改一下：
  public frmDepositEdit()
        {
            InitializeComponent();
            dgvCardType.AutoGenerateColumns = false;
            dgvCardType.EditingControlShowing += new DataGridViewEditingControlShowingEventHandler(dgvCardType_EditingControlShowing);
        }

        void dgvCardType_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
            e.Control.KeyPress += new KeyPressEventHandler(Control_KeyPress);
        }

        //求实收金额。
        decimal addMoney = 0;
        void Control_KeyPress(object sender, KeyPressEventArgs e)
        {

　　　　　　//要处理的事件信息。

            decimal intNumberBoxes = 0;
            decimal someAmount = 0;
            if (char.IsNumber(e.KeyChar))
            {
                int column = dgvCardType.CurrentCellAddress.X;
                int row = dgvCardType.CurrentCellAddress.Y;

                if (column == 2)
                {
                    string cellValue = Char.ToString(e.KeyChar);
                    string test = dgvCardType[column, row].EditedFormattedValue.ToString();
                    test = test + cellValue;
                    intNumberBoxes = Convert.ToDecimal(test);
                    if (intNumberBoxes > 999)
                    {
                        e.Handled = true;
                    }
                    someAmount = decimal.Parse(dgvCardType.Rows[dgvCardType.CurrentCellAddress.Y].Cells[1].Value.ToString());
                    txt_现金金额.Text = (addMoney + intNumberBoxes * someAmount).ToString();
                }
            }
        }