Linq在select语句中调用方法非常慢

Question

我正在尝试查找是否有一种方法可以从我的 linq select 语句中调用一个方法来构建一个不会显着减慢它的对象列表。这背后的原因是我也想在尝试仅获取一个对象时调用相同的方法并且不想维护两个版本（即，如果我将另一个字段添加到对象或想要渲染其中一个字段不同，我不必在多个地方更改它）。

在下面的示例中，TEST 1 的运行速度比 TEST 2 快 100 倍：

// Start timer
var timer = new Stopwatch();
timer.Start();

var test = (from job in dc.Jobs
        where !job.archived
        select new JobExtended()
        {
            JobId = job.jobId,
            NodeId = job.nodeId,
            JobName = job.name != string.Empty ? job.name : "TBC",
            Salary = job.salary,
            RecruiterId = job.fkRecruiterId,
            RecruiterNodeId = job.JobRecruiter != null ? job.JobRecruiter.recruiterNodeId : null,
            RecruiterName = job.JobRecruiter != null ? job.JobRecruiter.name : string.Empty,
            LocationId = job.fkLocationId,
            Location = job.refJobLocation != null ? job.refJobLocation.jobLocation : "",
            ContractTypeId = job.fkContractTypeId,
            ContractType = job.refJobContractType != null ? job.refJobContractType.contractType : "",
            CategoryId = job.fkCategoryId,
            Category = job.refJobCategory != null ? job.refJobCategory.category : "",
            ClosingDate = job.closingDate,
            Featured = job.featured,
            JobOfTheWeek = job.jobOfTheWeek,
            PublishedDate = job.publishedDate,
            Url = "/jobs/" + job.name.Replace(" ", "-").Replace("&", "and").Replace("'", "") + (job.fkLocationId.HasValue ? "-in-" + job.refJobLocation.jobLocation.Replace(" ", "-").Replace("&", "and").Replace("'", "") : "") + "-jn" + job.jobId,
            CreatedOn = job.createdOnDate,
            PrintWidth = job.printWidth,
            PrintHeight = job.printHeight,
            UntilFilled = (job.untilFilled != null && job.untilFilled.Value),
            AdvertCost = job.advertCost,
            DatesToShow = job.relJobDates.Where(x => x.fkJobId == job.jobId).Select(x => x.date).ToList(),
            IsParentJob = job.relLinkedJobs != null && job.relLinkedJobs.Any(x => x.fkParentJobId == job.jobId),
            IsAlternateWeekJob = job.alternateWeek != null && job.alternateWeek.Value,
            Archived = job.archived,
            LastModifiedDate = job.lastModifiedDate,
            RecruiterContactDetails = job.recruiterContactDetails
        }).ToList();

// Stop timer
timer.Stop();

// Output info
litTest.Text = "TEST 1 in " + timer.Elapsed.TotalSeconds + " seconds<br/>";

//Start timer
timer = new Stopwatch();
timer.Start();

var test2 = (from job in dc.Jobs
            where !job.archived
            select GetJobDetails(job)).ToList();

//Stop timer
timer.Stop();

//Output info
litTest.Text += "TEST 2 in " + timer.Elapsed.TotalSeconds + " seconds<br/>";

这是 TEST 2 调用的方法，它应该创建与 TEST 1 中返回的对象相同的对象：

public static JobExtended GetJobDetails(Data.Job job)
{
    return new JobExtended()
    {
        JobId = job.jobId,
        NodeId = job.nodeId,
        JobName = job.name != string.Empty ? job.name : "TBC",
        Salary = job.salary,
        RecruiterId = job.fkRecruiterId,
        RecruiterNodeId = job.JobRecruiter != null ? job.JobRecruiter.recruiterNodeId : null,
        RecruiterName = job.JobRecruiter != null ? job.JobRecruiter.name : string.Empty,
        LocationId = job.fkLocationId,
        Location = job.refJobLocation != null ? job.refJobLocation.jobLocation : "",
        ContractTypeId = job.fkContractTypeId,
        ContractType = job.refJobContractType != null ? job.refJobContractType.contractType : "",
        CategoryId = job.fkCategoryId,
        Category = job.refJobCategory != null ? job.refJobCategory.category : "",
        ClosingDate = job.closingDate,
        Featured = job.featured,
        JobOfTheWeek = job.jobOfTheWeek,
        PublishedDate = job.publishedDate,
        Url = "/jobs/" + job.name.Replace(" ", "-").Replace("&", "and").Replace("'", "") + (job.fkLocationId.HasValue ? "-in-" + job.refJobLocation.jobLocation.Replace(" ", "-").Replace("&", "and").Replace("'", "") : "") + "-jn" + job.jobId,
        CreatedOn = job.createdOnDate,
        PrintWidth = job.printWidth,
        PrintHeight = job.printHeight,
        UntilFilled = (job.untilFilled != null && job.untilFilled.Value),
        AdvertCost = job.advertCost,
        DatesToShow = job.relJobDates.Where(x => x.fkJobId == job.jobId).Select(x => x.date).ToList(),
        IsParentJob = job.relLinkedJobs != null && job.relLinkedJobs.Any(x => x.fkParentJobId == job.jobId),
        IsAlternateWeekJob = job.alternateWeek != null && job.alternateWeek.Value,
        Archived = job.archived,
        LastModifiedDate = job.lastModifiedDate,
        RecruiterContactDetails = job.recruiterContactDetails
    };
}

这样做的原因是因为我希望能够调用“GetJobDetails”来返回单个作业，例如：

    public JobExtended GetJobDetails(int jobId)
    {
        using (DataContext dc = new DataContext())
        {
            return dc.Jobs.Where(x => x.jobId == jobId).Select(j => GetJobDetails(j)).FirstOrDefault();
        }
    }

这样做只会让我只需要更新“GetJobDetails”方法，例如，如果我决定添加一个新字段来更改“Url”值的生成方式，但这样做会慢很多.有没有办法解决这个问题，我已经尝试了以下似乎没有帮助的方法：

var test3 = (from job in dc.Jobs
                where !job.archived
                select job).AsEnumerable()
                .Select(GetJobDetails).ToList();

var test4 = (from job in dc.Jobs
                where !job.archived
                select GetJobDetails(job));
var test4a = test4.ToList();

Answer 1

TEST 1 更快的原因是查询在服务器上执行一次并且只返回选定的字段。

var test = (from job in dc.Jobs
    where !job.archived
    select new JobExtended()
    {
        JobId = job.jobId,
        NodeId = job.nodeId,
        ...
    }).ToList();

当您在 TEST 2 中调用 GetJobDetails 时，参数 j 需要先具体化，然后才能作为参数发送到 GetJobDetails。所以有多个完整对象的调用。

return dc.Jobs.Where(x => x.jobId == jobId).Select(j => GetJobDetails(j)).FirstOrDefault();

为了实现你想要的东西，你应该使用扩展方法。这个扩展了 IQueryable。

    public static IEnumerable<JobExtended> SelectJobExtended(this IQueryable<Data.Job> query)
    {
        return query
            .Select(o => new JobExtended()
            {
                JobId = job.jobId,
                NodeId = job.nodeId,
                ...
            }
    }

然后你可以调用：

dc.Jobs.Where(x => x.jobId == jobId).SelectJobExtended().FirstOrDefault();

Answer 2

我以前见过这种问题。如果我记得，我们所做的是“堆叠”查询。

public IEnumerable<JobExtended> ConvertToJobExtended(IEnumerable<Job> jobs)
{
    return
        from job in jobs
        select new JobExtended()
        {
            MyString = job.MyInt.ToString(),
            ...
        };
}

那么你可以通过以下方式调用它：

var query = (from job in dc.Jobs
        where !job.archived
        select job;

var test2 = ConvertToJobExtended(query).ToList();

这里有很多选择......我希望这朝着你想要做的正确方向发展。