a variable in a module triggers an error when called from a form

Question

I have a sub with two variables in a module. When called from the form, the second variable triggers the error "“Variable 'ala1' is used before it has been assigned a value. A null reference exception could result at runtime”. The result is a blank textbox. Would appreciate your help to find where my code is not correct. Here is my code:

Module1

Dim N10, N02b, A6, N07 As Double

Public Sub AxlPos(ByVal axl As Double, ByVal axla As String)
    If axl > (N10 - N02b / 1000) * modCos(A6) + N07  Then
        axla = "in section 1"
    ElseIf axl > (N10 - N02b / 1000) * modCos(A6) Then
        axla = "in section 2"
    ElseIf axl > (N10 / 1000) * modCos(A6) Then
        axla = "in section 3"
    Else
        axla = "in section 4"
    End If
End Sub

Form1

Dim N10, N02b, A6, N07, al1 As Double
Dim ala1 as string

Private Sub btnSolve_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSolve.Click
   axlpos(al1,ala1)
   Txtala1.Text = ala1
End sub

score 3 · Answer 1 · edited May 23 '17 at 12:22

I suggest you to change your sub to a function returning a string.

Dim N10, N02b, A6, N07 As Double

Public Function AxlPos(ByVal axl As Double) as String
    Dim axla As String
    If axl > (N10 - N02b / 1000) * modCos(A6) + N07  Then
        axla = "in section 1"
    ElseIf axl > (N10 - N02b / 1000) * modCos(A6) Then
        axla = "in section 2"
    ElseIf axl > (N10 / 1000) * modCos(A6) Then
        axla = "in section 3"
    Else
        axla = "in section 4"
    End If
    return axla 
End Sub

And then call the function with

Dim N10, N02b, A6, N07, al1 As Double

Private Sub btnSolve_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSolve.Click
   Txtala1.Text = axlpos(al1)

End sub

The error is caused by the fact that at the point in which you call the method AxlPos the variable passed to the method (ala1) has not been assigned. The compiler warns you that this could cause a null reference exception in the code.
However it seems that you want to set the passed variable to a different value based in your calculations so the best approach is to create a function the return the calculated string value.

If you still want to use your approach then your code should be changed to

Private Sub btnSolve_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSolve.Click

   ala1 = String.Empty
   axlpos(al1,ala1)
   Txtala1.Text = ala1

End sub

but more important you should change the ByVal in a ByRef in the called module method

Public Sub AxlPos(ByVal axl As Double, ByRef axla As String)

See ByVal vs ByRef clarification

Both solutions are working well but the function option is of course smarter. Many thanks. — Dom, Jun 06 '16 at 17:34

a variable in a module triggers an error when called from a form

1 Answers1