Overloaded New in MustInherit Class ...

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  • Joe HM
    #1

    Overloaded New in MustInherit Class ...

    Hello -

    I have a question regarding overloading the New() in a MustInherit
    Class. The following show the code in question ...

    MustInherit Class cAbstract
    Public MustOverride Sub print()

    Protected mName As String

    Public Sub New() ' Why do I have to override this?
    mName = "N/A"
    End Sub

    Public Sub New(ByVal aName As String)
    mName = aName
    End Sub
    End Class

    Class cChildOfAbstrac t
    Inherits cAbstract

    Public Overrides Sub print()
    Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
    mName & "'")
    End Sub
    End Class

    ....
    Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name") ' Why is
    this not working?
    ....

    First of all, I have to override the New(). Is this because I
    overloaded it with the New(ByVal aName As String)?

    Then I'm trying to call the overloaded constructor when I instantiate
    the cChildOfAbstrac t but I cannot do that because apparently I have
    too many arguments. This works if I override the New(ByVal aName As
    String) in the child class but why do I have to do that? I thought I
    would inherit that from the base class.

    Thanks!
    Joe
    Tags: None
    • James Hahn
      #2
      Re: Overloaded New in MustInherit Class ...

      What do you mean by 'have to override the New()' - what is the message you
      are getting? Constructors aren't ordinary methods and are inherited
      differently.

      If the base class has no constructors or has a a paramaterless constructor
      then you don't need a constructor. But if the base class does not have a
      paramaterless constructor then your derived class must have a constructor
      and the very first line of the constructor must be a call to the base class
      constructor. Perhaps it was this requirement (not met in your sample code)
      that made it appear that you were required to have a New(). It all depends
      on what the base class's constructors are.

      "Joe HM" <unixverse@yaho o.comwrote in message
      news:e0acbe71-34a0-4d80-bd2c-b7af41208783@r6 6g2000hsg.googl egroups.com...
      Hello -
      >
      I have a question regarding overloading the New() in a MustInherit
      Class. The following show the code in question ...
      >
      MustInherit Class cAbstract
      Public MustOverride Sub print()
      >
      Protected mName As String
      >
      Public Sub New() ' Why do I have to override this?
      mName = "N/A"
      End Sub
      >
      Public Sub New(ByVal aName As String)
      mName = aName
      End Sub
      End Class
      >
      Class cChildOfAbstrac t
      Inherits cAbstract
      >
      Public Overrides Sub print()
      Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
      mName & "'")
      End Sub
      End Class
      >
      ...
      Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name") ' Why is
      this not working?
      ...
      >
      First of all, I have to override the New(). Is this because I
      overloaded it with the New(ByVal aName As String)?
      >
      Then I'm trying to call the overloaded constructor when I instantiate
      the cChildOfAbstrac t but I cannot do that because apparently I have
      too many arguments. This works if I override the New(ByVal aName As
      String) in the child class but why do I have to do that? I thought I
      would inherit that from the base class.
      >
      Thanks!
      Joe

        Comment

        • Family Tree Mike
          #3
          Re: Overloaded New in MustInherit Class ...

          To the best of my knowledge, constructors are not inheritted in VB.Net. Add
          this constructor to your class cChildOfAbstrac t to accomplish what I think
          you want:

          Public Sub New(ByVal s As String)
          MyBase.New(s)
          End Sub

          I don't think you need the default constructor in your base class, at least
          in what I was running.

          "Joe HM" <unixverse@yaho o.comwrote in message
          news:e0acbe71-34a0-4d80-bd2c-b7af41208783@r6 6g2000hsg.googl egroups.com...
          Hello -
          >
          I have a question regarding overloading the New() in a MustInherit
          Class. The following show the code in question ...
          >
          MustInherit Class cAbstract
          Public MustOverride Sub print()
          >
          Protected mName As String
          >
          Public Sub New() ' Why do I have to override this?
          mName = "N/A"
          End Sub
          >
          Public Sub New(ByVal aName As String)
          mName = aName
          End Sub
          End Class
          >
          Class cChildOfAbstrac t
          Inherits cAbstract
          >
          Public Overrides Sub print()
          Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
          mName & "'")
          End Sub
          End Class
          >
          ...
          Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name") ' Why is
          this not working?
          ...
          >
          First of all, I have to override the New(). Is this because I
          overloaded it with the New(ByVal aName As String)?
          >
          Then I'm trying to call the overloaded constructor when I instantiate
          the cChildOfAbstrac t but I cannot do that because apparently I have
          too many arguments. This works if I override the New(ByVal aName As
          String) in the child class but why do I have to do that? I thought I
          would inherit that from the base class.
          >
          Thanks!
          Joe

            Comment

            • Chris Dunaway
              #4
              Re: Overloaded New in MustInherit Class ...

