Have two questions regarding the code below:
In the following code, the class Test implement the interface Test_Interafce. I have read in more than one articles that when a class implements an interface, we should assign the class instance to the interface. Going by that in the following code the statement Dim t As New Test() should be replaced by Dim t As Test_Interface = New Test(). However I do not understand the advantage or need for the same. When I instantiate the class Test by simply writing Dim t As New Test(), I am able to access all elements (even the procedures of the interface implemented by the class) through the instance "t" and the code seems to be working fine. So then why to assign a class instance to an interface?
Infact if I write Dim t As Test_Interface = New Test() then through "t" does not allow me to access the subroutine CHECK in the class Test. The error being displayed is: "CHECK is not a member of Test_Interface". So isnt that a disadvantage??!!
What is the use of the statement: Throw New NotImplementedException(). This statement comes automatically when I implement teh interface in a class/structure.
The code is found below:
Module Module1
Interface Test_Interface
Function Length(ByVal s As String) As Integer
Sub Details(ByVal age As Integer, ByVal name As String)
End Interface
Sub Main()
Dim t As New Test() 'Alternate definition: Dim t As Test_Interface = New Test()
t.Details(31, "Mounisha")
Console.WriteLine("Length of the string entered = {0}", t.Length("Hello"))
t.check()
Console.ReadLine()
End Sub
End Module
Class Test
Implements Test_Interface
Public Sub Details(age As Integer, name As String) Implements Test_Interface.Details
Console.WriteLine("Age of person = {0}", age)
Console.WriteLine("Name of person = {0}", name)
'Throw New NotImplementedException() ----> what does this do?
End Sub
Public Function Length(s As String) As Integer Implements Test_Interface.Length
Console.WriteLine("Original String: {0}", s)
Return s.Length()
'Throw New NotImplementedException()
End Function
Sub check()
Console.WriteLine("The sub can be accessed")
End Sub
End Class
Related
In VB, I have a class that does some standard validations. What I'd LIKE to do is to declare some variables, then create instances of a validator class that include pointers to the variables, and then at some later time execute the validators to test the values in the fields that are pointed to.
Something like this:
public class MyData
public property foo as string
public property bar as string
dim vfoo as validator
dim vbar as validator
public sub new()
vfoo=new validator(&foo) ' i.e. & operator like in C
vbar=new validator(&bar)
end sub
public sub validate()
vfoo.validate
vbar.validate
end sub
end class
public class validator
dim _field as string* ' i.e. * like in C
public sub new(field as string*)
_field=field
end sub
public sub validate
if string.isnullorempty(_field) then
throw SomeException
else if not SomeOtherTest(_field) then
throw SomeOtherException
end sub
The catch is that, to the best of my knowledge, there is nothing like C pointers in VB. Is there any reasonably easy way to do this?
At present I am passing in the field values at the time I call the validate() function, but this is not ideal because I would like to be able to create a List of validators specific to a given caller, and then loop through the List. But at the time I loop, how would I know which value from MyClass to pass in, unless I had a giant select statement keying off some "field code"? (And of course in real life, there are not just two fields like in this example, there are quite a few.)
Am I just having a brain freeze and there's an easy way to do this? Or can this not be done in VB because there are no such thing as pointers?
Like Java, VB doesn't make direct use of pointers (it compensates where it can with library/framework calls). In the context of a garbage-collected language, I can't imagine that this style of validation would work out well.
But for fun, maybe a lambda-based solution could suit?:
Public Class MyData
Public Property foo As String
Public Property bar As String
Dim vfoo As validator
Dim vbar As validator
Public Sub New()
vfoo = New validator(Function() foo)
vbar = New validator(Function() bar)
End Sub
Public Sub validate()
vfoo.validate()
vbar.validate()
End Sub
End Class
Public Class validator
ReadOnly _fieldFunc As Func(Of String)
Public Sub New(fieldFunc As Func(Of String))
_fieldFunc = fieldFunc
End Sub
Public Sub validate()
Dim _field = _fieldFunc()
If String.IsNullOrEmpty(_field) Then
Throw New Exception("NullOrEmpty")
ElseIf Not SomeOtherTest(_field) Then
Throw New Exception("SomeOtherTest")
End If
End Sub
Public Function SomeOtherTest(f As String) As Boolean
Return True
End Function
End Class
I have written a class (containing only properties). The code is shortened, here is only the part of the code what I want to do:
Public Class test
Public Overrides Function ToString() As String
'Das Objekt lebensmittel serialisieren
Dim ser As New Xml.Serialization.XmlSerializer(Me.GetType)
Dim sw As New IO.StringWriter
ser.Serialize(sw, Me)
Return sw.tostring
End Function
Public Sub New()
End Sub
Public Sub New(ByVal t As String)
Dim deser As New Xml.Serialization.XmlSerializer(Me.GetType)
Dim ms As New IO.MemoryStream(System.Text.Encoding.Unicode.GetBytes(t))
Me = CType(deser.Deserialize(ms), test) 'This throws an error
End Sub
End Class
What I want to do is to overload the New() operator and deserialize the string to this class, like:
Dim x As New test(string)
How can I do this? The marked line throws an error in Sub New.
