cal1.vb

Private Function calculate() As Double
  Select Case Dropdown.SelectedItem
  Case "+"
    Return CDbl(TA.Text.Trim) + CDbl(TB.Text.Trim)
  Case "-"
    Return CDbl(TA.Text.Trim) - CDbl(TB.Text.Trim)
  Case "*"
    Return CDbl(TA.Text.Trim) * CDbl(TB.Text.Trim)
  Case "/"
    Return CDbl(TA.Text.Trim) / CDbl(TB.Text.Trim)
  End Select
End Function

Public Sub TA_changed() Handles TA.TextChanged
  LabelResult.Text = calculate()
End Sub

Public Sub TB_changed() Handles TB.TextChanged
  LabelResult.Text = calculate()
End Sub

Public Sub Dropdown_changed() Handles Dropdown.SelectedIndexChanged
  LabelResult.Text = calculate()
End Sub

cal2.vb

Private Function calculate() As Double
  Select Case Dropdown.SelectedItem
  Case "+"
    Return CDbl(TA.Text.Trim) + CDbl(TB.Text.Trim)
  Case "-"
    Return CDbl(TA.Text.Trim) - CDbl(TB.Text.Trim)
  Case "*"
    Return CDbl(TA.Text.Trim) * CDbl(TB.Text.Trim)
  Case "/"
    Return CDbl(TA.Text.Trim) / CDbl(TB.Text.Trim)
  End Select
End Function

Public Sub LabelResult_Load() Handles LabelResult.Load
  LabelResult.Text = calculate()
End Sub