              On Oct 27, 9:30 am, Joe HM <unixve...@yaho o.comwrote:
              Hello -
              >
              I have a question regarding overloading the New() in a MustInherit
              Class. The following show the code in question ...
              >
              MustInherit Class cAbstract
              Public MustOverride Sub print()
              >
              Protected mName As String
              >
              Public Sub New() ' Why do I have to override this?
              mName = "N/A"
              End Sub
              >
              Public Sub New(ByVal aName As String)
              mName = aName
              End Sub
              End Class
              >
              Class cChildOfAbstrac t
              Inherits cAbstract
              >
              Public Overrides Sub print()
              Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
              mName & "'")
              End Sub
              End Class
              >
              ...
              Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name") ' Why is
              this not working?
              ...
              >
              First of all, I have to override the New(). Is this because I
              overloaded it with the New(ByVal aName As String)?
              >
              Then I'm trying to call the overloaded constructor when I instantiate
              the cChildOfAbstrac t but I cannot do that because apparently I have
              too many arguments. This works if I override the New(ByVal aName As
              String) in the child class but why do I have to do that? I thought I
              would inherit that from the base class.
              >
              Thanks!
              Joe
              Constructors are not inherited. You will have to provide the child
              constructors that you need.

              Chris

                Comment

                • Joe HM
                  #5
                  Re: Overloaded New in MustInherit Class ...

                  Hello -

                  The following will cause a "Class 'cChild' must declare a 'Sub New'
                  because its base class 'cParent' does not have an accessible 'Sub New'
                  that can be called with no arguments."

                  MustInherit Class cParent
                  Public Sub New(ByVal aArgument As String)
                  MyBase.New()
                  End Sub
                  End Class

                  Class cChild
                  Inherits cParent
                  End Class

                  I thought that by default every class has a parameterless constructor
                  but I guess I was mistaken. Looks like I have to spell it out.

                  Thanks,
                  Joe



                  On Oct 27, 6:05 pm, "James Hahn" <jh...@yahoo.co mwrote:
                  What do you mean by 'have to override the New()' - what is the message you
                  are getting?  Constructors aren't ordinary methods and are inherited
                  differently.
                  >
                  If the base class has no constructors or has a a paramaterless constructor
                  then you don't need a constructor. But if the base class does not have a
                  paramaterless constructor then your derived class must have a constructor
                  and the very first line of the constructor must be a call to the base class
                  constructor. Perhaps it was this requirement (not met in your sample code)
                  that made it appear that you were required to have a New(). It all depends
                  on what the base class's constructors are.
                  >
                  "Joe HM" <unixve...@yaho o.comwrote in message
                  >
                  news:e0acbe71-34a0-4d80-bd2c-b7af41208783@r6 6g2000hsg.googl egroups.com...
                  >
                  >
                  >
                  Hello -
                  >
                  I have a question regarding overloading the New() in a MustInherit
                  Class.  The following show the code in question ...
                  >
                  MustInherit Class cAbstract
                     Public MustOverride Sub print()
                  >
                     Protected mName As String
                  >
                     Public Sub New()  ' Why do I have to override this?
                         mName = "N/A"
                     End Sub
                  >
                     Public Sub New(ByVal aName As String)
                         mName = aName
                     End Sub
                  End Class
                  >
                  Class cChildOfAbstrac t
                     Inherits cAbstract
                  >
                     Public Overrides Sub print()
                         Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
                  mName & "'")
                     End Sub
                  End Class
                  >
                  ...
                  Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name")   ' Why is
                  this not working?
                  ...
                  >
                  First of all, I have to override the New().  Is this because I
                  overloaded it with the New(ByVal aName As String)?
                  >
                  Then I'm trying to call the overloaded constructor when I instantiate
                  the cChildOfAbstrac t but I cannot do that because apparently I have
                  too many arguments.  This works if I override the New(ByVal aName As
                  String) in the child class but why do I have to do that?  I thought I
                  would inherit that from the base class.
                  >
                  Thanks!
                  Joe- Hide quoted text -
                  >
                  - Show quoted text -

                    Comment

                    • Joe HM
                      #6
                      Re: Overloaded New in MustInherit Class ...

                      Hello -

                      I guess I just realized that I don't need an interface to the
                      overloaded New() with the parameter in the base class. The following
                      works ...

                      MustInherit Class cParent
                      ....
                      End Class

                      Class cChild
                      Inherits cParent

                      Public Sub New(ByVal aArgument As String)
                      MyBase.New()
                      ...
                      End Sub
                      End Class

                      ....

                      Dim lDummy As cParent
                      lDummy = New cChild("")

                      Thanks,
                      Joe



                      On Oct 27, 6:59 pm, "Family Tree Mike"
                      <FamilyTreeM... @ThisOldHouse.c omwrote:
                      To the best of my knowledge, constructors are not inheritted in VB.Net.  Add
                      this constructor to your class cChildOfAbstrac t to accomplish what I think
                      you want:
                      >
                       Public Sub New(ByVal s As String)
                        MyBase.New(s)
                       End Sub
                      >
                      I don't think you need the default constructor in your base class, at least
                      in what I was running.
                      >
                      "Joe HM" <unixve...@yaho o.comwrote in message
                      >
                      news:e0acbe71-34a0-4d80-bd2c-b7af41208783@r6 6g2000hsg.googl egroups.com...
                      >
                      >
                      >
                      Hello -
                      >
                      I have a question regarding overloading the New() in a MustInherit
                      Class.  The following show the code in question ...
                      >
                      MustInherit Class cAbstract
                         Public MustOverride Sub print()
                      >
                         Protected mName As String
                      >
                         Public Sub New()  ' Why do I have to override this?
                             mName = "N/A"
                         End Sub
                      >
                         Public Sub New(ByVal aName As String)
                             mName = aName
                         End Sub
                      End Class
                      >
                      Class cChildOfAbstrac t
                         Inherits cAbstract
                      >
                         Public Overrides Sub print()
                             Console.WriteLi ne("cChildOfAbs tract.print() mName = '" &
                      mName & "'")
                         End Sub
                      End Class
                      >
                      ...
                      Dim lInstanceOfChil d As New cChildOfAbstrac t("My Name")   ' Why is
                      this not working?
                      ...
                      >
                      First of all, I have to override the New().  Is this because I
                      overloaded it with the New(ByVal aName As String)?
                      >
                      Then I'm trying to call the overloaded constructor when I instantiate
                      the cChildOfAbstrac t but I cannot do that because apparently I have
                      too many arguments.  This works if I override the New(ByVal aName As
                      String) in the child class but why do I have to do that?  I thought I
                      would inherit that from the base class.
                      >
                      Thanks!
                      Joe- Hide quoted text -
                      >
                      - Show quoted text -

                        Comment

                        • Tom Shelton
                          #7
                          Re: Overloaded New in MustInherit Class ...

                          On 2008-10-28, Joe HM <unixverse@yaho o.comwrote:
                          Hello -
                          >
                          The following will cause a "Class 'cChild' must declare a 'Sub New'
                          because its base class 'cParent' does not have an accessible 'Sub New'
                          that can be called with no arguments."
                          >
                          MustInherit Class cParent
                          Public Sub New(ByVal aArgument As String)
                          MyBase.New()
                          End Sub
                          End Class
                          >
                          Class cChild
                          Inherits cParent
                          End Class
                          >
                          I thought that by default every class has a parameterless constructor
                          but I guess I was mistaken. Looks like I have to spell it out.
                          >
                          Thanks,
                          Joe
                          >
                          The compiler will only generate a default constructor if you have not
                          explicitly added one to the class. As soon as you add the constructor:

                          Public Sub New (ByVal aArgument As String)

                          The compiler assumes you have explicitly stated your desires, and no longer
                          generates a default constructor. This is how it works in pretty much all
                          statically compiled languages... And it makes sense, because a lot of times a
                          default no argument constructor doesn't make sense for a class.


                          --
                          Tom Shelton

                            Comment

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