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Thread: [RESOLVED] Properties, Enumeration, & Constructors

  1. #1

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    Resolved [RESOLVED] Properties, Enumeration, & Constructors

    Im doing step by step..My instructions are:


    The GroceryItem class should contain the following public properties:

    1. ScanNumber - Represents the unique serial code for the item on the shelf. This property should be read-only.

    2. BrandName - The name as described on the item’s packaging

    3. Description - A short description of the item

    4. Price - The amount of money it costs to buy the item. Make sure that only positive values can be assigned to this property.

    5. Aisle - This should indicate one of the following aisles: Bakery, CannedGoods, Drinks, Deli, DryGoods, FrozenFoods, and Produce.

    This is my Grocery Item code:

    Code:
    Public Class GroceryItem
       Public ReadOnly Property ScanNumber As Integer
    Get
      Return ScanNumber
    End Get
        End Property
    
        Public Property BrandName As String
     Get
       Return BrandName
     End Get
     Set (value As String)
       BrandName = value
     End Set
    End Property
    
        Public Property Description As String
    Get
    Return Description
    End Get
    Set (value As String)
     Description = value
    End Set
    End Property
    
        Public Property Price As Integer
    Get
    Return Price
    End Get
    Set (value As Integer)
    Price = value
    End Set
    End Property
    
        Public Property Aisle As String
    Get
    Return Aisle As String
    End Get
    Set (value As String)
    Aisle = value
    End Set
    End Property
    End Class

    It says you can use enumeration.

    The GroceryItem class should contain the following public constructors:

    1. A constructor that accepts and sets only the ScanNumber property. Remember: If a property is read-only, then you’ll need to set the variable, not use the property name.

    2. A constructor that accepts and sets the ScanNumber, BrandName and Price properties.

    Add a new public class named GroceryBasket to the project.

    1. Derive from the generic List class.
    2. Ensure that only GroceryItem objects are stored as items.

    Add the following variable declaration to the Main module: Friend basket As New GroceryBasket

    My Main Code:

    Code:
    Module Main
        Friend blnLoggedIn As Boolean
        Dim arrUsernames() As String = {"Admin", "Clerk", "Manager"}
        Dim arrPasswords() As String = {"P@ssword", "pa$$word", "passw0rd"}
        Dim userIndex As Integer
        Friend basket As New GroceryBasket
    Test your work.

    Note: You can add code to the btnLogin_Click event handler procedure to instantiate the GroceryItem class and add multiple objects to the GroceryBasket variable. Make sure you either remove or comment out this code after testing.
    Last edited by EmilyM1105; Jan 27th, 2018 at 04:36 AM.

  2. #2
    Super Moderator Shaggy Hiker's Avatar
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    Re: Properties, Enumeration, & Constructors

    You've got a spurious End Class in the middle of your GroceryItem class. That will certainly mess things up.

    Also, while you may be able to get away with assuming the default for a property is Public, it isn't a sound practice, so I agree with Sitten in the other thread that you should explicitly state Public, even if it is the default.

    Most likely, the point was not to have you use the default implementation of the properties, anyways, but to spell them out. Therefore, you need variables for each of the properties. Sitten showed that using underscores, but I personally hate underscores because of where they are located on the keyboard (two rows up on the weakest finger). Therefore, I would write one of the properties like this:
    Code:
    Private mBrandName as String
    
    Public Property BrandName As String
     Get
       Return mBrandName
     End Get
     Set (value As String)
       mBrandName = value
     End Set
    End Property
    Much of that gets filled out as soon as you type the Get.

    As for constructors, that's just Public Sub New(), with whatever arguments you need. Looks like you need two of them: One that takes one argument and one that takes three.
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  3. #3

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by Shaggy Hiker View Post
    You've got a spurious End Class in the middle of your GroceryItem class. That will certainly mess things up.

    Also, while you may be able to get away with assuming the default for a property is Public, it isn't a sound practice, so I agree with Sitten in the other thread that you should explicitly state Public, even if it is the default.

