Linq 提示多列的不重复字段

 

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： 

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
----------------