Linq Distinct

Linq 用来实现集合(List, DataTable等) 的二次操作十分简便，这里介绍下用 Linq 对集合进行 Distinct 操作的几种方法。
0. 准备数据：
public class User { public string A; public string B; public string C; public string D; public override string ToString() { return string.Format("{0},{1},{2},{3}", A, B, C, D); } public static List<User> GetData() { return new List<User> { new User { A = "a1", B = "b1", C = "c1", D = "d1" }, new User { A = "a1", B = "b1", C = "c1", D = "d1" }, new User { A = "a2", B = "b1", C = "c1", D = "d1" }, new User { A = "a1", B = "b2", C = "c1", D = "d1" }, new User { A = "a1", B = "b1", C = "c1", D = "d1" }, new User { A = "a1", B = "b1", C = "c2", D = "d1" }, new User { A = "a1", B = "b1", C = "c1", D = "d2" }, }; } }
1. 使用GroupBy：对需要Distinct的字段进行分组，取组内的第一条记录这样结果就是Distinct的数据了。
Console.WriteLine("Distinct1 By: A"); var query1 = from e in User.GetData() group e by new { e.A } into g select g.FirstOrDefault(); foreach (var u in query1) Console.WriteLine(u.ToString());
2. 使用Distinct()扩展方法：需要实现IEqualityComparer接口。
class UserCompare : IEqualityComparer<User> { public bool Equals(User x, User y) { return (x.A == y.A && x.B == y.B); } public int GetHashCode(User obj) { // return obj.GetHashCode(); return obj.ToString().ToLower().GetHashCode(); } }Console.WriteLine("Distinct2 By: A,B"); var compare = new UserCompare(); var query2 = User.GetData().Distinct(compare); foreach (var u in query2) Console.WriteLine(u.ToString());
上面的实现中要注意GetHashCode()方法直接用obj.GetHashCode()的话，Distinct不能正常运行。

3. 自定义扩展方法DistinctBy(this IEnumerable source, Func keySelector)
public static class MyEnumerableExtensions { public static IEnumerable<TSource> DistinctBy<TSource, TKey> (this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) { HashSet<TKey> seenKeys = new HashSet<TKey>(); foreach (TSource element in source) { if (seenKeys.Add(keySelector(element))) { yield return element; } } } }
Console.WriteLine("Distinct3 By: A,B,C"); var query3 = User.GetData().DistinctBy(x => new { x.A, x.B, x.C }); foreach (var u in query3) Console.WriteLine(u.ToString());

运行结果：
A B C D
a2,b1,c1,d1
a1,b2,c1,d1
a1,b1,c1,d1
a1,b1,c2,d1
a1,b1,c1,d2
----------------
Distinct1 By: A
a1,b1,c1,d1
a2,b1,c1,d1
----------------
Distinct2 By: A,B
a1,b1,c1,d1
a2,b1,c1,d1
a1,b2,c1,d1
a1,b1,c2,d1
a1,b1,c1,d2
----------------
Distinct3 By: A,B,C
a1,b1,c1,d1
a2,b1,c1,d1
a1,b2,c1,d1
a1,b1,c2,d1
----------------