通過LINQ的

Question

在訪問DataList控件對象，我有以下的Linq聲明

from DataListItem dli in dlAttachments.Items 
select new Objects.BHAttachment 
{ 
    Name = ((FileUpload)dli.FindControl("fuAttachment")).HasFile ? ((FileUpload)dli.FindControl("fuAttachment")).FileName : (((HyperLink)dli.FindControl("hypCurrentAttachment")).Text != null ? ((HyperLink)dli.FindControl("hypCurrentAttachment")).Text : ""), 
    Path = ((FileUpload)dli.FindControl("fuAttachment")).HasFile ? ((FileUpload)dli.FindControl("fuAttachment")).PostedFile.FileName : "", 
    FileUpload = ((FileUpload)dli.FindControl("fuAttachment")).HasFile ? ((FileUpload)dli.FindControl("fuAttachment")) : new FileUpload(), 
    DocumentType = ((Label)dli.FindControl("lblType")).Text, 
    URL = "" 
} 

和它的作品，非常好。我的問題是關於不斷重新引用FileUpload對象。我正在重新創建（並重新完成）6次。有沒有辦法在Linq中設置它一次並引用那個單個對象？

Answer 1

可以使用let語句創建一個變量

from DataListItem dli in dlAttachments.Items 
let x = ((FileUpload)dli.FindControl("fuAttachment")) 
select new Objects.BHAttachment 
{ 
    Name = (x.HasFile ? (x.FileName : (((HyperLink)dli.FindControl("hypCurrentAttachment")).Text != null ? ((HyperLink)dli.FindControl("hypCurrentAttachment")).Text : "") 
} 

Answer 2

是，通過let clause。你可以爲你的所有物體做到這一點：

from DataListItem dli in dlAttachments.Items 
let fileUpload = dli.FindControl("fuAttachment") as FileUpload 
let hyperlink = dli.FindControl("hypCurrentAttachment") as Hyperlink 
let label = dli.FindControl("lblType") as Label 
select new Objects.BHAttachment 
{ 
    Name = fileUpload.HasFile ? fileUpload.FileName : (hyperlink.Text ?? ""), 
    ...