    Most likely, the point was not to have you use the default implementation of the properties, anyways, but to spell them out. Therefore, you need variables for each of the properties. Sitten showed that using underscores, but I personally hate underscores because of where they are located on the keyboard (two rows up on the weakest finger). Therefore, I would write one of the properties like this:
    Code:
    Private mBrandName as String
    
    Public Property BrandName As String
     Get
       Return mBrandName
     End Get
     Set (value As String)
       mBrandName = value
     End Set
    End Property
    Much of that gets filled out as soon as you type the Get.

    As for constructors, that's just Public Sub New(), with whatever arguments you need. Looks like you need two of them: One that takes one argument and one that takes three.
    Where did mBrandName come from??

  4. #4
    Super Moderator Shaggy Hiker's Avatar
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    Re: Properties, Enumeration, & Constructors

    My own head.

    Prior to 2010, that was the only way you could write a property. You had to first declare a variable, then expose that variable via the property, so you had to write it like that. You wrote the variable, then wrote the getter and setter. There were no shortcuts.

    Starting with 2010, you could make a shortcut if you wanted a full property (as opposed to ReadOnly or WriteOnly). What you wrote was essentially the shortcut, except that you HAD to have Public/Private/Protected, or Friend as an access qualifier. You STILL had to write out ReadOnly properties, though.

    Starting sometime later, perhaps 2013 or 2015, VS was changed such that you didn't have to write out ReadOnly properties, either, which is what you are showing with your properties. There is still a variable hidden behind those properties, you just don't see it anymore because there isn't an explicit Get or Set block. You can't always use that simpler version of properties, though, so you can still write it out the old way, as I showed, because you HAVE to do that if you are doing more with either the Get or Set block.

    For example, I might have a class with a Count property. There isn't some variable back there holding the count, the Get block might look like this:
    Code:
    Get
      Return someCollection.Count
    End Get
    The property isn't just returning some value it is holding, it will check how many items are in someCollection, and return that number whenever the property is accessed. Also, there wouldn't be any point to a Set block in such a property, because what would that even mean?

    Another example is a class I have where you can set the current time. In the setter, I check to see whether the time being set is the current time, or some other time, and raise various events in either case. In other words, when the user sets the time, the class sends out alerts to other parts of the program to let them know that they need to do something. The default property wouldn't know to do that, so I had to go old-school and write my own Set method.
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by Shaggy Hiker View Post
    My own head.

    Prior to 2010, that was the only way you could write a property. You had to first declare a variable, then expose that variable via the property, so you had to write it like that. You wrote the variable, then wrote the getter and setter. There were no shortcuts.

    Starting with 2010, you could make a shortcut if you wanted a full property (as opposed to ReadOnly or WriteOnly). What you wrote was essentially the shortcut, except that you HAD to have Public/Private/Protected, or Friend as an access qualifier. You STILL had to write out ReadOnly properties, though.

    Starting sometime later, perhaps 2013 or 2015, VS was changed such that you didn't have to write out ReadOnly properties, either, which is what you are showing with your properties. There is still a variable hidden behind those properties, you just don't see it anymore because there isn't an explicit Get or Set block. You can't always use that simpler version of properties, though, so you can still write it out the old way, as I showed, because you HAVE to do that if you are doing more with either the Get or Set block.

    For example, I might have a class with a Count property. There isn't some variable back there holding the count, the Get block might look like this:
    Code:
    Get
      Return someCollection.Count
    End Get
    The property isn't just returning some value it is holding, it will check how many items are in someCollection, and return that number whenever the property is accessed. Also, there wouldn't be any point to a Set block in such a property, because what would that even mean?

    Another example is a class I have where you can set the current time. In the setter, I check to see whether the time being set is the current time, or some other time, and raise various events in either case. In other words, when the user sets the time, the class sends out alerts to other parts of the program to let them know that they need to do something. The default property wouldn't know to do that, so I had to go old-school and write my own Set method.
    Ok I changed it up top..Is that right so far?

