c# - 如何逐一生成IEnumerable的元素

Question

假设我有一个 IEnumerable<Thing> enumerable对象和每个 Thing生成此列表的成本很高。

当我去执行下面的代码时，当代码到达第 1 行时会有相当大的开销，因为据我所知，整个列表是在开始迭代之前就在那里生成的。

foreach (Thing t in enumerable)   // line 1
{                                 // line 2
    DoStuffWith(t);               // line 3
}

有没有办法生成每个 Thing仅在需要时？像这样

while(enumerable.HasMoreThings())
{
    var e = enumerable.GenerateNextThing();
    DoStuffWith(e);
}

原代码:

using (var db = new PolyPrintEntities())
{
    var sw = new Stopwatch();
    sw.Start();

    IEnumerable<NotificationDto> notifications = db.EmailServiceNotifications
        .Select(not => new NotificationDto
        {
            Notification_ID = not.Notification_ID,
            StoredProcedure = not.StoredProcedure,
            SubjectLine = not.SubjectLine,
            BodyPreface = not.BodyPreface,
            Frequency = not.Frequency,
            TakesDateRange = not.TakesDateRange,
            DateRangeIntervalHours = not.DateRangeIntervalHours,
            TakesAssociateId = not.TakesAssociateId,
            NotificationType = not.NotificationType,
            Associates = not.SubscriptionEvent.SubscriptionSubscribers
                .Select(ss => new AssociateDto
                {
                    Record_Number = ss.Associate.Record_Number,
                    Name = ss.Associate.Name,
                    Email = ss.Associate.EMail
                })
        });

    sw.Stop();
    Console.WriteLine($"Enumeration:{sw.ElapsedMilliseconds}"); //about .5 seconds
    sw.Reset();

    sw.Start();

    // List is generated here, this is where lag happens. 
    int i = 0;

    foreach (var n in notifications)
    {
        if (i == 0)
        {
            sw.Stop();
            Console.WriteLine($"Generation:{sw.ElapsedMilliseconds}"); //about 10 seconds
            i++;
        }

        var ret = n.GetEmails();
        db.EmailQueues.AddRange(ret);
    }

    db.SaveChanges();
}

Answer 1

简单yield他们。

while(enumerable.HasMoreThings()) 
{
    yield return enumerable.GenerateNextThing();
}

每次有人想要下一个项目(可能是此函数的调用者)时，此函数都会延迟创建和yield项目。