上面的代码抛出一个错误，即对象不能从dbnull转换为其他类型。我试着设置if-else的条件，但没有意义，可能是因为我的问题是我将设置if，else条件，可以检查var\u p\u amount值是否为空，然后lbl\u purchasing\u amount.text=“0”；请帮忙

private void Get_Purchasing_Amount()
            {
                try
                {
                    double var_P_Amount = 0;
                    int var_C_Code = 0;
                    string query = "select c_code as 'code' from `db_vegetable`.`tbl_payment_master`";
                    DataTable dt_C_Code = method_Class.method_Class.FetchRecords(query);
                    if (dt_C_Code.Rows.Count > 0)
                    {
                        for (int i = 0; i <= dt_C_Code.Rows.Count; i++)
                        {
                            var_C_Code = Convert.ToInt32(dt_C_Code.Rows[i]["code"]);
                            if (var_C_Code.Equals(Convert.ToInt32(txt_Customer_Code.Text)))
                            {
                                string get_P_Amount;
                                if (check_All.Checked.Equals(true))
                                {
                                    get_P_Amount = "SELECT `purchasing` AS 'purchasing' FROM `db_vegetable`.`tbl_payment_master` WHERE `c_code` = " + txt_Customer_Code.Text + "";
                                }
                                else
                                {
                                    get_P_Amount = "SELECT SUM(t_price) as 'purchasing' FROM `db_vegetable`.`tbl_order_details` WHERE `c_code` = " + txt_Customer_Code.Text + " AND (`date` BETWEEN '" + txt_From_Date.Text + "' AND '" + txt_To_Date.Text + "')";
                                }
                                DataTable dt = method_Class.method_Class.FetchRecords(get_P_Amount);
                                var_P_Amount = Convert.ToDouble(dt.Rows[0]["purchasing"]);
                                lbl_Purchasing_Amount.Text = var_P_Amount.ToString();

                                //In this side i use many DBNull methed but it can't  be work
                                //My Question is that If var_P_Amount value is null then
                                //lbl_Purchasing_Amount.Text = "0";
                            }
                            else
                            {
                                lbl_Purchasing_Amount.Text = "0";
                            }
                        }
                    }
                    else
                    {
                        lbl_Purchasing_Amount.Text = "0";
                    }
                }
                catch (Exception)
                {

                }
            }