Object reference not set to an instance of an object when variable is declared in class level

Question

Sometimes get this error "Object reference not set to an instance of an object" when I declare a variable in class level, and use that variable in a procedure.

For example, this is my latest project to create a program to display the state of substances in a specific temperature:

 Public Class Form1
   Const WATER_FP As Integer = 0
   Const WATER_BP As Integer = 100

   Const ETHANOL_FP As Integer = -114
   Const ETHANOL_BP As Integer = 78

   Const MERCURY_FP As Integer = -39
   Const MERCURY_BP As Integer = 357

   Const OXYGEN_FP As Integer = -219
    Const OXYGEN_BP As Integer = -183

    Dim strSolid As String = vbNullString
    Dim strGas As String = vbNullString
    Dim sngTemperature As Single = CSng(txtTemperature.Text)

   Private Sub btnDisplayState_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplayState.Click


      If sngTemperature <= WATER_FP Then
         strSolid &= "Water "
      ElseIf sngTemperature >= WATER_BP Then
         strGas &= "Water "
      End If

      If sngTemperature <= ETHANOL_FP Then
         strSolid &= "Ethanol "
      ElseIf sngTemperature >= ETHANOL_BP Then
         strGas &= "Ethanol "
      End If

      If sngTemperature <= MERCURY_FP Then
         strSolid &= "Mercury "
      ElseIf sngTemperature >= MERCURY_BP Then
         strGas &= "Mercury "
      End If

      If sngTemperature <= OXYGEN_FP Then
         strSolid &= "Oxygen "
      ElseIf sngTemperature >= OXYGEN_BP Then
         strGas &= "Oxygen "
      End If

      If strSolid <> vbNullString Then
         strSolid = "Substances that are in solid state are " & strSolid
      Else
         strSolid = "No substances are in solid state."
      End If

      If strGas <> vbNullString Then
         strGas = "Substances that are in gaseous state are " & strGas
      Else
         strGas = "No substances are in gaseous state."
      End If

      lblMessage.Text = strSolid & ControlChars.CrLf & strGas
   End Sub
End Class

Upon debugging, I get an error as described above, without any highlight of where the code is causing the problem. However, if I move the variables into btnDisplayState_Click, the program works perfectly. Now I know what caused the error, but furthermore I wanted to know why this problem occurs.

OneFineDay · Accepted Answer · 2013-10-17 02:52:57Z

2

You cannot declare variable with a control's property as the control has not been initialized yet. You need to set the value after some text has been entered not before there is a value.

 Dim sngTemperature As Single = CSng(txtTemperature.Text)'no value here yet

Just declare it class level:

 Dim sngTemperature As Single

Then set it in the button event.

answered Oct 17, 2013 at 2:52

OneFineDay

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nathanchere · Accepted Answer · 2013-10-17 02:59:58Z

0

Dim sngTemperature As Single = CSng(txtTemperature.Text)

Don't assign variables like that. You're assuming txtTemperature.Text is initialised BEFORE sngTempearature. Easy fix, change to:

Dim sngTemperature As Single = 0

and change your method to begin with:

Private Sub btnDisplayState_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnDisplayState.Click

  sngTemperature = CSng(txtTemperature.Text)

  If sngTemperature <= WATER_FP Then
     strSolid &= "Water "
  ' etc etc

answered Oct 17, 2013 at 2:59

nathanchere

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