完美解决c# distinct不好用的问题

#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

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;
　　　　　　　　}
　　　　　　}
　　　　　}
}

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();

var divisions = db.MarketingLookup.AsEnumerable()
   .Select(ml=>ml.Division)
   .OrderBy(division=>division) 
   .Distinct()
   .Select(division => new SelectListItem
   {
     Text = division,
     Value = division
   }).ToList();