OnTextChangeのオーバーライド

'---------------------------------------
'チェック機能を実装するクラス
'---------------------------------------
Public Class TextChecker

    Public Sub New(ByVal Parent As Form)
        Dim Ctrl As Control

        For Each Ctrl In Parent.Controls
            If (TypeOf Ctrl Is TextBox) Then
                AddHandler CType(Ctrl, TextBox).TextChanged, _
                    AddressOf Me.TextChanged
            End If
        Next
    End Sub

    Public Sub TextChanged(ByVal sender As Object, _
        ByVal e As EventArgs)

        If CType(sender, TextBox).Text = "a" Then
            CType(sender, TextBox).Text = "bb"
        End If
    End Sub

End Class

'---------------------------------------
'TextBoxを幾つか配置したフォーム
'---------------------------------------
Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows フォーム デザイナで生成されたコード "
    '省略
#End Region

    Private TextChecker1 As TextChecker

    Private Sub Form1_Load(ByVal sender As Object, _
        ByVal e As System.EventArgs) Handles MyBase.Load

        TextChecker1 = New TextChecker(Me)
    End Sub

End Class