这种更简单的方法，不使用组，只是获得区域元素，然后迭代它们，并检查它们在列表中的计数，如果它们的计数是>1，这意味着它出现超过1项，所以将其添加到Repeteditemlist

var mylist = new List<int>() { 1, 1, 2, 3, 3, 3, 4, 4, 4 };
            var distList=  mylist.Distinct().ToList();
            var Repeteditemlist = new List<int>();
            foreach (var item in distList)
            {
               if(mylist.Count(e => e == item) > 1)
                {
                    Repeteditemlist.Add(item);
                }
            }
            foreach (var item in Repeteditemlist)
            {
                Console.WriteLine(item);
            }

预期的输出:

1 3 4

另一种方法是使用HashSet:

var hash = new HashSet<int>();
var duplicates = list.Where(i => !hash.Add(i));

如果你想在你的重复列表中的唯一值:

var myhash = new HashSet<int>();
var mylist = new List<int>(){1,1,2,2,3,3,3,4,4,4};
var duplicates = mylist.Where(item => !myhash.Add(item)).Distinct().ToList();

下面是与通用扩展方法相同的解决方案:

public static class Extensions
{
  public static IEnumerable<TSource> GetDuplicates<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector, IEqualityComparer<TKey> comparer)
  {
    var hash = new HashSet<TKey>(comparer);
    return source.Where(item => !hash.Add(selector(item))).ToList();
  }

  public static IEnumerable<TSource> GetDuplicates<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)
  {
    return source.GetDuplicates(x => x, comparer);      
  }

  public static IEnumerable<TSource> GetDuplicates<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector)
  {
    return source.GetDuplicates(selector, null);
  }

  public static IEnumerable<TSource> GetDuplicates<TSource>(this IEnumerable<TSource> source)
  {
    return source.GetDuplicates(x => x, null);
  }
}

这种更简单的方法，不使用组，只是获得区域元素，然后迭代它们，并检查它们在列表中的计数，如果它们的计数是>1，这意味着它出现超过1项，所以将其添加到Repeteditemlist

var mylist = new List<int>() { 1, 1, 2, 3, 3, 3, 4, 4, 4 };
            var distList=  mylist.Distinct().ToList();
            var Repeteditemlist = new List<int>();
            foreach (var item in distList)
            {
               if(mylist.Count(e => e == item) > 1)
                {
                    Repeteditemlist.Add(item);
                }
            }
            foreach (var item in Repeteditemlist)
            {
                Console.WriteLine(item);
            }

预期的输出:

1 3 4

有一个答案，但我不明白为什么不工作;

var anyDuplicate = enumerable.GroupBy(x => x.Key).Any(g => g.Count() > 1);

我的解是这样的;

var duplicates = model.list
                    .GroupBy(s => s.SAME_ID)
                    .Where(g => g.Count() > 1).Count() > 0;
if(duplicates) {
    doSomething();
}

完整的集Linq到SQL扩展的重复功能检查在MS SQL Server。不使用. tolist()或IEnumerable。这些查询在SQL Server中执行，而不是在内存中。结果只在内存中返回。

public static class Linq2SqlExtensions {

    public class CountOfT<T> {
        public T Key { get; set; }
        public int Count { get; set; }
    }

    public static IQueryable<TKey> Duplicates<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> groupBy)
        => source.GroupBy(groupBy).Where(w => w.Count() > 1).Select(s => s.Key);

    public static IQueryable<TSource> GetDuplicates<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> groupBy)
        => source.GroupBy(groupBy).Where(w => w.Count() > 1).SelectMany(s => s);

    public static IQueryable<CountOfT<TKey>> DuplicatesCounts<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> groupBy)
        => source.GroupBy(groupBy).Where(w => w.Count() > 1).Select(y => new CountOfT<TKey> { Key = y.Key, Count = y.Count() });

    public static IQueryable<Tuple<TKey, int>> DuplicatesCountsAsTuble<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> groupBy)
        => source.GroupBy(groupBy).Where(w => w.Count() > 1).Select(s => Tuple.Create(s.Key, s.Count()));
}

Linq查询:

var query = from s2 in (from s in someList group s by new { s.Column1, s.Column2 } into sg select sg) where s2.Count() > 1 select s2;