private static double GetTextBoxDouble(TextBox t, double _default = 0.00)
{
   var text = t.Text.Replace(',', '.');
   var b = double.TryParse(text, NumberStyles.Any, CultureInfo.InvariantCulture, out var val);
   return (b ? val : _default);
}

private void TextBox2_KeyPress(object sender, KeyPressEventArgs e)
{
  if (e.KeyChar == (char)Keys.Enter)
  {
     if (TextBox1.Text.Length >= 1)
     {
        TextBox3.Focus();
     }

   double num = GetTextBoxDouble(TextBox1);
     if (num == 0.00)
     {
        MessageBox.Show("Il campo non ha il numero");
        TextBox2.Clear(); TextBox2.Focus();
     }
  }
}