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.

来源：https://www.cnblogs.com/daimaxuejia/p/10971869.html