完美解决c# distinct不好用的问题

当一个结合中想根据某一个字段做去重方法时使用以下代码

IQueryable 继承自IEnumerable

先举例:

#region linq to object

List<People> peopleList = new List<People>();

peopleList.Add(new People { UserName = "zzl", Email = "1" });

peopleList.Add(new People { UserName = "zzl", Email = "1" });

peopleList.Add(new People { UserName = "lr", Email = "2" });

peopleList.Add(new People { UserName = "lr", Email = "2" });

Console.WriteLine("用扩展方法可以过滤某个字段,然后把当前实体输出");

peopleList.DistinctBy(i => new { i.UserName }).ToList().ForEach(i => Console.WriteLine(i.UserName + i.Email));

Console.WriteLine("默认方法,集合中有多个字段,当所有字段发生重复时,distinct生效,这与SQLSERVER相同");

peopleList.Select(i => new { UserName = i.UserName, Email = i.Email }).OrderByDescending(k => k.Email).Distinct().ToList().ForEach(i => Console.WriteLine(i.UserName + i.Email));

Console.WriteLine("集合中有一个字段,将这个字段重复的过滤,并输出这个字段");

peopleList.Select(i => new { i.UserName }).Distinct().ToList().ForEach(i => Console.WriteLine(i.UserName));

#endregion

该扩展方法贴出:

public static class EnumerableExtensions

{

  public static IEnumerable<TSource> DistinctBy<TSource, Tkey>(this IEnumerable<TSource> source, Func<TSource, Tkey> keySelector)

    {

      HashSet<Tkey> hashSet = new HashSet<Tkey>();

      foreach (TSource item in source)

      {

        if (hashSet.Add(keySelector(item)))

        {

          yield return item;

        }

      }

     }

}

补充知识:c# – .Distinct()调用不过滤

我正在尝试使用AsEnumerable将Entity Framework DbContext查询拉入IEnumerable< SelectListItem>.这将用作填充视图中下拉列表的模型属性.

但是,尽管调用了Distinct(),但每个查询都会返回重复的条目.

public IEnumerable<SelectListItem> StateCodeList { get; set; }

public IEnumerable<SelectListItem> DivCodeList { get; set; }

DivCodeList =

db.MarketingLookup.AsEnumerable().OrderBy(x => x.Division).Distinct().Select(x => new SelectListItem

{

Text = x.Division,

Value = x.Division

}).ToList();

StateCodeList =

db.MarketingLookup.AsEnumerable().OrderBy(x => x.State).Distinct().Select(x => new SelectListItem

{

Text = x.State,

Value = x.State

}).ToList();

为了使Distinct生效,如果类型是自定义类型,则序列必须包含实现IEquatable接口的类型的对象.

正如here所述:

Distinct returns distinct elements from a sequence by using the

default equality comparer to compare values.

一个解决方法,为了避免上述情况,因为我可以得出结论,你不需要整个对象而不是它的一个属性,就是将序列的每个元素投影到Division,然后创建OrderBy并调用Distinct :

var divisions = db.MarketingLookup.AsEnumerable()

.Select(ml=>ml.Division)

.OrderBy(division=>division)

.Distinct()

.Select(division => new SelectListItem

{

Text = division,

Value = division

}).ToList();

有关此问题的进一步文档,请查看here.

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