使用linq删除列表中的重复项
-
05-07-2019 - |
题
我有一个 Items
的类属性(Id,Name,Code,Price)
。
Items
列表中填充了重复的项目。
例如:
1 Item1 IT00001 $100
2 Item2 IT00002 $200
3 Item3 IT00003 $150
1 Item1 IT00001 $100
3 Item3 IT00003 $150
如何使用linq删除列表中的重复项?
解决方案
var distinctItems = items.Distinct();
要仅匹配某些属性,请创建自定义相等比较器,例如:
class DistinctItemComparer : IEqualityComparer<Item> {
public bool Equals(Item x, Item y) {
return x.Id == y.Id &&
x.Name == y.Name &&
x.Code == y.Code &&
x.Price == y.Price;
}
public int GetHashCode(Item obj) {
return obj.Id.GetHashCode() ^
obj.Name.GetHashCode() ^
obj.Code.GetHashCode() ^
obj.Price.GetHashCode();
}
}
然后像这样使用它:
var distinctItems = items.Distinct(new DistinctItemComparer());
其他提示
var distinctItems = items.GroupBy(x => x.Id).Select(y => y.First());
如果您的Distinct查询丢失了,您可能需要查看 MoreLinq 并使用DistinctBy运算符并按id选择不同的对象。
var distinct = items.DistinctBy( i => i.Id );
这就是我能够与Linq分组的方式。希望它有所帮助。
var query = collection.GroupBy(x => x.title).Select(y => y.FirstOrDefault());
使用 Distinct()
但请记住,它使用默认的相等比较器来比较值,因此如果您想要超出该值,则需要实现自己的比较器。
请参阅 http://msdn.microsoft.com/en-us/ library / bb348436.aspx 的一个例子。
此处有三个选项可用于删除列表中的重复项目:
- 使用自定义相等比较器,然后使用
Distinct(new DistinctItemComparer())
作为 @Christian Hayter 提到。 -
使用
GroupBy
,但请注意GroupBy
,您应该按所有列进行分组,因为如果您只是按Id
分组它不会永远删除重复的项目。例如,请考虑以下示例:List<Item> a = new List<Item> { new Item {Id = 1, Name = "Item1", Code = "IT00001", Price = 100}, new Item {Id = 2, Name = "Item2", Code = "IT00002", Price = 200}, new Item {Id = 3, Name = "Item3", Code = "IT00003", Price = 150}, new Item {Id = 1, Name = "Item1", Code = "IT00001", Price = 100}, new Item {Id = 3, Name = "Item3", Code = "IT00003", Price = 150}, new Item {Id = 3, Name = "Item3", Code = "IT00004", Price = 250} }; var distinctItems = a.GroupBy(x => x.Id).Select(y => y.First());
此分组的结果将是:
{Id = 1, Name = "Item1", Code = "IT00001", Price = 100} {Id = 2, Name = "Item2", Code = "IT00002", Price = 200} {Id = 3, Name = "Item3", Code = "IT00003", Price = 150}
哪个不正确,因为它将
{Id = 3,Name =&quot; Item3&quot;,Code =&quot; IT00004&quot;,Price = 250}
视为重复。所以正确的查询是:var distinctItems = a.GroupBy(c => new { c.Id , c.Name , c.Code , c.Price}) .Select(c => c.First()).ToList();
3.在项目类中删除
Equal
和GetHashCode
:public class Item { public int Id { get; set; } public string Name { get; set; } public string Code { get; set; } public int Price { get; set; } public override bool Equals(object obj) { if (!(obj is Item)) return false; Item p = (Item)obj; return (p.Id == Id && p.Name == Name && p.Code == Code && p.Price == Price); } public override int GetHashCode() { return String.Format("{0}|{1}|{2}|{3}", Id, Name, Code, Price).GetHashCode(); } }
然后你可以像这样使用它:
var distinctItems = a.Distinct();
醇>
通用扩展方法:
public static class EnumerableExtensions
{
public static IEnumerable<T> DistinctBy<T, TKey>(this IEnumerable<T> enumerable, Func<T, TKey> keySelector)
{
return enumerable.GroupBy(keySelector).Select(grp => grp.First());
}
}
使用示例:
var lstDst = lst.DistinctBy(item => item.Key);
List<Employee> employees = new List<Employee>()
{
new Employee{Id =1,Name="AAAAA"}
, new Employee{Id =2,Name="BBBBB"}
, new Employee{Id =3,Name="AAAAA"}
, new Employee{Id =4,Name="CCCCC"}
, new Employee{Id =5,Name="AAAAA"}
};
List<Employee> duplicateEmployees = employees.Except(employees.GroupBy(i => i.Name)
.Select(ss => ss.FirstOrDefault()))
.ToList();
尝试使用此扩展方法。希望这可以提供帮助。
public static class DistinctHelper
{
public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{
var identifiedKeys = new HashSet<TKey>();
return source.Where(element => identifiedKeys.Add(keySelector(element)));
}
}
用法:
var outputList = sourceList.DistinctBy(x => x.TargetProperty);
如果您不想编写IEqualityComparer,可以尝试以下内容。
class Program
{
private static void Main(string[] args)
{
var items = new List<Item>();
items.Add(new Item {Id = 1, Name = "Item1"});
items.Add(new Item {Id = 2, Name = "Item2"});
items.Add(new Item {Id = 3, Name = "Item3"});
//Duplicate item
items.Add(new Item {Id = 4, Name = "Item4"});
//Duplicate item
items.Add(new Item {Id = 2, Name = "Item2"});
items.Add(new Item {Id = 3, Name = "Item3"});
var res = items.Select(i => new {i.Id, i.Name})
.Distinct().Select(x => new Item {Id = x.Id, Name = x.Name}).ToList();
// now res contains distinct records
}
}
public class Item
{
public int Id { get; set; }
public string Name { get; set; }
}
不隶属于 StackOverflow