txtbox_einheit1_" & i), TextBox)
Dim txtbox_max As TextBox = CType(frm_new_prot_2.Controls("txtbox_max1_" & i), TextBox)
Dim txtbox_min As TextBox = CType(frm_new_prot_2.Controls("txtbox_min1_" & i), TextBox)
Dim txtbox_tol As TextBox = CType(frm_new_prot_2.Controls("txtbox_tol1_" & i), TextBox)
Dim ckbox_ok As CheckBox = CType(frm_new_prot_2.Controls("ckbox_ok1_" & i), CheckBox)
I think you got a bit confused about how references work. :-)
Also I suspect that the data in the textboxes is the only place where you're storing the data. This isn't terribly good programming practice. For example...
Say, the data comes from an external device and you have code that reads data from the device and stores it in the textboxes. This isn't good. anyone could accidentally overwrite data in the textbox.
It would be much better to store the data in Lists or Lists of class objects. and populate the textboxes with the data. When it comes to storing the data in your db, use the data in the lists rather than the textboxes.
The user interface should never be used to store data. Hope this helps

Answer 2

It might be easier to debug without all the casting.
Public Class Form2
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim lst As New List(Of String)
For i As Integer = 1 To 3
Dim str1 As String = Form1.Controls("TextBox" & i).Text
lst.Add(str1)
Next
For Each s As String In lst
Debug.Print(s)
Next
End Sub
End Class
I just added the List for testing purposes.