Question

我收到此错误：

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.
Source Error: 
Line 251:            ManageAccount.Instance.GetUserAccounts(username), "accountID", "name", accountFrom);
Line 252:
Line 253:            Currencies = new SelectList(
Line 254:            ManageCurrency.Instance.getCurrencies(), "id", "name",_Account.currency1.id);
Line 255:

它在此viewModel

中给出错误

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ComponentModel.DataAnnotations;
using Common;
using Business;
using System.Web.Mvc;
using System.Web.Security;
using System.Runtime.Remoting.Contexts;

namespace internetBankingApplication.ViewModel
{

    public class NewFixedAccountViewModel
    {
        private account _Account { get; set; }

        private fixedAccount _FixedAccount { get; set; }



        public SelectList AccountTypes { get; set; }

        public SelectList Durations { get; set; }

        public SelectList AccountFromList { get; set; }

        public SelectList Currencies { get; set; }

        public int ID
        {
            get
            {
                return _Account.accountID;
            }
        }

        [Required]
        [Display(Name = "Account Name")]
        public string Name
        {
            get
            {
                return _Account.name;
            }
            set
            {
                _Account.name = value;
            }
        }

        [Required]
        [Display(Name = "Account From")]
        public int accountFrom { get;set;}



        public string AccountFromName
        {
            get
            {
                string result = string.Empty;
                try
                {
                    result = ManageAccount.Instance.GetAccountBYID(accountFrom).name;
                }
                catch { }
                return result;
            }
        }



        [Required]
        [Display(Name = "Duration")]
        public int duration
        {
            get
            {
                return _FixedAccount.duration;
            }
            set
            {
                _FixedAccount.duration = value;
            }
        }

        public string durationName
        {
            get
            {
                string result = string.Empty;
                try
                {
                    result = ManageDuration.Instance.GetDurationById(_FixedAccount.duration).duration1;
                }
                catch { }
                return result;
            }
        }

        [Required]
        [Display(Name = "Available Balance")]
        public decimal AvailableBalance
        {
            get
            {
                return _Account.availableBalance;
            }
            set
            {
                _Account.availableBalance = value;
            }
        }

        [Required]
        [Display(Name = "Currency")]
        public int currency
        {
            get
            {
                return _Account.currency;
            }
            set
            {
                _Account.currency = value;
            }
        }

        public string CurrencyName
        {
            get
            {
                string result = string.Empty;
                try
                {
                    result = ManageCurrency.Instance.getTypesByID(_Account.currency).name;
                }
                catch { }
                return result;
            }
        }


        [Required]
        [Display(Name = "Account Description")]
        public string Description
        {
            get
            {
                return _Account.description;
            }
            set
            {
                _Account.description = value;
            }
        }


        [Required]
        [Display(Name = "Account Renew")]
        public bool renew
        {
            get
            {
                if (_FixedAccount.renew == 0)
                {
                    return false;
                }
                return true;
            }
            set
            {
                if (value == false)
                {
                    _FixedAccount.renew = 0;
                }
                else
                {

                    _FixedAccount.renew = 1;
                }
            }
        }


        public NewFixedAccountViewModel()
        {
           // AccountTypes = new SelectList(
           //ManageAccountType.Instance.getTypes(), "id", "type", _Account.typeID);

            string username = HttpContext.Current.User.Identity.Name.ToString();
            AccountFromList = new SelectList(
            ManageAccount.Instance.GetUserAccounts(username), "accountID", "name", accountFrom);

            **Currencies = new SelectList(
            ManageCurrency.Instance.getCurrencies(), "id", "name",_Account.currency1.id);** //Giving the error

            Durations = new SelectList(
            ManageDuration.Instance.GetAllDurations(), "id", "duration", _FixedAccount.duration);



        }

        public NewFixedAccountViewModel(string username)
        {
            AccountTypes = new SelectList(
            ManageAccountType.Instance.getTypes(), "id", "type", _Account.typeID);


            AccountFromList = new SelectList(
            ManageAccount.Instance.GetUserAccounts(username), "accountID", "name", accountFrom);

            Currencies = new SelectList(
            ManageCurrency.Instance.getCurrencies(), "id", "name", _Account.currency);

            Durations = new SelectList(
            ManageDuration.Instance.GetAllDurations(), "id", "duration", _FixedAccount.duration);


        }

        public NewFixedAccountViewModel(int accountID, string username)
        {
            _Account = ManageAccount.Instance.GetAccountBYID(accountID);

            _FixedAccount = ManageFixedAccount.Instance.GetFixedAccountByID(accountID);

            AccountFromList = new SelectList(
            ManageAccount.Instance.GetUserAccounts(username), "accountID", "name", accountFrom);

            AccountTypes = new SelectList(
            ManageAccountType.Instance.getTypes(), "id", "type", _Account.accountType);

            Currencies = new SelectList(
            ManageCurrency.Instance.getCurrencies(), "id", "name", _Account.currency);

            Durations = new SelectList(
            ManageDuration.Instance.GetAllDurations(), "id", "duration", _FixedAccount.duration);


        }
    }
}

Answer 1

您获得的错误消息意味着您尝试在空值上调用方法或属性 - 这是您无法做到的。

从我所看到的情况来看，ManageCurrency为空或ManageCurrency.Instance为空。

如果ManageCurrency为null，则对属性Instance的调用失败，因为null对象没有方法或属性。同样，如果ManageCurrency.Instance为null，则对方法getCurrencies的调用因同样的原因而失败。

无论如何 - 调试起来并不难。正如skumar建议的那样，在该行上设置断点，在调试模式下运行，并查看null值的位置，然后向后工作以发现值为null的原因。

Answer 2

您在以下代码中传递的用户名可能为空或无效。这就是它返回NULL并发生异常的原因。

AccountFromList = new SelectList(
            ManageAccount.Instance.GetUserAccounts(username), "accountID", "name", accountFrom);

对象引用未设置为视图模型中对象的实例

2 个答案: