linq Distinct
- IEnumerable<List<Models.ReportName>> query1 = from e in jv
- group e by new { e.ReportId } into g
- select g.ToList();
这段代码可以得到一个范型的集合(根据reportId分组后的范型集合),通过foreach能够取出进行操作。
- foreach (List<ReportName> reportNames in query1)
- {
- reportNameTemp = new ReportName();
- reportNameTemp.ReportList = new List<ReportList>();
- foreach (ReportName reportName in reportNames)
- {
- childReportName = new ReportList();
-
- childReportName.Name = reportName.Name;
- reportNameTemp.ReportList.Add(childReportName);
- reportNameTemp.ReportSetName = reportName.ReportSetName;
- reportNameTemp.ReportId = reportName.ReportId;
- }
-
- emptyList.Add(reportNameTemp);
- }
参考文章:http://blog.csdn.net/fangxinggood/article/details/6187043
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
----------------
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