  6. #6
    Super Moderator Shaggy Hiker's Avatar
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    Re: Properties, Enumeration, & Constructors

    Yes, that'll do. It isn't clear whether you HAVE to write out the properties in the old-school style, but it does seem like the professor is likely looking for that....or not, I'm not sure. This may just be my bias. I almost always write out properties, and have only started using the shorthand way that you had initially in some newer stuff. I always write out ReadOnly properties in all cases. It doesn't hurt anything...unless you are getting graded on it. You'd have to decide whether the professor wants to see the long version of the properties, or doesn't care which you use.
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by Shaggy Hiker View Post
    Yes, that'll do. It isn't clear whether you HAVE to write out the properties in the old-school style, but it does seem like the professor is likely looking for that....or not, I'm not sure. This may just be my bias. I almost always write out properties, and have only started using the shorthand way that you had initially in some newer stuff. I always write out ReadOnly properties in all cases. It doesn't hurt anything...unless you are getting graded on it. You'd have to decide whether the professor wants to see the long version of the properties, or doesn't care which you use.
    Thats why I asked where that came from because the instructions I posted up there are the EXACT instructions from my instructor..It doesnt say anything about doing that..Also Im using VB2017..So maybe I should stick to what hes asking...If I do how should I write it then?

  8. #8
    Super Moderator Shaggy Hiker's Avatar
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    Re: Properties, Enumeration, & Constructors

    The way you had it originally, except that I do think that adding the Public access qualifier at the beginning is a good idea. I still use VS2010 for lots of stuff, so I often forget that you don't have to write out ReadOnly properties anymore, but in 2017 you do not.
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by Shaggy Hiker View Post
    The way you had it originally, except that I do think that adding the Public access qualifier at the beginning is a good idea. I still use VS2010 for lots of stuff, so I often forget that you don't have to write out ReadOnly properties anymore, but in 2017 you do not.
    Ok so I updated my code up top..Is it right? According to the directions? Thanks!

  10. #10
    Super Moderator Shaggy Hiker's Avatar
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    Re: Properties, Enumeration, & Constructors

    I thought you were not going to write out the properties?
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by EmilyM1105 View Post
    Ok so I updated my code up top
    I would suggest that you don't do that. It will confuse people who come in part way through the thread and it means that noone can see what you've actually changed along the way.
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by Shaggy Hiker View Post
    I thought you were not going to write out the properties?
    So I dont have to write Get and Set statements?

  13. #13
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    Re: Properties, Enumeration, & Constructors

    It's entirely up to you, I just thought you felt that the professor would not want you to. There's no advantage to one over the other as far as performance goes. As long as you know that you can do either, that's probably all that matters.
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    Re: Properties, Enumeration, & Constructors

    Nevermind im sorry you were right about:
    Code:
    Private mBrandName as String
    
    Public Property BrandName As String
     Get
       Return mBrandName
     End Get
     Set (value As String)
       mBrandName = value
     End Set
    End Property

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by EmilyM1105 View Post
    Nevermind im sorry you were right about:
    Code:
    Private mBrandName as String
    
    Public Property BrandName As String
     Get
       Return mBrandName
     End Get
     Set (value As String)
       mBrandName = value
     End Set
    End Property
    Does your post above mean that the long way of defining a Property is what your instructor is expecting? If so, then your code at the very top in your first post looks somewhat ok, you just need to create a variable for the rest of the Properties, like you show above with mBrandName. Probably a good idea to just name these variables with the same format "mPropertyName", like mScanNumber, mDescription, etc, making sure the variable is of the same type as the Property (Integer variable for Integer Property, for example). And don't forget that in the other Properties, your Set statement need to be setting this "mPropertyName" variable = value.

    So lets look at your Price property. I would suggest changing the type from Integer to Double, since prices can be in dollars and cents, not just whole dollars.

