Collection.AsQueryable（） 在使用 AutoMapper 映射时返回 NullReferenceException

问：

我对编程还有点陌生。我的 LessonsService 中有以下方法：

public async Task<IEnumerable<T>> GetUserLessonsTableDataAsync<T>(string userId, string sortColumn, string sortColumnDirection, string searchValue)
{
    var user = await this.usersRepository
        .AllAsNoTracking()
        .Include(x => x.Trainer)
        .ThenInclude(t => t.Lessons)
        .Include(x => x.Member)
        .ThenInclude(m => m.Lessons)
        .ThenInclude(lm => lm.Lesson)
        .FirstOrDefaultAsync(x => x.Id == userId);

    IQueryable<Lesson> lessons;

    if (user.Trainer != null)
    {
        lessons = user.Trainer.Lessons.AsQueryable();
    }
    else
    {
        var lessonsMembers = user.Member.Lessons.Where(x => x.MemberId == user.MemberId);
        var lessonsList = new List<Lesson>();

        foreach (var lessonMember in lessonsMembers)
        {
            lessonsList.Add(lessonMember.Lesson);
        }

        lessons = lessonsList.AsQueryable();
    }

    var lessonData = from lesson in lessons select lesson;

    if (!(string.IsNullOrEmpty(sortColumn) && string.IsNullOrEmpty(sortColumnDirection)))
    {
        lessonData = lessonData.OrderBy(sortColumn + " " + sortColumnDirection);
    }

    if (!string.IsNullOrEmpty(searchValue))
    {
        lessonData = lessonData.Where(l => l.Topic.Contains(searchValue)
                            || l.StartingTime.Equals(searchValue)
                            || l.Group.Name.Contains(searchValue)
                            || l.Trainer.User.FirstName.Contains(searchValue)
                            || l.Trainer.User.LastName.Contains(searchValue));
    }

    return lessonData.To<T>().ToList();
}

出于某种原因，此代码会引发 System.NullReferenceException，并显示消息“对象引用未设置为对象的实例”。我调试并注意到 lessonData 不是 null，它充满了来自数据库的数据。显然，映射方法.To< T >（） 无法正常工作，因为它返回以下内容：

我的控制器是：

[HttpPost]
public async Task<IActionResult> GetUserLessons()
{
    try
    {
        var userId = this.HttpContext.Session.GetString("userId");
        var draw = this.Request.Form["draw"].FirstOrDefault();
        var start = this.Request.Form["start"].FirstOrDefault();
        var length = this.Request.Form["length"].FirstOrDefault();
        var sortColumn = this.Request.Form["columns[" + this.Request.Form["order[0][column]"].FirstOrDefault() + "][name]"].FirstOrDefault();
        var sortColumnDirection = this.Request.Form["order[0][dir]"].FirstOrDefault();
        var searchValue = this.Request.Form["search[value]"].FirstOrDefault();
        int pageSize = length != null ? Convert.ToInt32(length) : 0;
        int skip = start != null ? Convert.ToInt32(start) : 0;
        int recordsTotal = 0;

        var lessonData = await this.lessonsService.GetUserLessonsTableDataAsync<LessonViewModel>(userId, sortColumn, sortColumnDirection, searchValue);

        recordsTotal = lessonData.Count();

        var data = lessonData.Skip(skip).Take(pageSize).ToList();
        var jsonData = new { draw, recordsFiltered = recordsTotal, recordsTotal, data };

        return this.Ok(jsonData);
    }
    catch (Exception e)
    {
        throw;
    }
}

堆栈跟踪中的异常消息如下：

   at System.Linq.Enumerable.SelectIPartitionIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at ChessBurgas64.Services.Data.LessonsService.<GetUserLessonsTableDataAsync>d__15`1.MoveNext() in F:\ChessclubBurgas64WebApp\ChessBurgas64\Services\ChessBurgas64.Services.Data\LessonsService.cs:line 211
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at ChessBurgas64.Web.Controllers.UsersController.<GetUserLessons>d__17.MoveNext() in F:\ChessclubBurgas64WebApp\ChessBurgas64\Web\ChessBurgas64.Web\Controllers\UsersController.cs:line 171

我有一个静态类（其中方法.To< T >（） 被定位），在 lessonsService 中引用：

public static class QueryableMappingExtensions
{
    public static IQueryable<TDestination> To<TDestination>(
        this IQueryable source,
        params Expression<Func<TDestination, object>>[] membersToExpand)
    {
        if (source == null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        return source.ProjectTo(AutoMapperConfig.MapperInstance.ConfigurationProvider, null, membersToExpand);
    }

    public static IQueryable<TDestination> To<TDestination>(
        this IQueryable source,
        object parameters)
    {
        if (source == null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        return source.ProjectTo<TDestination>(AutoMapperConfig.MapperInstance.ConfigurationProvider, parameters);
    }
}

我注意到这仅发生在导航属性上（在本例中为用户。Trainer.Lessons））。 如果我直接从存储库中获取课程，lessons = this.lessonsRepository.All（）。Where（...）;，不会有问题，一切都会正常工作，因为集合将是 Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable 而不是 System.Collections.Generic.HashSet'1[Lesson]。所以我想问题在于方法.AsQueryable（） 并没有真正将课程转换为 Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable，这是映射方法所必需的。要< >（） 才能正常工作？解决此问题的最佳方法是什么？