Question

我正在将POCO映射到模型中，代码如下所示。

// NOT NEEDED var noneRequiredUserDocuments = new List<NoneRequiredUserDocument>();
//var docs = studentDocuments.Where(x => x.RequiredUserDocumentId == null); // NOT NEEDED .ToList();
//var noneRequiredUserDocuments = docs.Select(x => new NoneRequiredUserDocument

// You can chain LINQ methods (i.e. Where and Select)
var noneRequiredUserDocuments = studentDocuments
    .Where(x => x.RequiredUserDocumentId == null)
    .Select(x => new NoneRequiredUserDocument
    {
        StudentDocument = x,
        Src = _storageService.GetFileUrl(x.FileName),
        ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(Src, 75)

    }).ToList();

我的问题在于这一行：

ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(Src, 75)

Src在上下文中不存在。

我有没有办法在select中声明一个我可以在LINQ select中重用的变量？

我不想两次致电_storageService.GetFileUrl。

Answer 1

您可以在Select内声明一个变量，如下所示： -

noneRequiredUserDocuments = docs.Select(x => 
  {
      var src= _storageService.GetFileUrl(x.FileName);
      return new NoneRequiredUserDocument
     {
        StudentDocument = x,
        Src = src,
        ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(src, 75);
     };
  }).ToList();

在query syntax这样做相当于： -

from x in docs
let src= _storageService.GetFileUrl(x.FileName)
select and so on..

Answer 2

您可以使用“let”关键字：

var list = (from x in docs
            let temp = _storageService.GetFileUrl(x.FileName)
            select new NoneRequiredUserDocument
            {
                StudentDocument = x,
                Src = temp,
                ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(temp, 75)
            }).ToList();

Answer 3

你可以创建一个常规代码块，而不是在lambda表达式中运行单个语句，这样你就可以声明一个变量＆＃34; src＆＃34;并且它将在整个块中可用 - 遵循常规范围规则。

noneRequiredUserDocuments = docs.Select(x => {
      var  src = _storageService.GetFileUrl(x.FileName);
      return 
             new NoneRequiredUserDocument
             {
                 StudentDocument = x,
                 Src = src,
                 ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(Src, 75)
              };
    }).ToList();

Answer 4

您可以使用所有必要的数据引入匿名类型的临时集合：

noneRequiredUserDocuments = docs
   .Select(x => new { Data = x, Src = _storageService.GetFileUrl(x.FileName) }
   .Select(x => new NoneRequiredUserDocument
   {
       StudentDocument = x.Data,
       Src = x.Src
       ThumbnailImageUrl = ImageHelper.ThumbnailImageUrl(x.Src, 75)
   }).ToList();

在LINQ select中声明变量（x =＆gt; new

4 个答案: