SelectMany inside SelectMany

我尝试了这两种方法，但都不起作用？怎么做？

.SelectMany(x => x.SectionEvents).SelectMany(t => t.Section)
.SelectMany(x => x.SectionEvents.SelectMany(t => t.Section))

错误：

方法的类型参数'System.Linq.Enumerable.SelectMany<TSource,TResult>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,System.Collections.Generic.IEnumerable<TResult>>)'无法根据用法推断。尝试指定类型参数明确地

EEvent.List<EEvent>("CalendarPeriodUId", ECalendarPeriod.Current().UId).Value.ToList()
                .SelectMany(x => x.SectionEvents.SelectMany(t => t.Section)).ToFCollection().Bind(ddlSection, "SectionName");

public class Event
{
    public ICollection<SectionEvent> SectionEvents { get; set; }
}
public class SectionEvent
{
    public Event Event { get; set; }
    public Section Section { get; set; }
}
public class Section
{
    public ICollection<SectionEvent> SectionEvents { get; set; }
}

var q = events.SelectMany(e => e.SectionEvents).Select(se => se.Section);