C# Language Select with Func selector - Use to get ranking of elements


Example

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<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}
};