{A first chance exception of type 'System.NullReferenceException' occurred in calculator.exe} error during declaring variable - vb.net

Question

How to write this code in a right way?
Public Class Form1
Dim y As String = lbl_1.Text
It says:
{A first chance exception of type 'System.NullReferenceException' occurred in calculator.exe}
Can you help me guys?
this is a sample from the code
Public Class Form1
Dim y As String = lbl_1.Text
Private Sub btn_diff_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_diff.Click
lbl_1.Text = y & "-*"
End Sub
Private Sub btn_1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_1.Click
lbl_1.Text = y & "1"
End Sub
Private Sub lbl_1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles lbl_1.Click
Dim y As String = lbl_1.Text
lbl_1.Text = y
End Sub
Private Sub btn_n_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_n.Click
lbl_1.Text = ""
lbl_1.Focus()
End Sub
Private Sub btn_2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_2.Click
Dim y As String = lbl_1.Text
lbl_1.Text = y & "2"
End Sub
Private Sub btn_equal_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btn_equal.Click
lbl_1.Text = Val(lbl_1.Text)
End Sub
End Class
i want to make a calculator
but what should i write in the last button (btn_equal)? i have tried val but it doesnt work as i want
also when i declare y in each conrol it works but in puplic it doesnt work

Answer 1

The controls in the class Form1 need to be initialized before being used. If you want to use a control in that way you need to explicitly add the parameterless constructor to Form1 class
Public Class Form1
Dim y as String
Public Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
y = lbl_1.Text
....
End Sub
End Class
In your actual code, the reading of the label control's text happens before the label itself has been created in the InitializeComponent hidden call. If you declare explicitly the parameterless constructor (Public Sub New()) then the VS IDE adds the call to the InitializeComponent and you could place the initialization of your string variable after the creation of the label.
(You could find the InitializeComponent method inside the Form1.Designer.vb file if you click Show All Files in the properties window)
For future knowledge consider to read this QA where cases of NullReferenceException are discussed thoroughly.