System.NullReferenceException未將對象引用設置為對象的實例。 關於模型實例化 - 有時候

Question

我收到System.NullReferenceException對象引用未設置為模型實例化時的對象實例，但有時只是。 我的用戶有時會遇到錯誤，但一旦他們重新提交，則不會發生異常。 我無法弄清楚錯誤發生的原因。

這是ELMAH的錯誤日志：

System.NullReferenceException: Object reference not set to an instance of an object.


at Plus.Controllers.Review.ReviewNoteController.Save(ReviewNoteViewModel viewModel) in C:\Users\Rhys\Documents\Development\witplus-master\Plus\Controllers\Review\ReviewNoteController.cs:line 139
   at lambda_method(Closure , ControllerBase , Object[] )
   at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters)
   at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
   at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.<>c__DisplayClass2b.<BeginInvokeAction>b__1c()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult)
   at System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult)
   at System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult)
   at System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult)
   at System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult)
   at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResultBase`1.End()
   at System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult)
   at System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

這是我的代碼：

// Update Note
                if (viewModel.ID != 0)
                {
                    ReviewNote rn = _repository.Get<ReviewNote>(viewModel.ID);
                    MapViewModelToModel(rn, viewModel);
                    _repository.Update<ReviewNote>(rn);
                }
                // Create Note
                else
                {
                    ReviewAnswer ra = _repository.Get<ReviewAnswer>(viewModel.AnswerId);
                    bool newAnswer = false;
                    // Check if practice or checklists
                    if (viewModel.Referrer == Referrer.ReviewPractice)
                    {
                        // Create a new answer if it hasn't been answered yet. Could have made a bool in the GET method, but it's best to recheck to make sure no-one has answered in the mean time.
                        if (hasNoAnswer_PracticeLevel(viewModel.QuestionId))
                        {
                            ra = new ReviewAnswer();
                            ra.entityLevelAudit = true;
                            ra.entityId = this.LoggedInEntity.EntityId;
                            ra.questionId = viewModel.QuestionId;
                            ra.lastUpdateDate = DateTime.Now;
                            _repository.Save<ReviewAnswer>(ra);
                            newAnswer = true;
                            viewModel.AnswerId = ra.Id;
                        }
                    }
                    else
                    {
                        // Create a new answer if it hasn't been answered yet.
                        if (hasNoAnswer_Checklist(viewModel.QuestionId))
                        {
                            ra = new ReviewAnswer();
                            ra.entityLevelAudit = false;
                            ra.entityId = this.LoggedInEntity.EntityId;
                            ra.questionId = viewModel.QuestionId;
                            ra.userId = this.SelectedUser.UserId;
                            ra.lastUpdateDate = DateTime.Now;
                            _repository.Save<ReviewAnswer>(ra);
                            newAnswer = true;
                            viewModel.AnswerId = ra.Id;
                        }
                    }

                    ReviewNote rn = new ReviewNote();
                    rn.auditID = ra.Id;
                    rn.Type = (int)this.LoggedInUser.ReviewTypeId; //Audit Note type is the user audit type
                    MapViewModelToModel(rn, viewModel);
                    _repository.Save<ReviewNote>(rn);
                }

錯誤發生在第139行，即：

ReviewNote rn = new ReviewNote(); // 6th last line.

ReviewNote模型代碼是：

using Plus.Models.UserModel;
using System;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;
using System.Web;
using System.Web.Mvc;

namespace Plus.Models.Review
{
    public class ReviewNote : BaseModel
    {
        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity), Column("AuditNoteId")]
        public int ID { get; set; } // AuditNoteId

        public int auditID { get; set; }
        [ForeignKey("auditID")]
        public virtual ReviewAnswer Answer { get; set; }

        [StringLength(10000), AllowHtml, Column("noteText")]
        public string Text { get; set; } //noteText

        [StringLength(200), Column("noteFile")]
        public string URL { get; set; } //noteFile

        [DataType(DataType.DateTime), Column("lastUpdateDate")]
        public DateTime LastUpdated { get; set; }//lastUpdateDate

        [Column("showNoteToAll")]
        public bool VisibleToAll { get; set; } //ShowNoteToAll

        [Required]
        public int NoteBy { get; set; } //NoteBy
        [ForeignKey("NoteBy")]
        public virtual User By { get; set; }

        [Column("noteType")]
        public int Type { get; set; } //NoteType
    }
}

這是BaseModel代碼：

public abstract class BaseModel
    {
        [NotMapped, JsonIgnore]
        public Alias Alias { get; set; }

        [NotMapped, JsonIgnore]
        public IDictionary<string, object> AliasDictionary { get; set; }

        [NotMapped, JsonIgnore]
        public AppUserDTO LoggedInAppUser { get; set; }

        [NotMapped, JsonIgnore]
        public UserDTO LoggedInUser { get; set; }

        [NotMapped, JsonIgnore]
        public EntityDTO LoggedInEntity { get; set; }

        [NotMapped, JsonIgnore]
        public ResourceDTO LoggedInUsersResource { get; set; }

        [NotMapped, JsonIgnore]
        public virtual ICollection<MenuItemClaim> MenuSystemTreeClaims { get; set; }

        [NotMapped, JsonIgnore]
        public IStateProvider UserSession { get; set; }

        [NotMapped, JsonIgnore]
        public UserDTO SelectedUser { get; set; }

        [NotMapped, JsonIgnore]
        public TermsAndConditions TAC { get; set; }

        [NotMapped, JsonIgnore]
        public string PageClaimValue { get; set; }
    }

我無法理解為什么在ReviewNote的實例化中會出現NullReferenceException。 如果有人能提出建議，那將是最有幫助的，因為它對我的用戶來說越來越成為一個問題。

我的項目是帶有Identity 2和Entity Framework 6.2的ASP.NET MVC5。 我認為錯誤只是在我將Entity Framework 6.1升級到6.2之后才開始，但這只是猜測。

謝謝。

Answer 1

正如評論者所指出的那樣，在構造ReviewNote似乎沒有任何東西可以產生NullReferenceException ，並且在你自己的代碼中沒有任何代碼出現在stacktrace中。

過去我遇到過一些問題，堆棧跟蹤中給出的行號與發生實際錯誤的位置相距幾行。 如果您無法使用附加的調試器來捕獲它，我可以建議您添加某種信息來補充行號。 一個簡單的方法是遞增整數：

int debugId = 0;
try
{
    ReviewNote rn = new ReviewNote(); // debugId == 0
    ++debugId;
    rn.auditID = ra.Id; // debugId == 1
    ++debugId;
    rn.Type = (int)this.LoggedInUser.ReviewTypeId; // debugId == 2
    ++debugId;
    MapViewModelToModel(rn, viewModel); // debugId == 3
    ++debugId;
    _repository.Save<ReviewNote>(rn); // debugId == 4
}
catch (NullReferenceException nre)
{
    throw new Exception("debugId:" + debugId, nre);
}

debugId的值將告訴您發生故障的行