    Code:
    Private mPrice As Double
    
    Public Property Price As Double
      Get
        Return mPrice
      End Get
      Set (value As Double)
        mPrice = value
      End Set
    End Property
    The instructions say to ensure that only positive values can be assigned to the price. When your Property has limitations on what values can be used, the way to test for this is with appropriate logic inside of the Properties Set statement. I'll show you a similar example. Imagine that we had created a Class that stores information about people. And one of the Properties of this Class is the person's Age. You would agree that a person's age can't be negative, and there is also some upper limit to where above that number a person's age doesn't make sense.

    Example: Replace an invalid age with the closest valid value:

    Code:
    Private mAge As Integer
    Private minAge As Integer = 0
    Private maxAge As Integer = 140
    
    Public Property Age As Integer
      Get
        Return mAge
      End Get
      Set (value As Integer)
        If value < minAge Then
          mAge = minAge
        ElseIf value > maxAge Then
          mAge = maxAge
        Else
          mAge = value
        End If
      End Set
    End Property
    There are other ways of handling an invalid property value being passed, like displaying a Messagebox to the user:

    Code:
      Set (value As Integer)
        If value < minAge OrElse value > maxAge Then
          MessageBox.Show("Invalid Age", "Error")
        Else
          mAge = value
        End If
      End Set
    or Throwing a custom exception:

    Code:
      Set (value As Integer)
        If value < minAge OrElse value > maxAge Then
          Throw New AgeException("Invalid Age")
        Else
          mAge = value
        End If
      End Set
    Notice that in those last two examples, in the case of an invalid age we're not setting mAge to anything. That should demonstrate to you how to add logic to a Set statement to ensure that the value passed to the property is valid, and some different ways of handling it if it isn't valid.

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by OptionBase1 View Post
    Does your post above mean that the long way of defining a Property is what your instructor is expecting? If so, then your code at the very top in your first post looks somewhat ok, you just need to create a variable for the rest of the Properties, like you show above with mBrandName. Probably a good idea to just name these variables with the same format "mPropertyName", like mScanNumber, mDescription, etc, making sure the variable is of the same type as the Property (Integer variable for Integer Property, for example). And don't forget that in the other Properties, your Set statement need to be setting this "mPropertyName" variable = value.

    So lets look at your Price property. I would suggest changing the type from Integer to Double, since prices can be in dollars and cents, not just whole dollars.

    Code:
    Private mPrice As Double
    
    Public Property Price As Double
      Get
        Return mPrice
      End Get
      Set (value As Double)
        mPrice = value
      End Set
    End Property
    The instructions say to ensure that only positive values can be assigned to the price. When your Property has limitations on what values can be used, the way to test for this is with appropriate logic inside of the Properties Set statement. I'll show you a similar example. Imagine that we had created a Class that stores information about people. And one of the Properties of this Class is the person's Age. You would agree that a person's age can't be negative, and there is also some upper limit to where above that number a person's age doesn't make sense.

    Example: Replace an invalid age with the closest valid value:

    Code:
    Private mAge As Integer
    Private minAge As Integer = 0
    Private maxAge As Integer = 140
    
    Public Property Age As Integer
      Get
        Return mAge
      End Get
      Set (value As Integer)
        If value < minAge Then
          mAge = minAge
        ElseIf value > maxAge Then
          mAge = maxAge
        Else
          mAge = value
        End If
      End Set
    End Property
    There are other ways of handling an invalid property value being passed, like displaying a Messagebox to the user:

    Code:
      Set (value As Integer)
        If value < minAge OrElse value > maxAge Then
          MessageBox.Show("Invalid Age", "Error")
        Else
          mAge = value
        End If
      End Set
    or Throwing a custom exception:

    Code:
      Set (value As Integer)
        If value < minAge OrElse value > maxAge Then
          Throw New AgeException("Invalid Age")
        Else
          mAge = value
        End If
      End Set
    Notice that in those last two examples, in the case of an invalid age we're not setting mAge to anything. That should demonstrate to you how to add logic to a Set statement to ensure that the value passed to the property is valid, and some different ways of handling it if it isn't valid.
    ok so I have made some changes.. How does this look so far?

    This is my Grocery Item code:

    Code:
    Public Class GroceryItem
        Public ReadOnly Property ScanNumber As Integer
            Get
                Return ScanNumber
            End Get
        End Property
    
        Private mBrandName As String
        Public Property BrandName As String
            Get
                Return mBrandName
            End Get
            Set(value As String)
                mBrandName = value
            End Set
        End Property
    
        Private mDescription As String
        Public Property Description As String
            Get
                Return mDescription
            End Get
            Set(value As String)
                mDescription = value
            End Set
        End Property
    
        Private mPrice As Double
        Public Property Price As Double
            Get
                Return mPrice
            End Get
            Set(value As Double)
                If value > 0 Then
                    mPrice = value
                End If
            End Set
        End Property
    
        Private mAisle As String
        Public Property Aisle As String
            Get
                Return mAisle
            End Get
            Set(value As String)
                mAisle = value
            End Set
        End Property
    End Class
    So for price It doesnt tell me anything about a price limit in the instructions?? so how would I know what to use on the part where I say what happens if the price reaches a limit??
    Last edited by EmilyM1105; Jan 31st, 2018 at 04:03 PM.

  17. #17
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    Re: Properties, Enumeration, & Constructors

    The only issue I see is in the ScanNumber property. Although it is read-only, you still want a Private variable to hold what that value is, and then return the contents of that variable.


    Code:
        Private mScanNumber As Integer
        Public ReadOnly Property ScanNumber As Integer
            Get
                Return mScanNumber
            End Get
        End Property
    The rest looks good!

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by OptionBase1 View Post
    The only issue I see is in the ScanNumber property. Although it is read-only, you still want a Private variable to hold what that value is, and then return the contents of that variable.


    Code:
        Private mScanNumber As Integer
        Public ReadOnly Property ScanNumber As Integer
            Get
                Return mScanNumber
            End Get
        End Property
    The rest looks good!
    ok So heres the update lol:

    Code:
     Public Class GroceryItem
        Private mScanNumber As Integer
        Public ReadOnly Property ScanNumber As Integer
            Get
                Return mScanNumber
            End Get
        End Property
    
        Private mBrandName As String
        Public Property BrandName As String
            Get
                Return mBrandName
            End Get
            Set(value As String)
                mBrandName = value
            End Set
        End Property
    
        Private mDescription As String
        Public Property Description As String
            Get
                Return mDescription
            End Get
            Set(value As String)
                mDescription = value
            End Set
        End Property
    
        Private mPrice As Double
        Public Property Price As Double
            Get
                Return mPrice
            End Get
            Set(value As Double)
                If value > 0 Then
                    mPrice = value
                End If
            End Set
        End Property
    
        Private mAisle As String
        Public Property Aisle As String
            Get
                Return mAisle
            End Get
            Set(value As String)
                mAisle = value
            End Set
        End Property
    End Class
    So what number on the instructions up top do you think im on now??...I was confused on this part??

    Aisle - This should indicate one of the following aisles: Bakery, CannedGoods, Drinks, Deli, DryGoods, FrozenFoods, and Produce.
    It says you can use enumeration?
    Last edited by EmilyM1105; Jan 31st, 2018 at 04:35 PM.

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    Re: Properties, Enumeration, & Constructors

    There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

    Code:
    myGroceryItem.Aisle = "Bakery"
    Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

    Code:
    myGroceryItem.Aisle = "AisleThatDoesNotExist"
    but the instructions you posted didn't specify that you need to worry about that.

    So I would say you are up to the point where you need to set up the 2 Constructors.

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by OptionBase1 View Post
    There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

    Code:
    myGroceryItem.Aisle = "Bakery"
    Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

    Code:
    myGroceryItem.Aisle = "AisleThatDoesNotExist"
    but the instructions you posted didn't specify that you need to worry about that.

    So I would say you are up to the point where you need to set up the 2 Constructors.
    So would I have to do this for each Aisle?

    Code:
    myGroceryItem.Aisle = "Bakery"
    Also, I thought that the constructors is what was in my GroceryItem code with Get & Set statements?? So constructors are something different?

  21. #21
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by EmilyM1105 View Post
    Also, I thought that the constructors is what was in my GroceryItem code with Get & Set statements?? So constructors are something different?
    The code with the Get an Set are properties.

    A constructor in VB is a method named New. When you use the New keyword in code to create an instance of a type, you are invoking a constructor. Every type has a parameterless constructor by default, which would be the equivalent of this:
    vb.net Code:
    1. Public Sub New()
    2. End Sub
    If you add a constructor yourself then that default constructor is removed, so if you add a constructor with parameters and you want a parameterless constructor too then you must add both.
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by OptionBase1 View Post
    There are several ways that you could set up the Aisle property. There are pros and cons to all of them, some involve more lines of code, like using an enumeration, and more complex code. The way you've done it by defining Aisle as a String is probably the easiest way to do it. You would then assign an aisle to it like this:

    Code:
    myGroceryItem.Aisle = "Bakery"
    Note, however, that the way the Aisle property is configured, there's nothing stopping you from assigning a product to an Aisle not in the list, like this:

    Code:
    myGroceryItem.Aisle = "AisleThatDoesNotExist"
    but the instructions you posted didn't specify that you need to worry about that.

    So I would say you are up to the point where you need to set up the 2 Constructors.
    Do I write those constructors under my properties??

    Like this?

    Code:
    Public Sub New(mScanNumber As Integer, Brandname As String, Price As Double)
    Me.mScanNumber = Number
    Me.Brandname = Brandname
    Me.Price = Number
    End Sub
    Did I do that right?

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    Re: Properties, Enumeration, & Constructors

    That's the general idea but your implementation isn't really correct there. For one thing, you're assigning Number twice but you have no Number parameter. If you have properties named 'ScanNumber', 'BrandName' and 'Price' then your constructor parameters should be 'scanNumber', 'brandName' and 'price':
    vb.net Code:
    1. Public Sub New(scanNumber As Integer, brandName As String, price As Double)
    2.     Me.ScanNumber = scanNumber
    3.     Me.BrandName = brandName
    4.     Me.Price = price
    5. End Sub
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  24. #24

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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by jmcilhinney View Post
    That's the general idea but your implementation isn't really correct there. For one thing, you're assigning Number twice but you have no Number parameter. If you have properties named 'ScanNumber', 'BrandName' and 'Price' then your constructor parameters should be 'scanNumber', 'brandName' and 'price':
    vb.net Code:
    1. Public Sub New(scanNumber As Integer, brandName As String, price As Double)
    2.     Me.ScanNumber = scanNumber
    3.     Me.BrandName = brandName
    4.     Me.Price = price
    5. End Sub
    ScanNumber is set as read only. So It keeps giving me an error message on the ScanNumber Part saying "it is ReadOnly"


    Code:
    Public Sub New(scanNumber As Integer, brandName As String, price As Double)
        Me.ScanNumber = scanNumber
        Me.BrandName = brandName
        Me.Price = price
    End Sub
    Thats why I put Me.MScanNumber.. when I do that the error goes away.


    Next is the instructions....

    Add a new public class named GroceryBasket to the project.

    1. Derive from the generic List class.
    2. Ensure that only GroceryItem objects are stored as items.

    Heres my GroceryBasket Code:

    Code:
    Public Class GroceryBasket
        Inherits List(Of GroceryItem)
    End Class
    Is that right?? Sorry I had a hard time understanding this part
    Last edited by EmilyM1105; Feb 2nd, 2018 at 12:23 AM.

  25. #25
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    Re: Properties, Enumeration, & Constructors

    Quote Originally Posted by EmilyM1105 View Post
    Oh ok I thought it had to be different because ScanNumber is read only??
    If the property is ReadOnly then yes, you would have to set the backing field instead. I hadn't examined all previous posts so I didn't realise that that was the case.
    Quote Originally Posted by EmilyM1105 View Post
    Is that all I have to do for the constructor instructions?
    The constructor needs to contain all the code that needs to be executed when an object is created. Do you need to do anything other than set the property values when you create an object? If so, add code to do that. If not, don't.
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