我想在CashDisc = 0时使用total-cashDisc

Question

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    public void Cashdis() {
      decimal t = Convert.ToDecimal(TxtToatal.Text);
      if (TxtcashDisc.Text == String.Empty) {
        CashDisc = 0;
        index = lstvItems.FocusedItem.Index;
        removetotal = Convert.ToDecimal(this.lstvItems.Items[index].SubItems[6].Text);
      } else {
        CashDisc = Convert.ToDecimal(TxtcashDisc.Text);
      }
      bal = t - CashDisc;
      TxtToatal.Text = bal.ToString();
      TxtPaid.Text = bal.ToString();
    }

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我想当CashDisc = 0时，total-cashDisc但该时间错误已经出现＆＃34;附加信息：对象引用未设置为对象的实例。＆＃34;