Using the Specification Design Pattern

Public MustInherit Class Specification(Of T)

#Region "Declarations"
    Private AndSpec As Specification(Of T)
    Private OrSpec As Specification(Of T)
    Private IsNegated As Boolean
#End Region

#Region "Must Override"
    Protected MustOverride Function SpecificationRule(ByVal obj As T) As Boolean
#End Region

#Region "Public Methods"
    Public Function IsSatisfiedBy(ByVal obj As T) As Boolean

        ' Test this Spec
        Dim blnIsSatisfied As Boolean = SpecificationRule(obj)

        ' Test the AND Branch of the Tree
        If Not AndSpec Is Nothing Then
            blnIsSatisfied = blnIsSatisfied And AndSpec.IsSatisfiedBy(obj)
        End If

        ' Test the OR Branch of the Tree
        If Not OrSpec Is Nothing Then
            blnIsSatisfied = blnIsSatisfied Or OrSpec.IsSatisfiedBy(obj)
        End If

        ' Apply the Not Flag
        blnIsSatisfied = blnIsSatisfied Xor IsNegated

        Return blnIsSatisfied
    End Function
#End Region

#Region "Operators"
    Protected Shared Function AndOperator(ByVal a As Specification(Of T), ByVal b As Specification(Of T)) As Specification(Of T)
        Dim newSpec As Specification(Of T) = a.Copy
        newSpec.SetAnd(b.Copy)
        Return newSpec
    End Function

    Protected Shared Function OrOperator(ByVal a As Specification(Of T), ByVal b As Specification(Of T)) As Specification(Of T)
        Dim newSpec As Specification(Of T) = a.Copy
        newSpec.SetOr(b.Copy)
        Return newSpec
    End Function

    Protected Shared Function NotOperator(ByVal a As Specification(Of T)) As Specification(Of T)
        Dim newSpec As Specification(Of T) = a.Copy
        newSpec.SetNot(Not a.IsNegated)
        Return newSpec
    End Function
#End Region

#Region "Private"
    Private Sub SetAnd(ByVal spec As Specification(Of T))
        ' If setting to nothing, set it directly
        If spec Is Nothing Then
            AndSpec = spec
            Exit Sub
        End If

        ' If setting to an actual spec, then attach to the end of any existing spec
        If AndSpec Is Nothing Then
            ' No existing spec, so set directly
            AndSpec = spec
        Else
            ' Recursively call SetAnd until we get to the end of the chain of specs
            AndSpec.SetAnd(spec)
        End If
    End Sub

    Private Sub SetOr(ByVal spec As Specification(Of T))
        ' If setting to nothing, set it directly
        If spec Is Nothing Then
            OrSpec = spec
            Exit Sub
        End If

        ' If setting to an actual spec, then attach to the end of any existing spec
        If OrSpec Is Nothing Then
            ' No existing spec, so set directly
            OrSpec = spec
        Else
            ' Recursively call SetOr until we get to the end of the chain of specs
            OrSpec.SetOr(spec)
        End If
    End Sub

    Private Sub SetNot(ByVal value As Boolean)
        IsNegated = value
    End Sub

    Private Function Copy() As Specification(Of T)

        ' Copy this spec
        Dim newSpec As Specification(Of T)
        newSpec = CType(Me.MemberwiseClone, Specification(Of T))

        ' Clear the andSpec and orSpec so they can be pointed at copies of the existing andSpec and orSpec
        ' After the MemberwiseClone the newSpec points to the same subSpecs
        newSpec.SetAnd(Nothing)
        newSpec.SetOr(Nothing)

        If Not AndSpec Is Nothing Then
            newSpec.SetAnd(AndSpec.Copy)
        End If

        If Not OrSpec Is Nothing Then
            newSpec.SetOr(OrSpec.Copy)
        End If

        Return newSpec
    End Function
#End Region

End Class