GetType上的InvalidOperationException

Question

I try to serialize a Class in VB using XMLSerializer. But when I call GetType for my Class I got a InvalidOperationException error.

Dim Playlist_serialize As New XmlSerializer(p.GetType)

Here is my class :

Public Class Playlist
Private p_name As String
Private p_elements As List(Of Playlist_element)

Sub New()
    p_elements = New List(Of Playlist_element)
End Sub

Public Property Name() As String
    Get
        Name = p_name
    End Get
    Set(value As String)
        p_name = value
    End Set
End Property

Public Property Elements() As List(Of Playlist_element)
    Get
        Elements = p_elements
    End Get
    Set(value As List(Of Playlist_element))
        p_elements = value
    End Set
End Property

---

Here is my Playlist_element :

Public Class Playlist_element
Private p_Name As String
Private p_Type As String
Private p_Genre As String

Public Property Name() As String
    Get
        Name = p_Name
    End Get
    Set(value As String)
        p_Name = value
    End Set
End Property

Public Property Type() As String
    Get
        Type = p_Type
    End Get
    Set(value As String)
        p_Type = value
    End Set
End Property

Public Property Genre() As String
    Get
        Genre = p_Genre
    End Get
    Set(value As String)
        p_Genre = value
    End Set
End Property

Sub New(ByVal name As String, ByVal type As String, ByVal genre As String)
    Me.Name = name
    Me.Genre = genre
    Me.Type = Type
End Sub
End Class

Answer 1

There are several issues with the way Playlist_element is coded. First your property getters are wrong. They need to return the backing field:

Public Property Name() As String
    Get
        ' this does nothing:
        'Name = p_Name
        Return p_Name
    End Get
    Set(value As String)
        p_Name = value
    End Set
End Property

Next, I would not use Type as a property name even if you can. If you drill into the inner exception and view the message, it tells you that it cannot serialize PlayList_element because it does not have a simple constructor. All serializers require this because they do not know how to use:

Sub New(ByVal name As String, ByVal type As String, ByVal genre As String)
    p_Name = name
    p_Genre = genre
    p_Type = type
End Sub

' add:
Public Sub New()

End Sub

It should work fine. I should note that as of VS2010, you can use auto-implemented properties and skip a lot of that code:

Public Class Element

    Public Property Name() As String
    Public Property Type() As String
    Public Property Genre() As String

    Sub New(name As String, type As String, genre As String)
        _Name = name
        _Genre = genre
        _Type = type
    End Sub

    Public Sub New()

    End Sub
End Class

VS provides a "hidden" backing field as with _Name , _Genre etc.