使用LINQ，来自List<int>，如何检索包含重复多次的条目及其值的列表？
答案 0 :(得分：429)
解决问题的最简单方法是根据元素的值对元素进行分组，然后在组中有多个元素的情况下选择组的代表。在LINQ中，这转换为：
var query = lst.GroupBy(x => x)
.Where(g => g.Count() > 1)
.Select(y => y.Key)
.ToList();
如果您想知道重复元素的次数，可以使用：
var query = lst.GroupBy(x => x)
.Where(g => g.Count() > 1)
.Select(y => new { Element = y.Key, Counter = y.Count() })
.ToList();
这将返回匿名类型的List，每个元素都将包含属性Element和Counter，以检索您需要的信息。
最后，如果它是您正在寻找的字典，您可以使用
var query = lst.GroupBy(x => x)
.Where(g => g.Count() > 1)
.ToDictionary(x => x.Key, y => y.Count());
这将返回一个字典，其元素为关键字，以及它作为值重复的次数。
答案 1 :(得分：99)
查明可枚举是否包含任何重复：
var anyDuplicate = enumerable.GroupBy(x => x.Key).Any(g => g.Count() > 1);
确定可枚举中的所有值唯一：
var allUnique = enumerable.GroupBy(x => x.Key).All(g => g.Count() == 1);
答案 2 :(得分：19)
另一种方法是使用HashSet：
var hash = new HashSet<int>();
var duplicates = list.Where(i => !hash.Add(i));
如果您需要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)).ToList().Distinct().ToList();
这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);
}
}
答案 3 :(得分：10)
你可以这样做：
var list = new[] {1,2,3,1,4,2};
var duplicateItems = list.Duplicates();
使用这些扩展方法：
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 moreThan1 = grouped.Where(i => i.IsMultiple());
return moreThan1.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();
}
}
在Duplicates方法中使用IsMultiple（）比Count（）更快，因为这不会迭代整个集合。
答案 4 :(得分：6)
我创建了一个扩展响应，你可以在你的项目中包含它，我认为当你在List或Linq中搜索重复项时，这会返回最多的情况。
示例：
//Dummy class to compare in list
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;
}
}
//The extention static class
public static class Extention
{
public static IEnumerable<T> getMoreThanOnceRepeated<T>(this IEnumerable<T> extList, Func<T, object> groupProps) where T : class
{ //Return only the second and next reptition
return extList
.GroupBy(groupProps)
.SelectMany(z => z.Skip(1)); //Skip the first occur and return all the others that repeats
}
public static IEnumerable<T> getAllRepeated<T>(this IEnumerable<T> extList, Func<T, object> groupProps) where T : class
{
//Get All the lines that has repeating
return extList
.GroupBy(groupProps)
.Where(z => z.Count() > 1) //Filter only the distinct one
.SelectMany(z => z);//All in where has to be retuned
}
}
//how to use it:
void DuplicateExample()
{
//Populate List
List<Person> PersonsLst = new List<Person>(){
new Person(1,"Ricardo","Figueiredo"), //fist Duplicate to the example
new Person(2,"Ana","Figueiredo"),
new Person(3,"Ricardo","Figueiredo"),//second Duplicate to the example
new Person(4,"Margarida","Figueiredo"),
new Person(5,"Ricardo","Figueiredo")//third Duplicate to the example
};
Console.WriteLine("All:");
PersonsLst.ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname));
/* OUTPUT:
All:
1 -> Ricardo Figueiredo
2 -> Ana Figueiredo
3 -> Ricardo Figueiredo
4 -> Margarida Figueiredo
5 -> Ricardo Figueiredo
*/
Console.WriteLine("All lines with repeated data");
PersonsLst.getAllRepeated(z => new { z.Name, z.Surname })
.ToList()
.ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname));
/* OUTPUT:
All lines with repeated data
1 -> Ricardo Figueiredo
3 -> Ricardo Figueiredo
5 -> Ricardo Figueiredo
*/
Console.WriteLine("Only Repeated more than once");
PersonsLst.getMoreThanOnceRepeated(z => new { z.Name, z.Surname })
.ToList()
.ForEach(z => Console.WriteLine("{0} -> {1} {2}", z.Id, z.Name, z.Surname));
/* OUTPUT:
Only Repeated more than once
3 -> Ricardo Figueiredo
5 -> Ricardo Figueiredo
*/
}
答案 5 :(得分：2)
仅查找重复值：
var duplicates = list.GroupBy(x => x.Key).Any(g => g.Count() > 1);
例如。 var list = new [] {1,2,3,1,4,2};
so group by将按其键对数字进行分组，并保持其计数（重复的次数）。之后，我们只检查重复多次的值。
仅查找uniuqe值：
var unique = list.GroupBy(x => x.Key).All(g => g.Count() == 1);
例如。 var list = new [] {1,2,3,1,4,2};
so group by将按其键对数字进行分组，并保持其计数（重复的次数）。之后，我们只检查仅重复一次均值唯一的值。
答案 6 :(得分：1)
在Microsoft SQL Server中检查的完整的Linq to Duplicates函数的SQL扩展集。不使用.ToList（）或IEnumerable。 这些查询在SQL Server中而不是在内存中执行。。结果仅在内存中返回。
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()));
}
答案 7 :(得分：0)
有一个答案，但是我不明白为什么不起作用；
var anyDuplicate = enumerable.GroupBy(x => x.Key).Any(g => g.Count() > 1);
在这种情况下，我的解决方案就是这样；
var duplicates = model.list
.GroupBy(s => s.SAME_ID)
.Where(g => g.Count() > 1).Count() > 0;
if(duplicates) {
doSomething();
}
答案 8 :(得分：0)
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;
答案 9 :(得分：0)
这种不使用组的更简单方法只需获取 District 元素，然后遍历它们并检查它们在列表中的计数，如果它们的 计数 >1 这意味着它出现超过 1 个项目，因此将其添加到 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);
}
预期产出：
1 3 4
答案 10 :(得分：-1)
按键删除重复项
myTupleList = myTupleList.GroupBy(tuple => tuple.Item1).Select(group => group.First()).ToList();