.NET[C#]LINQ查询List集合中所有重复的元素如何实现？(转载)

方案一

var query = lst.GroupBy(x=>x)
              .Where(g=>g.Count()>1)
              .Select(y=>y.Key)
              .ToList();

如果还需要找出重复的元素个数：

var query = lst.GroupBy(x=>x)
              .Where(g=>g.Count()>1)
              .Select(y=> new { Element = y.Key, Counter = y.Count()})
              .ToList();

如果需要返回结果为一个字典：

var query = lst.GroupBy(x=>x)
              .Where(g=>g.Count()>1)
              .ToDictionary(x=>x.Key,y=>y.Count());

方案二

查找List集合中任意一个重复的元素：

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

查找List集合中所有重复的元素：

var allUnique = enumerable.GroupBy(x => x.Key).All(g => g.Count() == 1);

方案三

使用 HashSet ：

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); } } 

用法：

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

方案四

public static class Extensions
{
    public static IEnumerable<TSource> Duplicates<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector) { var grouped = source.GroupBy(selector); var moreThen1 = grouped.Where(i => i.IsMultiple()); return moreThen1.SelectMany(i => i); } public static IEnumerable<TSource> Duplicates<TSource, TKey>(this IEnumerable<TSource> source) { return source.Duplicates(i => i); } public static bool IsMultiple<T>(this IEnumerable<T> source) { var enumerator = source.GetEnumerator(); return enumerator.MoveNext() && enumerator.MoveNext(); } } 

用法：

var list = new[] {1,2,3,1,4,2}; var duplicateItems = list.Duplicates(); 

方案五

public class Person
{
    public int Id { get; set; } public string Name { get; set; } public string Surname { get; set; } public Person(int id, string name, string surname) { this.Id = id; this.Name = name; this.Surname = surname; } } //静态扩展类 public static class Extention { public static IEnumerable<T> getMoreThanOnceRepeated<T>(this IEnumerable<T> extList, Func<T, object> groupProps) where T : class { //返回第二个以后面的重复的元素集合 return extList .GroupBy(groupProps) .SelectMany(z => z.Skip(1)); //跳过第一个重复的元素 } public static IEnumerable<T> getAllRepeated<T>(this IEnumerable<T> extList, Func<T, object> groupProps) where T : class { //返回所有重复的元素集合 return extList .GroupBy(groupProps) .Where(z => z.Count() > 1) //Filter only the distinct one .SelectMany(z => z);//All in where has to be retuned } } //使用示例程序: void DuplicateExample() { //填充数据 List<Person> PersonsLst = new List<Person>(){ new Person(1,"Ricardo","Figueiredo"), //第一个重复数据 new Person(2,"Ana","Figueiredo"), new Person(3,"Ricardo","Figueiredo"),//第二个重复数据 new Person(4,"Margarida","Figueiredo"), new Person(5,"Ricardo","Figueiredo")//第三个重复数据 }; Console.WriteLine("所有集合数据:"); PersonsLst.ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname)); /* 输出: 所有集合数据: 1 -> Ricardo Figueiredo 2 -> Ana Figueiredo 3 -> Ricardo Figueiredo 4 -> Margarida Figueiredo 5 -> Ricardo Figueiredo */ Console.WriteLine("所有重复集合数据"); PersonsLst.getAllRepeated(z => new { z.Name, z.Surname }) .ToList() .ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname)); /* 输出: 所有重复集合数据 1 -> Ricardo Figueiredo 3 -> Ricardo Figueiredo 5 -> Ricardo Figueiredo */ Console.WriteLine("重复了一次以上的集合数据"); PersonsLst.getMoreThanOnceRepeated(z => new { z.Name, z.Surname }) .ToList() .ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname)); /* 输出: 重复了一次以上的集合数据 3 -> Ricardo Figueiredo 5 -> Ricardo Figueiredo */ }