Problem

A Textbox should only allow numbers and decimals

Approach

Using the Keypress method will validate on each key that is entered.
Call the code below onKeyPress Event of each Textbox with

validateNumber(sender, e);

after each

Solution

private void validateNumber(object sender, KeyPressEventArgs e)
{
  if ((e.KeyChar == ',') && (((TextBox)sender).Text.IndexOf(',') > -1))
  {
    e.Handled = true;
    return;
  }

  if (!Char.IsDigit(e.KeyChar))
  {
    if ((e.KeyChar != ',') &&
        (e.KeyChar != Convert.ToChar(Keys.Back)))
    {
        e.Handled = true;
        return;
    }
 }
}