Collection error

[Snowblind47]

Hi,

I am trying to build a class that works with a form, but I keep getting the dreaded "Object reference not set to an instance of an object" Error.

The form basically collects test grades and assigns them to a collection.

My class code is as follows:

Public Class Scores
    Private colScoreCollection As New Collection
    Public mintSum As Integer
    Public mdecAverage As Decimal
    Public mintMinimum As Integer
    Public mintMaximum As Integer
    Public mintNumberOfScores As Integer

    Public Sub New()
        colScoreCollection = Nothing
        mintSum = 0
        mdecAverage = 0
        mintMinimum = 0
        mintMaximum = 0
        mintNumberOfScores = 0
    End Sub
    Public ReadOnly Property Sum() As Integer
        Get
            Return mintSum
        End Get
    End Property

    Public ReadOnly Property Average() As Decimal
        Get
            Return mdecAverage
        End Get
    End Property

    Public ReadOnly Property Minimum() As Integer
        Get
            Return mintMinimum
        End Get
    End Property

    Public ReadOnly Property Maximum() As Integer
        Get
            Return mintMaximum
        End Get
    End Property

    Public ReadOnly Property NumberOfScores() As Integer
        Get
            Return mintNumberOfScores
        End Get
    End Property

    Public Sub AddScore(ByVal intGrades As Integer)
        colScoreCollection.Add(intGrades)
    End Sub
End Class

My form code is:

Imports TestScores
Public Class Form1
    Inherits System.Windows.Forms.Form

    Private Sub btnAddScore_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAddScore.Click
        Dim intGrade As Integer


        intGrade = Convert.ToInt32(nudScores.Value)

        gobjScores.AddScore(intGrade)

    End Sub
End Class

I had a similar project I was working on that had a collection in it, but now that one is giving me the same error. It there something with my version of Visual Studio that is causing this error possibly?

Thanks for any help.

[ForumAccount]

Look at the first line of code in your constructor. Now look at your error message. What do you see?

[MarMan]

You need to:

gobjScores = New Scores

before you do:

gobjScores.AddScore(intGrade)

Not necessarily in the same place, just before.

[NickThissen]

The error tells you that some object is Nothing. Sometimes it is hard to figure out which object is Nothing, but in this case you should be able to spot which one it is, mainly because you have typed it, literally, yourself.

EDIT
Besides, you should really be using a List(Of Integer) instead of a Collection. It works the same way except for the fact that you can only add Integers and that it returns Integers instead of Objects. It will make life easier for you down the line, while you don't have the change anything (besides the name 'Collection' to 'List(of Integer)') now.

[Snowblind47]

Thanks for the replies! I was under the impression that a constructor had to be created for the collection I needed to use, I guess this is false?

But wouldn't a constructor be necessary to create a new instance of the class using a collection? That is the part I'm a bit cloudy on at the moment.

I realize it would be wiser to use a sorted list or even an array, but I'm actually using an old text book from college (trying to get back into coding again), and this was an assignment I never had to do, and the assignment specifically called for a collection.

Thanks again for the help.

[kleinma]

You have a few problems with your class design.

1) scores itself should BE a collection, not contain an internal collection
2) all your fields are public, and you also expose read only properties for them. If the fields are public, then can be directly accessed and modified outside the class, which means the readonly fields are useless and don't encapsulate the internal values of the class as they should
3) Your constructor is useless because all it does is set numeric variables to 0 which are actually already 0 since numeric value types have a default value of 0. If you don't specify a constructor (sub new) in a class, a parameterless one is automatically generated when compiled.

[MarMan]

I looked at it again and the problem could also be with colScoreCollection. Nowhere in your code do I see you setting them.

You use a constructor when you wish to do more than just create an instance. For example, you can set an internal variable that causes the object to behave differently. Listen to Nick, he knows what he is talking about.

I don't know what you mean by

But wouldn't a constructor be necessary to create a new instance of the class using a collection?

You can create a class. You can create a collection. You can have a constructor for either of them. It basically is like calling a method at the same time.

But before you use ANY object, you have to set them to something.

[NickThissen]

I was under the impression that a constructor had to be created for the collection I needed to use, I guess this is false?

But wouldn't a constructor be necessary to create a new instance of the class using a collection? That is the part I'm a bit cloudy on at the moment.

When you do not specify a 'Public Sub New' method (a constructor), a class has a default constructor which is 'implicit', eg hidden from the code. If you simply create an empty class:

Public Class SomeClass
End Class

then you can still create a new instance by invoking its constructor:

Dim c As New SomeClass()

This will call the default constructor, which does nothing at all except create a new instance.

When you want your constructor to do something, then you create your own. You could either set some initial values, or force the user to pass some object to your class in the constructor like this

Public Class SomeClass
   
   Public Sub New(ByVal i As Integer)
      ' do something with i
   End Sub

End Class

Now you use it as

Dim c As New SomeClass(3)

and the default (parameterless) constructor is now no longer available (so calling New SomeClass() without parameters is not allowed anymore).

I realize it would be wiser to use a sorted list or even an array, but I'm actually using an old text book from college (trying to get back into coding again), and this was an assignment I never had to do, and the assignment specifically called for a collection.

Thanks again for the help.

I just saw you are using VS02/03 in which case i don't think a List(Of T) is available so you can disregard that comment, although really, you should upgrade to at least VS2008 or VS2010. The express versions are free and I doubt you would need any of the features the non-free professional versions offer at this point.

You use a constructor when you wish to do more than just create an instance.

What you should have said is: you use a custom constructor (eg: create your own) if you wish to do more than just create an instance. The default parameterless constructor is always available and you always have to use it if you didn't specify your own, so even without creating your own Public Sub New you are always using a constructor (the default one).
It's just a minor 'correction' but since this is exactly the point the OP is having trouble with I thought I should point it out