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

how write code in right way?

public class form1    dim y string = lbl_1.text 

it says:

{a first chance exception of type 'system.nullreferenceexception' occurred in calculator.exe} 

can me guys?

this sample code

public class form1 dim y string = lbl_1.text  private sub btn_diff_click(byval sender system.object, byval e system.eventargs) handles btn_diff.click      lbl_1.text = y & "-*" end sub   private sub btn_1_click(byval sender system.object, byval e system.eventargs) handles btn_1.click      lbl_1.text = y & "1" end sub  private sub lbl_1_click(byval sender system.object, byval e system.eventargs) handles lbl_1.click     dim y string = lbl_1.text     lbl_1.text = y end sub  private sub btn_n_click(byval sender system.object, byval e system.eventargs) handles btn_n.click     lbl_1.text = ""     lbl_1.focus() end sub  private sub btn_2_click(byval sender system.object, byval e system.eventargs) handles btn_2.click     dim y string = lbl_1.text     lbl_1.text = y & "2" end sub  private sub btn_equal_click(byval sender system.object, byval e system.eventargs) handles btn_equal.click     lbl_1.text = val(lbl_1.text) end sub 

end class

i want make calculator

but should write in last button (btn_equal)? have tried val doesnt work want

also when declare y in each conrol works in puplic doesnt work

the controls in class form1 need initialized before being used. if want use control in way need explicitly add parameterless constructor form1 class

public class form1      dim y string      public sub new()          ' call required designer.        initializecomponent()         ' add initialization after initializecomponent() call.        y = lbl_1.text        ....    end sub  end class 

in actual code, reading of label control's text happens before label has been created in initializecomponent hidden call. if declare explicitly parameterless constructor (public sub new()) vs ide adds call initializecomponent , place initialization of string variable after creation of label.
(you find initializecomponent method inside form1.designer.vb file if click show files in properties window)

for future knowledge consider read qa cases of nullreferenceexception discussed thoroughly.