I'm trying to make a application, in this application I have a List(of T) collection that holds an object.
When processing the object I need to know it's Index from the list.
Example:
Public Class
Public oList as New List(of TestObject)
Private Sub Test()
Dim NewObject As New TestObject
oList.add(NewObject)
Index(NewObject)
End Sub
Private Sub Index(Byval TestObject As TestObject)
debug.print(Testobject.index)
End Sub
End Class
Is something like this possible? Ive seen it available in a reference file I used some time ago, but now I would like to make this available within my own class.
Can someone provide a sample?
PS: I know I can get the index using the List(Of T).IndexOf Method (T) but for future possibilities I would like to make the call from the object itself.
What usually happen is that they have a custom list, they don't directly used List(Of T) and store the list inside the object when they add that item to the list.
Module Module1
Sub Main()
Dim someList As New CustomList
someList.Add(New CustomItem())
someList.Add(New CustomItem())
someList.Add(New CustomItem())
Console.WriteLine(someList(1).Index)
Console.ReadLine()
End Sub
End Module
Class CustomItem
' Friend since we don't want anyone else to see/change it.
Friend IncludedInList As CustomList
Public ReadOnly Property Index
Get
If IncludedInList Is Nothing Then
Return -1
End If
Return IncludedInList.IndexOf(Me)
End Get
End Property
End Class
Class CustomList
Inherits System.Collections.ObjectModel.Collection(Of CustomItem)
Protected Overrides Sub InsertItem(index As Integer, item As CustomItem)
If item.IncludedInList IsNot Nothing Then
Throw New ArgumentException("Item already in a list")
End If
item.IncludedInList = Me
MyBase.InsertItem(index, item)
End Sub
Protected Overrides Sub RemoveItem(index As Integer)
Me(index).IncludedInList = Nothing
MyBase.RemoveItem(index)
End Sub
End Class
It looks like this
Public oList As New List(Of TestObject)
Private Sub Test()
Dim NewObject As New TestObject(oList.Count)
oList.add(NewObject)
End Sub
Public Class TestObject
Public index As Integer
Public Sub New(IndxOfObj As Integer)
Me.index = IndxOfObj
End Sub
End Class
If you necessarily need to have it as a property on the object I would suggest the following:
Public Class Main
Public oList As New List(Of TestObject)
Public Sub New()
' This call is required by the designer.
InitializeComponent()
' Add any initialization after the InitializeComponent() call.
Dim NewObject As New TestObject(Me)
oList.Add(NewObject)
Dim NewObject2 As New TestObject(Me)
oList.Add(NewObject2)
MsgBox(NewObject2.Index)
End Sub
Public Function Index(ByVal TestObject As TestObject) As Integer
Return oList.IndexOf(TestObject)
End Function
End Class
Public Class TestObject
Private _main As Main
Public ReadOnly Property Index() As Integer
Get
Return _main.Index(Me)
End Get
End Property
Public Sub New(RootClass As Main)
_main = RootClass
End Sub
End Class
If you happen to have the Main class as a Singleton you can skip the whole sending 'Me' into the constructor business. Then you can just call Main.Index without storing it as a property on all TestObjects.
I have a simple class List.vb which is the following:
Public Class List
Public fList As List(Of Integer)
Public Sub New()
fList = New List(Of Integer)
fList.Add(1)
fList.Add(2)
fList.Add(3)
fList.Add(4)
fList.Add(5)
End Sub
End Class
The Console application is using this class like the following:
Module Module1
Sub Main()
Dim fObject As List = New List
Dim cnt As Integer = 0
For Each x As Integer In fObject.fList
Console.WriteLine("hello; {0}", fObject.fList.Item(cnt).ToString())
cnt = cnt + 1
Next
Console.WriteLine("press [enter] to exit")
Console.Read()
End Sub
End Module
Can I change the class code so that List.vb is a list(of integer) type?
This would mean that in the Console code I could replace In fObject.fList with just In fObject?
Or am I barking up the wrong tree - should classes be single objects and lists should be collections of classes ?
Yes, you can do that. In order for an object to be compatible with For Each, it must have a GetEnumerator function:
Public Function GetEnumerator() As IEnumerator _
Implements IEnumerable.GetEnumerator
Return New IntListEnum(fList)
End Function
The IntListEnum class must, in turn, implement IEnumerator, like this:
Public Class IntListEnum Implements IEnumerator
Private listInt As List(Of Integer)
Dim position As Integer = -1
Public Sub New(ByVal fList As List(Of Integer))
listInt = fList
End Sub
Public Function MoveNext() As Boolean Implements IEnumerator.MoveNext
position = position + 1
Return (position < listInt.Count)
End Function
Public Sub Reset() Implements IEnumerator.Reset
position = -1
End Sub
Public ReadOnly Property Current() As Object Implements IEnumerator.Current
Get
Try
Return listInt(position)
Catch ex As IndexOutOfRangeException
Throw New InvalidOperationException()
End Try
End Get
End Property
End Class
Now you can make fList private, and iterate your List as follows:
For Each x As Integer In fObject
You can see a complete example here.
The answer that dasblinkenlight has provided is excellent, but if all you need is a list that of integers that is pre-populated, you can just inherit from List(Of Integer) and then have the class populate itself in the constructor:
Public Class List
Inherits List(Of Integer)
Public Sub New()
Add(1)
Add(2)
Add(3)
Add(4)
Add(5)
End Sub
End Class
When you inherit from List(Of Integer), your class automatically gets all of the functionality implemented by that type, so your class also becomes a list class that works the same way. Then, you can just use it like this:
Dim fObject As New List()
For Each x As Integer In fObject
Console.WriteLine("hello; {0}", x)
Next
I find myself calling functions from lambdas frequently as the provided delegate does not match or does not have sufficient parameters. It is irritating that I cannot do lambda on subroutines. Each time I want to do this I have to wrap my subroutine in a function which returns nothing. Not pretty, but it works.
Is there another way of doing this that makes this smoother/prettier?
I have read that this whole lambda inadequacy will probably be fixed in VS2010/VB10 so my question is more out of curiosity.
A simple Example:
Public Class ProcessingClass
Public Delegate Sub ProcessData(ByVal index As Integer)
Public Function ProcessList(ByVal processData As ProcessData)
' for each in some list processData(index) or whatever'
End Function
End Class
Public Class Main
Private Sub ProcessingSub(ByVal index As Integer, _
ByRef result As Integer)
' (...) My custom processing '
End Sub
Private Function ProcessingFunction(ByVal index As Integer, _
ByRef result As Integer) As Object
ProcessingSub(index, result)
Return Nothing
End Function
Public Sub Main()
Dim processingClass As New ProcessingClass
Dim result As Integer
' The following throws a compiler error as '
' ProcessingSub does not produce a value'
processingClass.ProcessList( _
Function(index As Integer) ProcessingSub(index, result))
' The following is the workaround that'
' I find myself using too frequently.'
processingClass.ProcessList( _
Function(index As Integer) ProcessingFunction(index, result))
End Sub
End Class
If you find that you are doing it too often and generally with the same type of data, you can wrap the delegate in a class.
Create a base class that converts to the delegate:
Public MustInherit Class ProcessDataBase
Public Shared Widening Operator CType(operand As ProcessDataBase) as ProcessingClass.ProcessData
Return AddressOf operand.Process
End Sub
Protected MustOverride Sub Process(index As Integer)
End Class
Inherit from the class:
Public Class ProcessResult
Inherits ProcessDataBase
Public Result As Integer
Protected Overrides Sub Process(index as Integer)
' Your processing, result is modified.
End SUb
End Class
Use it:
Public Class Main()
Public Sub Main()
Dim processingClass As New ProcessingClass
Dim processor As New ProcessResult
processingClass.ProcessList(processor)
Dim result as integer=processor.Result
End Sub
End Class
It IS fixed in VB10, the VS10 Beta is available, if it's an option for you to use it. In VB10 you have lambdas without a return value, and inline subs/functions.
For now, maybe you could just forget lambdas and work with delegates instead? Something like:
processingClass.ProcessList(AddressOf ProcessingSub)