A simple Business Rules Manager

I read a seriously cool article today on Msdn about creating a "Rules Manager". In it, Rockford Lhotka demonstrates how to create a central controller for validation rules. The rules manager allows you to create rules at all sorts of levels and helps to abstract the task of managing validation away from individual Forms so that the validation logic is all in one place. As an example of this, imagine creating a "Human" class and assigning some rules such as:

• Name must not be empty
• Date Of Birth must be earlier than Date Of Death

Now, when you come to use a Human in the application you can simply write validation code such as:

     myPerson.CheckRules()
     If myPerson.IsValid() Then
        ...
     Else
         For Each brokenRule As Rule In myPerson.BrokenRules
              ...
         Next
     End If

You could also add Rules at Form level which would allow you to create complex associations between arbitrary objects.

One of the really neat things about this article is that it demonstrates some excellent, real-world use of Delegates and Reflection. I might write about those some more tomorrow, but, one of the things that opened my eyes was the use of the Visual Basic CallByName method. CallByName is a function which allows you to invoke members on objects. Those objects can be anything... Forms, Controls, Custom Classes, etc.

Here is a small demo using CallByName which invokes a method on a custom class:

' When the MyForm Form loads the CallByName function
' is called on an instance of the Foo class passing
' an argument to the Speak method.
Public Class MyForm
    Inherits System.Windows.Forms.Form
    Private Sub MyForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim foo As New Foo
        CallByName(foo, "Speak", CallType.Method, New String() {"Hello World"})
    End Sub
End Class

Public Class Foo
    Public Sub Speak(ByVal theWord As String)
        MessageBox.Show(theWord)
    End Sub
End Class