Don't the KeyCode property to detect modifier keys in combination with other keys.
vb.net Code:
Private Sub TextBox1_KeyDown(sender As Object, e As KeyEventArgs) Handles TextBox1.KeyDown
Dim depressedKeys As New List(Of String)
If e.Control Then
depressedKeys.Add("Ctrl")
End If
If e.Shift Then
depressedKeys.Add("Shift")
End If
If e.Alt Then
depressedKeys.Add("Alt")
End If
If e.KeyCode >= Keys.A AndAlso e.KeyCode <= Keys.Z Then
depressedKeys.Add(e.KeyCode.ToString())
End If
Me.TextBox1.Text = String.Join("+", depressedKeys)
End Sub