C# · 12月 28, 2021

c# – 为什么我在System.Data.Entity.dll(实体框架)中得到一个NullReferenceException?

我有一个列表<状态>的国家实体对象,每个状态对象都有其他对象的集合,如许可证,税金,债券等

还有一个ExpiredObjects的集合,它是任何已经过期且需要更新的对象的列表.由于某种原因,当我尝试访问一个特定状态(内华达州)时,这个属性给我一个NullReferenceException,但是对于我来说,我不知道实际上是什么,因为我没有看到任何空值任何地方.

这是我的代码抛出异常.它遍历所有状态,并将所有ExpiredObject添加到显示的视图特定集合中.我的测试代码仍然包含在内

private List<ExpiredObject> LoadAllExpiredObjects() { var list = new List<ExpiredObject>(); foreach (var state in States) { // This tells me the state is not null,and neither is state.ExpiredObjects Debug.WriteLine(string.Format(“{0}|{1}|{2}”,state.Name,state == null,state.ExpiredObjects == null)); try { var x = state.ExpiredObjects; Debug.WriteLine(x == null); // Exception occurs on the “for” line on the Nevada state only // Contents in for loop do not execute foreach (var item in x) { Debug.WriteLine(string.Format(“{0}”,item)); list.Add(item); } // This used to be the only line of code before I started testing // It fails on Nevada //list.AddRange(state.ExpiredObjects); } catch (Exception ex) { Debug.WriteLine(ex.Message); Debug.WriteLine(ex.StackTrace); } } return list; }

堆栈跟踪的错误是这样的:

A first chance exception of type ‘System.NullReferenceException’ occurred in System.Data.Entity.dllObject reference not set to an instance of an object. at System.Data.EntityKey.AddHashValue(Int32 hashCode,Object keyValue) at System.Data.EntityKey.GetHashCode() at System.Collections.Generic.GenericEqualityComparer`1.GetHashCode(T obj) at System.Collections.Generic.Dictionary`2.FindEntry(TKey key) at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key,TValue& value) at System.Data.Objects.ObjectStateManager.TryGetEntityEntry(EntityKey key,EntityEntry& entry) at System.Data.Common.Internal.Materialization.Shaper.HandleEntityAppendOnly[TEntity](Func`2 constructEntityDelegate,EntityKey entityKey,EntitySet entitySet) at lambda_method(Closure,Shaper ) at System.Data.Common.Internal.Materialization.Coordinator`1.ReadNextElement(Shaper shaper) at System.Data.Common.Internal.Materialization.Shaper`1.SimpleEnumerator.MoveNext() at System.Data.Objects.DataClasses.RelatedEnd.Merge[TEntity](IEnumerable`1 collection,MergeOption mergeOption,Boolean setIsLoaded) at System.Data.Objects.DataClasses.EntityCollection`1.Load(List`1 collection,MergeOption mergeOption) at System.Data.Objects.DataClasses.EntityCollection`1.Load(MergeOption mergeOption) at System.Data.Objects.DataClasses.RelatedEnd.Load() at System.Data.Objects.DataClasses.RelatedEnd.DeferredLoad() at System.Data.Objects.DataClasses.EntityCollection`1.GetEnumerator() at MyNamespace.Statesviewmodel.LoadAllExpiredObjects() in C:\Users\me\Desktop\MySolution\Statesviewmodel.cs:line 217

当我选择内华达州,并且尝试将DataGrid绑定到ExpiredObjects集合(如果我注释该绑定,该代码工作正常),我也得到完全相同的错误

有谁知道可能导致这个错误?

解决方法 如果只是内华达,那么它必须是数据问题,请仔细检查数据库.

总而言之,核心问题是:

>这是db-first.> …正在为在EF中标记为不为空的字段之一返回一个空值