这有什么区别？ [英] What is the difference between this?

问题描述

我是MVC的初学者，现在我尝试使用asp.net mvc教程学习mvc。从那里我找到了一个代码来从dbcontext（实体框架）生成一个下拉列表。

< pre lang =c＃> public ActionResult Index（ string movieGenre， string searchString）
{
var GenreLst = new 列表与LT;串GT;（）;

var GenreQry = 来自 d in db.Movies
orderby d.Genre
选择 d.Genre;

GenreLst.AddRange（GenreQry.Distinct（））;
ViewBag.movi​​eGenre = new SelectList（GenreLst）;

var movies = 来自 m in db.Movies
选择 m;

if （！String.IsNullOrEmpty（searchString））
{
movies = movies.Where（s = < span class =code-keyword>> s.Title.Contains（searchString））;
}

if （！string.IsNullOrEmpty（movieGenre））
{
movies = movies。 Where（x = > x.Genre == movieGenre）;
}

return 查看（电影）;
}

i已更改此代码并重新确认为

  public  ActionResult Index（ string  movieGenre， string  searchString）
 {
  //   List<串GT; GenreList = new List< string>（）;  
  var  GenreQry = 来自 d   db.Movies  orderby  d.Genre  select  d.Genre; 
 GenreQry = GenreQry.Distinct（）; 
  //   GenreList.AddRange（GenreQry.Distinct（））;  
 ViewBag .movi​​eGenre =  new  SelectList（GenreQry）; 
 
  var  movies = 来自 m  in  db.Movies 选择 m; 
  if （！string.IsNullOrEmpty（searchString））
 {
 movies = movies.Where（m = >  m.Title.Contains（searchString））; 
} 
 
  if （！string.IsNullOrEmpty（movieGenre））
 {
 movies = movies。 Where（x = >  x.Genre == movieGenre）; 
} 
 
  //   return View（db.Movies.ToList（）） ;  
  return 查看（电影）; 
} 

我的代码也可以正常工作。那么GenreLst的需求是列表类型。我只是想知道区别，因为现在处于学习阶段

解决方案

Linq查询（或Linq方法，例如代码中的Distinct）返回一个IEnumerable集合，而不是List< T> - 如果您的SelectList构造函数采用IEnumerable而不是特定集合（例如List（实现IEnumerable）），您可以将它传递给任何集合。



A List< T>是一个实现IEnumerable的特定集合--SelectList构造函数的原始示例可能只接受List< T>。因此有必要创建一个专门用于通过。



但是，你总是可以生成一个：

 GenreList = GenreQry.ToList（）; 

这样做会更简单。

Iam a beginner in MVC and now iam trying to study mvc using asp.net mvc tutorials.From there i had find a code to generate a dropdown list from dbcontext(entity framework).

public ActionResult Index(string movieGenre, string searchString)
{
    var GenreLst = new List<string>();

    var GenreQry = from d in db.Movies
                   orderby d.Genre
                   select d.Genre;

    GenreLst.AddRange(GenreQry.Distinct());
    ViewBag.movieGenre = new SelectList(GenreLst);

    var movies = from m in db.Movies
                 select m;

    if (!String.IsNullOrEmpty(searchString))
    {
        movies = movies.Where(s => s.Title.Contains(searchString));
    }

    if (!string.IsNullOrEmpty(movieGenre))
    {
        movies = movies.Where(x => x.Genre == movieGenre);
    }

    return View(movies);
}

i had changed this code and re-witten as

public ActionResult Index(string movieGenre, string searchString)
        {
           // List<string> GenreList = new List<string>();
            var GenreQry = from d in db.Movies orderby d.Genre select d.Genre;
            GenreQry = GenreQry.Distinct();
            //GenreList.AddRange(GenreQry.Distinct());
            ViewBag.movieGenre = new SelectList(GenreQry);

            var movies = from m in db.Movies select m;
            if (!string.IsNullOrEmpty(searchString))
            {
                movies = movies.Where(m => m.Title.Contains(searchString));
            }

            if(!string.IsNullOrEmpty(movieGenre))
            {
                movies = movies.Where(x => x.Genre == movieGenre);
            }

            //return View(db.Movies.ToList());
            return View(movies);
        }

My code also works fine.Then what is the need of GenreLst that is of list type. I just want to know the difference because now am in a learning stage

解决方案

A Linq query (or a Linq method, such as Distinct in your code) returns an IEnumerable collection, rather than a List<T> - provided that your SelectList constructor takes an IEnumerable rather than a specific collection such as List (which implements IEnumerable) you can pass it any collection.

A List<T> is a specific collection which implements IEnumerable - it's possible that the original example of the SelectList constructor only accepted a List<T> so it was necessary to create one specifically to pass through.

However, you can always generate one:

GenreList = GenreQry.ToList();

Will do it rather simpler.

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