private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (!char.IsDigit(e.KeyChar)
            && e.KeyChar != (char)Keys.Return
            && e.KeyChar != (char)Keys.Back
            && e.KeyChar != (char)Keys.Space
            && e.KeyChar != ','
            && e.KeyChar != '.'
            && textBox1.Text.Length > 0)
        {
            var lines = textBox1.Text.Split(Environment.NewLine.ToCharArray());
            if (lines.Count() > 1)
            {
                e.Handled = true;
            }
        }
    }