Entity framework Include（），其中where子句失败

这很奇怪，有人能解释一下我为什么这样做很好：

_db.Tasks.Include(t => t.Category).ThenInclude(c => c.ApplicationUser)
                .ToList();

虽然这会引发异常：

_db.Tasks.Include(t=>t.Category).ThenInclude(c=>c.ApplicationUser)
                .Where(t => t.Category.ApplicationUser.UserName == username)
                .ToList();

   at lambda_method(Closure , InternalEntityEntry )
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.SimpleNonNullableDependentKeyValueFactory`1.TryCreateFromCurrentValues(InternalEntityEntry entry, TKey& key)
   at Microsoft.EntityFrameworkCore.Query.Internal.WeakReferenceIdentityMap`1.CreateIncludeKeyComparer(INavigation navigation, InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.IncludeCore(Object entity, INavigation navigation)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.Include(QueryContext queryContext, Object entity, IReadOnlyList`1 navigationPath, IReadOnlyList`1 relatedEntitiesLoaders, Int32 currentNavigationIndex, Boolean queryStateManager)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.Include(QueryContext queryContext, Object entity, IReadOnlyList`1 navigationPath, IReadOnlyList`1 relatedEntitiesLoaders, Boolean queryStateManager)
   at Microsoft.EntityFrameworkCore.Query.Internal.GroupJoinInclude.Include(Object entity)
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.<_GroupJoin>d__26`4.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__163`3.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.<_TrackEntities>d__15`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at _:line 56

由于某些原因，.Include（）不适用于.Where子句。有什么想法吗

例外情况：

_db.Tasks.Include(t=>t.Category).ThenInclude(c=>c.ApplicationUser)
                .Where(t => t.Category.ApplicationUser.UserName == username)
                .ToList();

   at lambda_method(Closure , InternalEntityEntry )
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.SimpleNonNullableDependentKeyValueFactory`1.TryCreateFromCurrentValues(InternalEntityEntry entry, TKey& key)
   at Microsoft.EntityFrameworkCore.Query.Internal.WeakReferenceIdentityMap`1.CreateIncludeKeyComparer(INavigation navigation, InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.IncludeCore(Object entity, INavigation navigation)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.Include(QueryContext queryContext, Object entity, IReadOnlyList`1 navigationPath, IReadOnlyList`1 relatedEntitiesLoaders, Int32 currentNavigationIndex, Boolean queryStateManager)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryBuffer.Include(QueryContext queryContext, Object entity, IReadOnlyList`1 navigationPath, IReadOnlyList`1 relatedEntitiesLoaders, Boolean queryStateManager)
   at Microsoft.EntityFrameworkCore.Query.Internal.GroupJoinInclude.Include(Object entity)
   at Microsoft.EntityFrameworkCore.Query.QueryMethodProvider.<_GroupJoin>d__26`4.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__163`3.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.<_TrackEntities>d__15`2.MoveNext()
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at _:line 56

型号：

public class Task
{
    [Key]
    public int TaskId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public DateTime Timestamp { get; set; }
    public int CategoryId { get; set; }

    public virtual Category Category { get; set; }
}

public class Category
{
    [Key]
    public int CategoryId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public DateTime Timestamp { get; set; }

    public string UserId { get; set; }
    [ForeignKey("UserId")]
    public virtual ApplicationUser ApplicationUser { get; set; }

    public virtual ICollection<Task> Tasks { get; set; }
}

public class ApplicationUser : IdentityUser
{
    public virtual ICollection<Category> Categories { get; set; }
}

公共类任务
{
[关键]
public int TaskId{get；set；}
公共字符串名称{get；set；}
公共字符串说明{get；set；}
公共日期时间时间戳{get；set；}
public int CategoryId{get；set；}
公共虚拟类别{get；set；}
}
公共类类别
{
[关键]
public int CategoryId{get；set；}
公共字符串名称{get；set；}
公共字符串说明{get；set；}
公共日期时间时间戳{get；set；}
公共字符串用户标识{get；set；}
[外键（“用户ID”）]
公共虚拟应用程序用户应用程序用户{get；set；}
公共虚拟ICollection任务{get；set；}
}
公共类应用程序用户：IdentityUser
{
公共虚拟ICollection类别{get；set；}
}

在浪费了半天之后，我终于解决了这个问题。我无法使用.Include（）和.Where（）实现所需的结果，但这正是我所需要的。如果您能找到方法使其与.Include（）和.Where（）一起工作，请告诉我，因为我非常好奇：

return _db.Tasks
                    .Where(t => t.Category.ApplicationUser.UserName == username)
                    .Select(t => new
                    {
                        t,
                        t.Category
                    })
                    .AsEnumerable()
                    .Select(t => t.t)
                    .ToList();

---

在浪费了半天之后，我终于解决了这个问题。我无法使用.Include（）和.Where（）实现所需的结果，但这正是我所需要的。如果您能找到方法使其与.Include（）和.Where（）一起工作，请告诉我，因为我非常好奇：

return _db.Tasks
                    .Where(t => t.Category.ApplicationUser.UserName == username)
                    .Select(t => new
                    {
                        t,
                        t.Category
                    })
                    .AsEnumerable()
                    .Select(t => t.t)
                    .ToList();

---

例外情况是什么？还有，您使用的是什么版本的EF？显示您的

类别

和

应用程序用户

它是EF核心，异常附件您只显示堆栈跟踪。显示完整的异常消息“对象引用未设置为对象的实例”。异常是什么？还有，您使用的是什么版本的EF？显示您的

类别

和

应用程序用户

它是EF核心，异常附件您只显示堆栈跟踪。显示完整的异常消息“对象引用未设置为对象的实例”