Overrides and mybase ??

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Hello,

I have seen the following routines in a class and don't understand how they
work. For one, why two new() routines? Which is executed? Why not just use
one routine? What does the mybase do in this case? Bit confusing! :) Thanks

'<remarks/>
Public Sub New()
MyBase.New()
Me.Url =
System.Configuration.ConfigurationSettings.AppSettings("ReportServerURL") +
"/ReportService.asmx"
End Sub

'<remarks/>
Public Sub New(ByVal ReportServerURL As String)
MyBase.New()
Me.Url = ReportServerURL + "/ReportService.asmx"
End Sub
 
These constructors have 2 different parameters. Someone might want to create
an instance of this class and pass in the URL for the report server, or not.

If they don't pass it in, they get the report server from the configuration
file.

Otherwise, whatever they passed in is used.

In both cases, the base class's constructor is called, to make sure that the
entire instance gets instantiated properly. The base class constructor may
do some work without which the class isn't usable.
 
TheBee said:
Hello,

I have seen the following routines in a class and don't understand how they
work. For one, why two new() routines? Which is executed? Why not just use
one routine? What does the mybase do in this case? Bit confusing! :) Thanks

'<remarks/>
Public Sub New()
MyBase.New()
Me.Url =
System.Configuration.ConfigurationSettings.AppSettings("ReportServerURL") +
"/ReportService.asmx"
End Sub

'<remarks/>
Public Sub New(ByVal ReportServerURL As String)
MyBase.New()
Me.Url = ReportServerURL + "/ReportService.asmx"
End Sub
Click to expand...

MyBase.New in this case is not strictly necessary, since it is simply
calling the base classes default constructor. The only time that
MyBase.New is required is if the Base class can't be constucted without
parameters - in other words, there is no constructor that doesn't take
arguments. You may optionally use it if you want to pass arguments to
the base class constuctor... Maybe a small example to make this clear:

Option Strict On
Option Explicit On

Imports System

Module Module1

' Base with only a default constructor
Private Class Base
Public Sub New(ByVal param As String)
Console.WriteLine("Base New - {0}", param)
End Sub
End Class

Private Class Descendant
Inherits Base

Public Sub New()
Console.WriteLine("Descendant New")
End Sub

Public Sub New(ByVal param As String)
Console.WriteLine("Descendant New - {0}", param)
End Sub

End Class

Sub Main()
Dim a As New Descendant
Dim b As New Descendant("hi")
End Sub

End Module

Now... If you were to add a constructor to the Base class that took
parameters...

Imports System

Module Module1

Private Class Base
Public Sub New()
Console.WriteLine("Base New")
End Sub

Public Sub New(ByVal param As String)
Console.WriteLine("Base New - {0}", param)
End Sub
End Class

Private Class Descendant
Inherits Base

Public Sub New()
Console.WriteLine("Descendant New")
End Sub

Public Sub New(ByVal param As String)
' for fun, lets pass on param to the base!
MyBase.New(param)
Console.WriteLine("Descendant New - {0}", param)
End Sub

End Class
Sub Main()
Dim a As New Descendant
Dim b As New Descendant("hi")
End Sub

End Module

No, if you remove the Base.New () :

Option Strict On
Option Explicit On

Imports System

Module Module1

Private Class Base

Public Sub New(ByVal param As String)
Console.WriteLine("Base New - {0}", param)
End Sub
End Class

Private Class Descendant
Inherits Base

Public Sub New()
MyBase.New("default")
Console.WriteLine("Descendant New")
End Sub

Public Sub New(ByVal param As String)
MyBase.New(param)
Console.WriteLine("Descendant New - {0}", param)
End Sub

End Class
Sub Main()
Dim a As New Descendant
Dim b As New Descendant("hi")
End Sub

End Module

Anyway... Only the last example REQUIRES the call to MyBase.New. Most
times, this is simply a convienient way to base arguments to a base
class constructor. I think a lot of people still explicitly call
MyBase.New() simply because in some of the early VB.NET's (pre 1.0) it
was required...
 
Tom said:
MyBase.New in this case is not strictly necessary, since it is simply
calling the base classes default constructor. The only time that
MyBase.New is required is if the Base class can't be constucted without
parameters - in other words, there is no constructor that doesn't take
arguments. You may optionally use it if you want to pass arguments to
the base class constuctor... Maybe a small example to make this clear:

Option Strict On
Option Explicit On

Imports System

Module Module1

' Base with only a default constructor
Private Class Base
Public Sub New(ByVal param As String)
Console.WriteLine("Base New - {0}", param)
End Sub
End Class
Click to expand...

Crap... I copied in the wrong constructor.. That should simply be:

' Base with only a default constructor
Public Sub New()
Console.WriteLine("Base New")
End Sub

Sorry for the confusion!
 
Thank you very much for the time you spent replying. It is greatly
appreciated.

So the person who wrote the code really didn't need to do both. They could
have just used the first one? So you can duplicate a name of a routine many
times without issue? Does compiler than automatically choose which to use
based on parameters passed? What if both routines have same parameters with
types? What happens then?

Thanks again
 
The Bee,
| So you can duplicate a name of a routine many
| times without issue?
Yes this is known as overloading.

| Does compiler than automatically choose which to use
| based on parameters passed?
Yes the compiler looks at the number of parameters & their types to decide
which overload to call.

| What if both routines have same parameters with
| types?
The signatures of the methods (routines) need to be unique on number & type
of parameters. Overloading on return value or parameter name will cause a
compile error.


Advanced information:

..NET 2.0 (VS 2005) allows you to overload generic classes on the number of
type parameters, however you cannot overload a generic simply on the
constraint. For example:

System.Nullable - has shared methods pertaining to all nullable types.

System.Nullable(Of T) - creates a specific nullable type, such as
Nullable(Of Integer) & Nullable(Of Double).

The reason for the two Nullable types is to simplify calling of the methods
on System.Nullable...

--
Hope this helps
Jay B. Harlow [MVP - Outlook]
..NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net


| Thank you very much for the time you spent replying. It is greatly
| appreciated.
|
| So the person who wrote the code really didn't need to do both. They
could
| have just used the first one? So you can duplicate a name of a routine
many
| times without issue? Does compiler than automatically choose which to use
| based on parameters passed? What if both routines have same parameters
with
| types? What happens then?
|
| Thanks again
|
| | >
| > Tom Shelton wrote:
| >> TheBee wrote:
| >> > Hello,
| >> >
| >> > I have seen the following routines in a class and don't understand
how
| >> > they
| >> > work. For one, why two new() routines? Which is executed? Why not
| >> > just use
| >> > one routine? What does the mybase do in this case? Bit confusing!
:)
| >> > Thanks
| >> >
| >> > '<remarks/>
| >> > Public Sub New()
| >> > MyBase.New()
| >> > Me.Url =
| >> >
System.Configuration.ConfigurationSettings.AppSettings("ReportServerURL")
| >> > +
| >> > "/ReportService.asmx"
| >> > End Sub
| >> >
| >> > '<remarks/>
| >> > Public Sub New(ByVal ReportServerURL As String)
| >> > MyBase.New()
| >> > Me.Url = ReportServerURL + "/ReportService.asmx"
| >> > End Sub
| >>
| >> MyBase.New in this case is not strictly necessary, since it is simply
| >> calling the base classes default constructor. The only time that
| >> MyBase.New is required is if the Base class can't be constucted without
| >> parameters - in other words, there is no constructor that doesn't take
| >> arguments. You may optionally use it if you want to pass arguments to
| >> the base class constuctor... Maybe a small example to make this clear:
| >>
| >> Option Strict On
| >> Option Explicit On
| >>
| >> Imports System
| >>
| >> Module Module1
| >>
| >> ' Base with only a default constructor
| >> Private Class Base
| >> Public Sub New(ByVal param As String)
| >> Console.WriteLine("Base New - {0}", param)
| >> End Sub
| >> End Class
| >>
| >
| > Crap... I copied in the wrong constructor.. That should simply be:
| >
| > ' Base with only a default constructor
| > Public Sub New()
| > Console.WriteLine("Base New")
| > End Sub
| >
| > Sorry for the confusion!
| >
| > --
| > Tom Shelton [MVP]
| >
|
|
 
Back
Top