vb.net No parameterless constructor defined for this object - vb.net

Question

I am download a Json libary here : http://www.pozzware.com/pozzware/Corsi/Programmazione/VB.NET/JSON%20Library.aspx
This is my class on my project :
Imports PW.JSON
Public Class Prova
Private _id As Integer
Private _name As String
Private _valido As Boolean
Private _subObject As Prova
Private _numero As Integer
Private _numeroDec As Double
Private _array() As String
Public Property ID() As Integer
Get
Return _id
End Get
Set(ByVal value As Integer)
_id = value
End Set
End Property
Public Property Name() As String
Get
Return _name
End Get
Set(ByVal value As String)
_name = value
End Set
End Property
Public Property Valido() As Boolean
Get
Return _valido
End Get
Set(ByVal value As Boolean)
_valido = value
End Set
End Property
Public Property SubObject() As Prova
Get
Return _subObject
End Get
Set(ByVal value As Prova)
_subObject = value
End Set
End Property
Public Property NumeroDec() As Double
Get
Return _numeroDec
End Get
Set(ByVal value As Double)
_numeroDec = value
End Set
End Property
Public Property Array() As String()
Get
Return _array
End Get
Set(ByVal value As String())
_array = value
End Set
End Property
Public Sub New(ByVal ID As Integer, ByVal Name As String)
_id = ID
_name = Name
End Sub
Public Function SomeMethod() As String
Return "Method: " & _id
End Function
End Class
And this is my code :
Sub PasteJsonExam()
Dim strJSON As String = "{""NumeroDec"": 100.34, ""Name"": ""Nome Object"", " & _
" ""Array"": [""A"", ""E"", ""I"", ""O"", ""U""], " & _
" ""SubObject"": {""NumeroDec"": 0, ""Name"": ""Nome - SubObject"", " & _
" ""Array"": null, ""SubObject"": null, ""Valido"": false, ""ID"": 2}, " & _
" ""Valido"": true, ""ID"": 1}"
Dim objprova As Prova
objprova = PW.JSON.JSONHelper.StringToObject(strJSON, GetType(Prova))
MsgBox(objprova.Name)
MsgBox(objprova.SubObject.Name)
End Sub
When i call that sub i got error :
No parameterless constructor defined for this object.
At this line :
objprova = PW.JSON.JSONHelper.StringToObject(strJSON, GetType(Prova))
I am not an professonal in VB.net so i hope someone explain for me why i got that error and how can i fix this.

Answer 1

Probably StringToObject method try to create an instance of Prova. It does this by calling the default(parameterless) constructor of the type.
But in the class you defined there isn't any parameterless constructor, you have only:
Public Class Prova
' Other fields and methods
Public Sub New(ByVal ID As Integer, ByVal Name As String)
_id = ID
_name = Name
End Sub
' Other fields and methods
End Class
To let the method work you need to define a parameterless constructor like:
Public Class Prova
' Other fields and methods
Public Sub New()
End Sub
Public Sub New(ByVal ID As Integer, ByVal Name As String)
_id = ID
_name = Name
End Sub
' Other fields and methods
End Class

Answer 2

Well i think that the library you are using is not the best choice, but, if you want to run your program you must add a default constructor (a constructor without parameter). That's because the library probably uses Reflection
This is the class with the Default Constructor:
Imports PW.JSON
Public Class Prova
Private _id As Integer
Private _name As String
Private _valido As Boolean
Private _subObject As Prova
Private _numero As Integer
Private _numeroDec As Double
Private _array() As String
Public Property ID() As Integer
Get
Return _id
End Get
Set(ByVal value As Integer)
_id = value
End Set
End Property
Public Property Name() As String
Get
Return _name
End Get
Set(ByVal value As String)
_name = value
End Set
End Property
Public Property Valido() As Boolean
Get
Return _valido
End Get
Set(ByVal value As Boolean)
_valido = value
End Set
End Property
Public Property SubObject() As Prova
Get
Return _subObject
End Get
Set(ByVal value As Prova)
_subObject = value
End Set
End Property
Public Property NumeroDec() As Double
Get
Return _numeroDec
End Get
Set(ByVal value As Double)
_numeroDec = value
End Set
End Property
Public Property Array() As String()
Get
Return _array
End Get
Set(ByVal value As String())
_array = value
End Set
End Property
' This is the default constructor the library should need'
Public Sub New()
End Sub
Public Sub New(ByVal ID As Integer, ByVal Name As String)
_id = ID
_name = Name
End Sub
Public Function SomeMethod() As String
Return "Method: " & _id
End Function
End Class