MVP-VM (Model View Presenter - ViewModel) with Data Binding and BackgroundWorker in Windows Forms applications

Imports System.Collections.Generic
Imports System.Text

Public Class Validation
    Private ReadOnly m_ErrorList As New Dictionary(Of String, String)

    Public ReadOnly Property ErrorCount As Integer
        Get
            Return m_ErrorList.Count
        End Get
    End Property

    Public ReadOnly Property Item(ByVal propertyName As String) As String
        Get
            If m_ErrorList.ContainsKey(propertyName) Then
                Return m_ErrorList(propertyName)
            End If

            Return Nothing
        End Get
    End Property

    Public Overrides Function ToString() As String
        Dim sb As New StringBuilder
        For Each s In m_ErrorList.Values
            sb.AppendLine(s)
        Next

        Return sb.ToString()
    End Function

    Public Sub Clear(ByVal propertyName As String)
        If m_ErrorList.ContainsKey(propertyName) Then
            m_ErrorList.Remove(propertyName)
        End If
    End Sub

    Protected Sub AddValidationError(ByVal propertyName As String, ByVal message As String)
        If m_ErrorList.ContainsKey(propertyName) Then
            Dim existingMessage = m_ErrorList(propertyName)
            If Not existingMessage.Contains(message) Then
                m_ErrorList(propertyName) &= ";" & message
            End If
        Else
            m_ErrorList.Add(propertyName, message)
        End If
    End Sub
End Class