Result Validator is always nothing, how do I get it with message with prop?

Question

I have a model like that:

Imports System.ComponentModel
Imports System.ComponentModel.DataAnnotations

Public Class Product

    Private _id As Integer
    Private _name As String
    Private _description As String
    Private _unitPrice As Decimal

    <Required(AllowEmptyStrings:=False)>
    Public Property Title() As String

    <Required(AllowEmptyStrings:=False)>
    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 Description() As String
        Get
            Return _description
        End Get
        Set(ByVal value As String)
            _description = value
        End Set
    End Property

    Public Property UnitPrice() As Decimal
        Get
            Return _unitPrice
        End Get
        Set(ByVal value As Decimal)
            _unitPrice = value
        End Set
    End Property

End Class

And my code to validate:

Dim results As List(Of ValidationResult) = Nothing
Dim product As New Product
If Validator.TryValidateObject(product, New ValidationContext(product), results, True) Then
    MessageBox.Show(123)
Else
    MessageBox.Show(results.ToString())
End If

My questions are:

1. Why is the results variable always Nothing and how can I get the error message from result and know which property has an error?

2. Why can Title() be validated but Id() cannot? And how can I make Id() validated?

Answer 1

Why is the results variable always Nothing and how can I get the error message from result and know which property has an error?

When the validationResults parameter is null, the validation stops at the first error and no errors are added to the collection. If you want it to hold a collection of failed validations, you must initialize the collection before passing its reference:

Dim results As New List(Of ValidationResult)

Why can Title() be validated but Id() cannot? And how can I make Id() validated?

Title is a String, which is a reference type whose default value is null (AKA, Nothing), while Id is an Integer, which is a value type whose default value is 0. So, the RequiredAttribute makes no sense here since the property will always have a value. You may change its type to a Nullable(Of Integer) if you want it to work with the RequiredAttribute:

Public Property Id As Integer?