IQueryable 繼承自IEnumerable
先舉例：
#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("用擴展方法可以過濾某個字段，然后把當前實體輸出");
peopleList.DistinctBy(i => new { i.UserName }).ToList().ForEach(i => Console.WriteLine(i.UserName + i.Email));
Console.WriteLine("默認方法，集合中有多個字段，當所有字段發生重復時，distinct生效，這與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("集合中有一個字段，將這個字段重復的過濾，並輸出這個字段");
peopleList.Select(i => new { i.UserName }).Distinct().ToList().ForEach(i => Console.WriteLine(i.UserName));
#endregion
該擴展方法貼出：
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;
}
}
}
}