Select with Func C# with Example

selector - Use to get ranking of elements 
On of the overloads of the SELECT extension methods also passes the index of the current item in the collection 
being SELECTed. These are a few uses of it. 
Get the "row number" of the items 
var rowNumbers = collection.OrderBy(item => item.Property1) 
.ThenBy(item => item.Property2) 
.ThenByDescending(item => item.Property3) 
.Select((item, index) => new { Item = item, RowNumber = index }) 
.ToList(); 
Get the rank of an item within its group 
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(); 
Get the ranking of groups (also known in Oracle as dense_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(); 
For testing this you can use: 
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); 
} 
} 
And data: 
List collection = new List 
 

{ 
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} 
};