Question

我有这段代码：

IQueryable<Dealer> dealers = db.Dealers.Include(x => x.Statuses);
this.view.Table = dealers
    .Select(x => new DealerRow
    {
        Id = x.Id,
        LastStatus = x.Statuses.FirstOrDefault()
    })
    .ToList();

以this.view.Table = dealers开头的行是发生此异常的地方：

对象引用未设置为对象的实例。

堆栈跟踪如下：

[NullReferenceException: Object reference not set to an instance of an object.]
   MySql.Data.Entity.SelectStatement.AddDefaultColumns(Scope scope) +293
   MySql.Data.Entity.SelectStatement.Wrap(Scope scope) +36
   MySql.Data.Entity.SelectGenerator.Visit(DbApplyExpression expression) +236
   System.Data.Entity.Core.Common.CommandTrees.DbApplyExpression.Accept(DbExpressionVisitor`1 visitor) +64
   MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type) +32
   MySql.Data.Entity.SelectGenerator.HandleJoinExpression(DbExpressionBinding left, DbExpressionBinding right, DbExpressionKind joinType, DbExpression joinCondition) +77
   MySql.Data.Entity.SelectGenerator.Visit(DbJoinExpression expression) +48
   System.Data.Entity.Core.Common.CommandTrees.DbJoinExpression.Accept(DbExpressionVisitor`1 visitor) +64
   MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type) +32
   MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type) +22
   MySql.Data.Entity.SelectGenerator.Visit(DbSortExpression expression) +76
   System.Data.Entity.Core.Common.CommandTrees.DbSortExpression.Accept(DbExpressionVisitor`1 visitor) +64
   MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type) +32
   MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type) +22
   MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression expression) +53
   System.Data.Entity.Core.Common.CommandTrees.DbProjectExpression.Accept(DbExpressionVisitor`1 visitor) +64
   MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree tree) +68
   MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree) +328
   System.Data.Entity.Core.Common.DbProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext) +13
   System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext) +127
   System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver, BridgeDataReaderFactory bridgeDataReaderFactory, ColumnMapFactory columnMapFactory) +1420
   System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext) +103
   System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext) +127
   System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.CreateCommandDefinition(ObjectContext context, DbQueryCommandTree tree) +151
   System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.Prepare(ObjectContext context, DbQueryCommandTree tree, Type elementType, MergeOption mergeOption, Boolean streaming, Span span, IEnumerable`1 compiledQueryParameters, AliasGenerator aliasGenerator) +161
   System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption) +1027
   System.Data.Entity.Core.Objects.<>c__DisplayClass7.<GetResults>b__6() +39
   System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction(Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess) +288
   System.Data.Entity.Core.Objects.<>c__DisplayClass7.<GetResults>b__5() +155
   System.Data.Entity.Infrastructure.DefaultExecutionStrategy.Execute(Func`1 operation) +9
   System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption) +281
   System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0() +11
   System.Data.Entity.Internal.LazyEnumerator`1.MoveNext() +45
   System.Collections.Generic.List`1..ctor(IEnumerable`1 collection) +387
   System.Linq.Enumerable.ToList(IEnumerable`1 source) +58
   DealersPresenter.RefreshTable() in File1.cs:98
   DealersListPage.Filter(Object sender, EventArgs e) in File2.aspx.cs:37
   System.Web.UI.WebControls.Button.OnClick(EventArgs e) +9659822
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +108
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +12
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +15
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +35
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1639

具体来说，如果我从选择中注释LastStatus，程序编译并完美运行。所以我假设Statuses是问题所在。

一对多关系设置如下：

[Table("dealers")]
public class Dealer
{

    public Dealer()
    {
        Statuses = new List<DealerStatus>();
    }

    [Key]
    [Column("id")]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    public virtual ICollection<DealerStatus> Statuses { get; set; }

}

和

[Table("dealer_statuses")]
public class DealerStatus
{

    [Key]
    [Column("id")]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    [Required]
    [ForeignKey("Dealer")]
    [Column("id_dealer")]
    public int DealerId { get; set; }
    public virtual Dealer Dealer { get; set; }

    [Required]
    [Column("note")]
    public string Note { get; set; }

}

我已经阅读了其他5个问题。我试着打电话给Include和类似的人。我还检查了Dealer的默认构造函数初始化一个空列表。

造成这种异常的原因是什么？

Answer 1

尝试从集合中移除virtual以及其他类型的热切加载后，我在Select调用之后移动了ToList（这将会急切地加载所有内容）但是至少它编译并运行。

更新到最新的6.9.9 MySql.Data.Entity对我来说也不起作用。

导航属性上的“对象引用未设置为对象的实例”

1 个答案: