C＃使用For循环列出添加项目

Question

我在使用c＃列表时遇到问题，不确定将新对象添加到Managepagesid列表时缺少了什么地方！

 public class Clients
    {
        [BsonId]
        public string Id { get; set; } //Object ID managed by MongoDb

        public string Name { get; set; } //Client Name

        public string Phone { get; set; } //Client Phone

        public string Email { get; set; } //Client E-mail

        public string Username { get; set; } //Client Username

        public DateTime LastModified { get; set; } //Client Last Login

        public string FB_User_Id { get; set; } //Client FB User ID

        public AccessToken UserAccessToken { get; set; } //AccessToken which is stored while user is logged in.

        public List<ManagePages> Manage_Pages_id { get; set; } //The pages maintained by the specific client
     }

---

我正在尝试访问将新项目添加到ManagePage_id列表中，但它抛出了一些空异常。

Clients client = new Clients();

            client.FB_User_Id = FBData.id;
            client.Name = FBData.name;
            client.Email = FBData.email;
            client.Username = FBData.username;

            for (int index = 0; index < FBData.accounts["data"].Count; index++)
            {
                ManagePages pagedetails = new ManagePages()
                {
                    page_id = FBData.accounts["data"][index].id,
                    page_name = FBData.accounts["data"][index].name,
                    ManagePages_AccessTokens = new AccessToken()
                    {
                        AccessToken_accessToken = FBData.accounts["data"][index].access_token,
                        AccessToken_expiryDate = DateTime.Now
                    },
                    ManagePages_category = FBData.accounts["data"][index].category
                };

                var category = pagedetails.ManagePages_category;

                client.Manage_Pages_id.Add(pagedetails);
            }

堆栈跟踪已添加！

Exception> System.NullReferenceExceptionObject引用未设置为对象的实例。 在g：\\ Development \\ Vega_MongoDB \\ Vega_MongoDB \\ FBDataAccess \\ ClientFBRepository.cs：第48行的Vega_MongoDB.FBDataAccess.ClientFBRepository.ClientsData（字符串accessToken）处，在g：\\ Development \\ Vega_MongoDB的Vega_MongoDB.Models.ClientRepository..ctor（字符串连接）处的第48行\\ Vega_MongoDB \\ Models \\ Clients \\ ClientRepository.cs：第47行，位于g：\\ Development \\ Vega_MongoDB \\ Vega_MongoDB \\ Models \\ Clients \\ ClientRepository.cs：Vega_MongoDB.Models.ClientRepository.ctor（）中的Vega_MongoDB.Controllers.ClientsController g。\\ Development \\ Vega_MongoDB \\ Vega_MongoDB \\ Controllers \\ ClientsController.cs中的..cctor（）：第18行

我已经检查了pagedetails对象，它包含所有数据。

谢谢

毗湿奴

Answer 1

您应该在添加项目之前创建列表的实例：

client.Manage_Pages_id = new List<ManagePages>();
for (int index = 0; index < FBData.accounts["data"].Count; index++)
{
    ManagePages pagedetails = new ManagePages()
    {
        page_id = FBData.accounts["data"][index].id,
        page_name = FBData.accounts["data"][index].name,
        ManagePages_AccessTokens = new AccessToken()
        {
            AccessToken_accessToken = FBData.accounts["data"][index].access_token,
            AccessToken_expiryDate = DateTime.Now
        },
        ManagePages_category = FBData.accounts["data"][index].category
    };

    var category = pagedetails.ManagePages_category;

    client.Manage_Pages_id.Add(pagedetails);
}

Answer 2

尝试将其添加到您的班级：

public Clients()
{
  this.Manage_Pages_id = new List<ManagePages>();
}

Answer 3

您需要先初始化列表，然后再添加任何内容。 您可以在Clients类的构造函数中或在您的调用代码中进行此操作（如Artem建议的那样）。

public class Clients
{
    //properties

    public Clients()
    {
         Manage_Pages_id = new List<ManagePages>();
    }
}