hi,
I think you are not checking resultTextbox.Text for null or empty try this before assigning value to total(you can do the changes accordingly):
private void rdCash_CheckedChanged(object sender, EventArgs e)
{
if (rdCash.Checked)
{
if (string.IsNullOrEmpty(resultTextbox.Text) != true )
{
decimal total = Convert.ToDecimal(resultTextbox.Text);
}
else
{
decimal total = Convert.ToDecimal("0.00");
}
decimal total = string.IsNullOrEmpty(resultTextbox.Text) ?
0: Convert.ToDecimal(resultTextbox.Text);
decimal paid = Convert.ToDecimal(txtPaid.Text);
decimal results = Decimal.Add(total, paid);
txtChange.Text = results.ToString();
}
}