VB.NET use variable in textbox name - vb.net

Question

OK, so I have a form called quote_guidelines and i would like to use a procedure to update my database with values entered into the quote_guidelines form. The values are the descriptions and costs of decorations, food/drink and entertainment. Each tab has been assigned one of these additional servies. I wanted to make a procedure which would update the database with these new values.
The problem was that the query would have to contain a different table and different textbox names. I tried to solve this by saving the names of the tables and textboxes in variables, which will then be passed into the procedure.
the table variable works fine in the sql statement however, the textbox names dont.
I tried this:
quote_guidelines.Controls(description_textbox).Text
But this doesnt work.
Here is my query:
query = "UPDATE " & additional & " SET description='" & quote_guidelines.Controls("" & description_textbox & "").Text & "', cost= " & quote_guidelines.Controls("" & cost_textbox & "").Text & " WHERE " & additional & "ID='" & y & "'"
When I run the program, i get the following error "An unhandled exception of type 'System.NullReferenceException' occurred in EHCC_BookingSystem.exe
Additional information: Object reference not set to an instance of an object."
I just realised that it might be some other stupid mistake i've made so here's the whole procedure:
Sub UpdateAdditionals(textbox3 As System.Object, textbox4 As System.Object, textbox5 As System.Object, textbox6 As System.Object, textbox7 As System.Object, textbox8 As System.Object, ByRef additional As String, ByRef description_textbox As String, ByRef cost_textbox As String)
mysqlconn = New MySqlConnection
mysqlconn.ConnectionString = "Server=localhost;userid=root;password=root;database=comp4"
Dim reader As MySqlDataReader
MsgBox(additional & description_textbox & cost_textbox)
Try
mysqlconn.Open()
Dim query As String
query = "UPDATE " & additional & " SET description='" & quote_guidelines.Controls.Find(description_textbox, True).FirstOfDefault().Text & "', cost= " & quote_guidelines.Controls.Find(cost_textbox, True).FirstOfDefault().Text & " WHERE " & additional & "ID='" & y & "'"
MsgBox(query)
command = New MySqlCommand(query, mysqlconn)
reader = command.ExecuteReader()
mysqlconn.Close()
Catch ex As MySqlException
MessageBox.Show(ex.Message)
Finally
mysqlconn.Dispose()
End Try
End Sub

Answer 1

Try this - Replaced Controls in Controls.Find (Note the `True parameter to search the controls children):
query = "UPDATE " & additional & " SET description='" & TryCast(quote_guidelines.Controls.Find(description_textbox, True)(0), TextBox).Text & "', cost= " & TryCast(quote_guidelines.Controls.Find(cost_textbox, True)(0).Text & " WHERE " & additional & "ID='" & y & "'"
Also note you could do .FirstOfDefault() to get the first control (don't forget to cast):
Controls.Find("nameofcontrol", True).FirstOfDefault()