Public Class frmTabDemo
 
    Dim tc As New TabControl With _
            {.Parent = Me, .Dock = DockStyle.Fill}
 
    Dim tp As New TabPage With _
            {.Parent = tc, .Dock = DockStyle.Fill, .Text = "TheTab"}
 
 
 
    Private Sub frmTabDemo_Load(sender As System.Object, _
                         e As System.EventArgs) Handles MyBase.Load
 
        Me.Width = 500
 
        Dim tbo As New TextBox With _
                      {.Parent = tp, .Font = New Font("Arial", 24), _
                       .Dock = DockStyle.Top, .Name = "TheBox"}
 
        Dim btn As New Button With _
                     {.Parent = tp, .Text = "Click", .Top = 100}
        AddHandler btn.Click, AddressOf btn_Click
 
    End Sub
 
 
 
    Public Sub btn_Click(sender As Object, e As EventArgs)
 
        For Each ctrl As Control In tp.Controls
            If TypeOf ctrl Is TextBox AndAlso ctrl.Name = "TheBox" Then
                Dim tb As TextBox = DirectCast(ctrl, TextBox)
                tb.Text = "Da tut sich schon etwas"
            End If
        Next ctrl
    End Sub
End Class