重载解析失败，因为没有可访问的“新”是这些参数最具体的：

Question

I have a usercontrol with a repeater. Originally in page load I had code which grabbed data from the database and bound to the repeater. I now want to take this functionality outside of the usercontrol so that I can have more than one on the page and have them bind to different data.

My code now is:

Imports System.ComponentModel

Public Class UpdateList
    Inherits System.Web.UI.UserControl

    Private m_dataSource As Object

    <TypeConverter("System.Windows.Forms.Design.DataSourceConverter, System.Design")> _
    <Category("Data")> _
    <DefaultValue(Nothing)> _
    Public Property DataSource() As Object
        Get
            Return Me.m_dataSource
        End Get
        Set(value As Object)
            If Me.m_dataSource <> value Then
                m_dataSource = value
                tryDataBinding()
            End If
        End Set
    End Property

    Public ReadOnly Property UpdateCount As Integer
        Get
            Return m_UpdateCount
        End Get
    End Property


    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load


    End Sub

    Protected Sub tryDataBinding()

        rep_Updates.DataSource = Me.m_dataSource
        rep_Updates.DataBind()

    End Sub

End Class

I get a wavy line at <DefaultValue(Nothing)> and get an error:

Overload resolution failed because no accessible 'New' is most specific for these arguments:

'Public Sub New(value As Boolean)': Not most specific.

'Public Sub New(value As Byte)': Not most specific.

'Public Sub New(value As Char)': Not most specific.

What does this mean? Thanks

UPDATE

The fix was to change the property declaration for datasource to...

    Private m_dataSource As Object

    Public Property DataSource() As Object
        Get
            Return Me.m_dataSource
        End Get
        Set(value As Object)
            m_dataSource = value
            tryDataBinding()
        End Set
    End Property

Answer 1

at least in WinForms the DefaultValue attribute ctor cant be Nothing (there is no such definition in Object Browser). DefaultAttribute doesnt define an initial starting value (in spite of the name), but the comparison value for when to persist a property value. This seems dubious anyway in the case of a webform and datasource, so just remove the attribute.

As you noted the TypeConverter is probably out of place as well.

Answer 2

I get a wavy line at <DefaultValue(Nothing)> and […] an error:

Overload resolution failed because no accessible 'New' is most specific for these arguments:

Public Sub New(value As Boolean) : Not most specific.
Public Sub New(value As Byte) : Not most specific.
Public Sub New(value As Char) : Not most specific.

DefaultValueAttribute constructor is overloaded. This error means that the VB.NET compiler has trouble deciding which of the overloads to invoke for <DefaultValue(Nothing)> . The problem is that Nothing in VB.NET can actually mean two things:

• the null reference ( null in C#)
• the default value of the type to which it is assigned ( default(T) in C#)

Because of that, every single one of the available constructor overloads could be chosen. You obviously intend VB.NET to choose the Public Sub New(value As Object) overload , but the compiler simply isn't smart enough to recognise this.

Unfortunately, it doesn't seem to be possible to make Nothing more specific, eg via CObj(Nothing) , because only constant values are allowed as custom attribute arguments.

If I find a solution to this VB.NET-specific problem, I'll update my answer.