Linq 选择而不是 foreach

Question

我有两个收藏

var campaigns = new List<Campaigns>();
IEnumerable<CampaignsDb> campaignsFromDB = db.Campaigns
        .Where(c => (c.IsDeleted == false))
        .OrderBy(c => c.ScheduleTime)
        .ToArray();

接下来我将使用foreach() 填充另一个集合：

foreach (var campaign in campaignsFromDB)
{
    campaigns.Add(new Campaigns { CampaignID = campaign.CampaignID, OwnerID = campaign.CreatedBy, AccountID = campaign.AccountID });
}

我可以使用Select() linq 方法代替foreach 循环吗？

Answer 1

除非您需要以原始空列表开头（例如，首先添加一些其他广告系列），否则您可以使用：

var campaigns = campaignsFromDB
    .Select(c => new Campaigns
           {
               CampaignID = c.CampaignID,
               OwnerID = c.CreatedBy,
               AccountID = c.AccountID
           })
    .ToList();

但是，为了填充campaignsFromDB，它仍会从数据库中获取完整的活动信息。如果您出于其他原因需要该数组，那很好，但如果不需要，您可以通过将投影放入查询中来提高效率：

var campaigns = db.Campaigns
    .Where(c => !c.IsDeleted)
    .OrderBy(c => c.ScheduleTime)
    .Select(c => new Campaigns
           {
               CampaignID = c.CampaignID,
               OwnerID = c.CreatedBy,
               AccountID = c.AccountID
           })
    .ToList();

Answer 2

是的。只是

var campaigns = db.Campaigns
        .Where(c => (c.IsDeleted == false))
        .OrderBy(c => c.ScheduleTime)
        .Select(c => new Campaigns { CampaignID = c.CampaignID, OwnerID = c.CreatedBy, AccountID = c.AccountID })
        .ToList();

或者如果您也需要数据库中的源集

var campaignsFromDB = db.Campaigns
        .Where(c => (c.IsDeleted == false))
        .OrderBy(c => c.ScheduleTime)
        .ToArray();

var campaigns = campaignsFromDB
        .Select(c => new Campaigns { CampaignID = c.CampaignID, OwnerID = c.CreatedBy, AccountID = c.AccountID })
        .ToList();

Answer 3

是的，像这样：

var campaigns = db.Campaigns
    .Where(c => !c.IsDeleted)
    .OrderBy(c => c.ScheduleTime)
    .Select(c => new Campaigns
    {
        CampaignID = c.CampaignID,
        OwnerID = c.CreatedBy, 
        AccountID = c.AccountID
    })
    .ToList();