Vahid Vahid - 1 month ago 6
C# Question

Group items in a list based on two properties using LINQ

I have a

Column
class as below:

public class Column
{
public int LocId { get; set; }
public int SecId { get; set; }
public double StartElevation { get; set; }
public double EndElevation { get; set; }
}


And a list of
Column
objects:

List<Column> Columns = new List<Column>();


For example:

Columns:
{
Column1: { LocId = 1 , SecId = 1, StartElevation = 0, EndElevation = 160 }
Column2: { LocId = 1 , SecId = 1, StartElevation = 160, EndElevation = 320 }
Column3: { LocId = 1 , SecId = 2, StartElevation = 320, EndElevation = 640 }
Column4: { LocId = 2 , SecId = 1, StartElevation = 0, EndElevation = 160 }
Column5: { LocId = 2 , SecId = 2, StartElevation = 160, EndElevation = 320 }
}


I want to apply the below algorithm to the above list using Linq.

Go through the
Columns
list and:


  • (A) Choose a list of items that have the the same
    LocId
    .

  • (B) Then from that list choose another list of items that have the same
    SecId
    .



This will give me a list hopefully which I will perform other things on it.

So applying the above algorithm on the data above will be like this.

Columns List
---------------------------------------------------------
| Column1 Column2 Column3 Column4 Column5 |
---------------------------------------------------------
|
|
(A)
|
|
--------------------------------------------
| |
GroupBasedOnLocId GroupBasedOnLocId
| |
----------- -----------
| Column1 | | Column4 |
| Column2 | | Column5 |
| Column3 | -----------
----------- |
| |
(B) (B)
| |
------------------------- -------------------------
| | | |
| | | |
GroupBasedOnSecId GroupBasedOnSecId GroupBasedOnSecId GroupBasedOnSecId
| | | |
| | | |
Column1 Column3 Column4 Column5
Column2


How can I accomplish this using LINQ?

Answer

Use .GroupBy with composite key.

Sample code:

List<Column> Columns = new List<Column>
{
   new Column { LocId = 1 , SecId = 1, StartElevation = 0, EndElevation = 160 },
   new Column { LocId = 1 , SecId = 1, StartElevation = 160, EndElevation = 320 },
   new Column { LocId = 1 , SecId = 2, StartElevation = 320, EndElevation = 640 },
   new Column { LocId = 2 , SecId = 1, StartElevation = 0, EndElevation = 160 },
   new Column { LocId = 2 , SecId = 2, StartElevation = 160, EndElevation = 320 }
};

foreach (var group in Columns.GroupBy(c => new { c.LocId, c.SecId }))
{
    Console.WriteLine("group: LocId = {0}, SecId = {1}", group.Key.LocId, group.Key.SecId);
    foreach(Column column in group)
        Console.WriteLine("  item: StartElevation = {0}, EndElevation = {1}", column.StartElevation, column.EndElevation);
}

You can transform the group any way you want:

foreach (var res in Columns.GroupBy(c => new { c.LocId, c.SecId })
                           .Select(g => new
                           {
                               g.Key.LocId,
                               g.Key.SecId,
                               MinStartElevation = g.Min(c => c.StartElevation),
                               MaxEndElevation = g.Max(c => c.EndElevation)
                           }))
{
    Console.WriteLine("LocId = {0}, SecId = {1}, MinStartElevation = {2}, MaxEndElevation = {3}",
        res.LocId, res.SecId, res.MinStartElevation, res.MinStartElevation);
}