InvalidOperationException error was unhandled..

[anac]

Hi all!

Struggling as hell with my first windows form programme. The task was to create a calculator, and the part i'm at is for fuel consumption (a further part is a calorie counter..)

While it seemed more easy for me to create all code in the main class, the task is specifically to create a separate class for each part of the calculator, while using the main form class only for the GUI part. I couldn't really understand how to call a class within another class and how to use the functions / variables from the fuelCalc in the MainForm part. Eventually I have tried to adapt a sample programme that our teacher provided us, but since i can't really understand all the lines, it is perhaps not surprising that I can't figure it out for my own code.
At the moment I get an "InvalidOperationException was unhandled" error. Tried to fix it in different ways but i really can't get around the proper way to call the class /use the object from the fuelCalc class.

The code probably has many flaws, but a good start would be to figure out how to initialize/ call the functions and vars from one class into the Mainform.

Oh and on a side note, I understood that using Public is a bad programming practice, but if I need to use the objects from a class into another one shouldn't everything be public? or does inherit mean that i could still use private?

Thanks a lot for taking your time to help a poor beginner !!

this is the code for fuelcalc class:

Option Strict On
Option Explicit On

Public Class fuelCalc

    Private reading0 As Double ' previous km reading, user input, has to be positive or zero
    Private reading1 As Double 'current km reading, user input, has to be higher than reading0
    Private volume As Double ' amount of fuel, user input
    Private price As Double 'user input
    Private dist As Double = reading1 - reading0
    Private litPerKm As Double = volume / dist


    Public Function ConsLitKm() As Double
        Dim kmPerLit As Double = dist / volume
        Return kmPerLit
    End Function
    Public Function ConsKmLit() As Double
        Dim litPerKm = volume / dist
        Return litPerKm
    End Function

    Public Function ConsLitMile() As Double

        Const kmToMile As Double = 0.621
        Dim litPerMile As Double = litPerKm / kmToMile
        Return litPerMile
    End Function

    Public Function ConsLitSMile() As Double
        Dim litPerSMil As Double = litPerKm * 10
        Return litPerSMil
    End Function

    Public Function Tcost() As Double
        Dim cost As Double = litPerKm * price
        Return cost
    End Function


End Class

and the code for the main form:

Option Strict On
Option Explicit On
''' This class is solely responsible for all user interactions.
''' Everything that has to to with GUI is placed here.
''' BUT it uses other objects to do other jobs. In this example
''' project, it uses an object of the class fuelCalc
Partial Public Class MainForm
    Inherits Form
    'instance variable 
    ' variables are read from the textboxes 

    Private reading0 As Double = Val(TxtReading0.Text)
    Private reading1 As Double = Val(txtReading1.Text)
    Private volume As Double = Val(TxtVol.Text)
    Private price As Double = Val(TxtPrice.Text)
    Private dist As Double = reading1 - reading0
    Private carMilage As fuelCalc


    Public Sub New()

        ' This call is required by the designer. 'VSs initialization
        InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.
        'My initializations
        InitializeGUI()

    End Sub

    Private Sub InitializeGUI()
        'initialization
        carMilage = New fuelCalc()

    End Sub
    Private Function valid() As Boolean ' tests whether user input is a valid number and whether 
        Dim ok As Boolean
        If (dist > 0 And reading0 >= 0 And Double.TryParse(TxtReading0.Text, reading0) And Double.TryParse(txtReading1.Text, reading1) And Double.TryParse(TxtVol.Text, volume) And Double.TryParse(TxtPrice.Text, price)) Then
            carMilage.ConsKmLit()
            carMilage.ConsLitKm()
            carMilage.ConsLitMile()
            carMilage.ConsLitSMile()
            carMilage.Tcost()
        Else : MsgBox("Please write a valid number")
        End If
        Return ok
    End Function

    Private Sub updateGUI() 'prints the output to the read only textboxes
        txtConsKmLit.Text = carMilage.ConsKmLit().ToString()
        txtConsLitKm.Text = carMilage.ConsLitKm().ToString()
        txtConsLitMile.Text = carMilage.ConsLitMile().ToString()
        txtConsLitSMile.Text = carMilage.ConsLitSMile().ToString()
        txtCost.Text = carMilage.Tcost().ToString()

    End Sub


    Private Sub btnFuel_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnFuel.Click
        
        Dim ok As Boolean = valid() 'if user input is a valid number, then display output when clicked on calculate
        If ok Then
            updateGUI()
        Else : MessageBox.Show("invalid input")
        End If

    End Sub

End Class

[coataktia92]

So mutch Textboxes...
at first please make more Notes what u want to do oO....
second one, upload ur whole Project, ill test it tomorrow. I tryed to reconstruct it, but its not as easy as u think to put all those different Textboxes in a Form and do a normal calculation....

[Evil_Giraffe]

Okay, lots of things to address here, I'll start with this one:

Oh and on a side note, I understood that using Public is a bad programming practice, but if I need to use the objects from a class into another one shouldn't everything be public? or does inherit mean that i could still use private?

I assume that you're referring to the practice of making the fields of a class Public. As you've figured, Public means that a member of a class is visible from outside the class. It would be a strange class if nothing was public (although not totally unheard of, but you're not going to hit those scenarios for a while).

The "bad programming practice" part comes in when you make the internals of the class visible outside it, clearly you need to make the interface parts Public. This is the principle of information hiding and lets you change how the class works internally without affecting all the consumers of that class (and is one of the key ways that OO methods manage complexity).

So, in the fuelCalc class, you have several fields marked as Private that hold the instance's state. That's good, those should be Private. You've also got some methods marked as Public that are designed to be used by the consumer classes to retrieve the calculated values. That's good, those should be Public.

I think you've just got a bit hung up on having anything Public at all, which is not what the "avoid making things Public" thing is about. What you're doing with regards to that is fine

[anac]

Yes, i know it's a lot of them but this is the task:/...I added more notes and attached the entire project file, but hopefully things will get more clear when you¨'ll see the form.


Thanks a lot again, highly appreciated!!