C#使用Func选择器进行选择-用于获取元素的排名
示例
在Select扩展方法的重载中，还传递了edindex集合中的当前项目select。这些是它的一些用途。
获取项目的“行号”
var rowNumbers = collection.OrderBy(item => item.Property1)
.ThenBy(item => item.Property2)
.ThenByDescending(item => item.Property3)
.Select((item, index) => new { Item = item, RowNumber = index })
.ToList();获取项目在其组中的排名
var rankInGroup = collection.GroupBy(item => item.Property1)
.OrderBy(group => group.Key)
.SelectMany(group => group.OrderBy(item => item.Property2)
.ThenByDescending(item => item.Property3)
.Select((item, index) => new
{
Item = item,
RankInGroup = index
})).ToList();获取组的排名（在Oracle中也称为density_rank）
var rankOfBelongingGroup = collection.GroupBy(item => item.Property1)
.OrderBy(group => group.Key)
.Select((group, index) => new
{
Items = group,
Rank = index
})
.SelectMany(v => v.Items, (s, i) => new
{
Item = i,
DenseRank = s.Rank
}).ToList();为了进行测试，您可以使用：
public class SomeObject
{
public int Property1 { get; set; }
public int Property2 { get; set; }
public int Property3 { get; set; }
public override string ToString()
{
return string.Join(", ", Property1, Property2, Property3);
}
}和数据：
List<SomeObject> collection = new List<SomeObject>
{
new SomeObject { Property1 = 1, Property2 = 1, Property3 = 1},
new SomeObject { Property1 = 1, Property2 = 2, Property3 = 1},
new SomeObject { Property1 = 1, Property2 = 2, Property3 = 2},
new SomeObject { Property1 = 2, Property2 = 1, Property3 = 1},
new SomeObject { Property1 = 2, Property2 = 2, Property3 = 1},
new SomeObject { Property1 = 2, Property2 = 2, Property3 = 1},
new SomeObject { Property1 = 2, Property2 = 3, Property3 = 1}
};