LINQ advice on selecting distinct elements from a collection

Question

I need to select distinct highways from a roads collection.

I suppose I could use LINQ for this.

I have (stub code)

  Dim roads As List(Of Roads) = myRegion.Roads
  Dim highways As New List(Of Highway)

  For Each road In roads
    If road.RoadType = RoadType.Highway Then
      highways.Add(DirectCast(road, Highway))
    End If
  Next ic

  ' Now I think sorting them by .Id and then remove duplicates '
  Dim myComparer As New HighwayByIdComparer
  highways.Sort(myComparer)

C# variants are accepted as well ;)

Answer 1

C#:

return myRegion.Roads
    .Where(x => x.RoadType == RoadType.Highway)
    .DistinctBy(x => x.Id);

(where DistinctBy is an extension method defined in Jon Skeet's excellent MoreLINQ project)

Answer 2

EDIT: I didn't get the original requirement, changed OrderBy to GroupBy/Select First. You can also simply use Distinct if you override the Equals method to compare the Ids

var highways = roads.OfType<Highway>()
          .GroupBy(x => x.Id)
          .Select(x => x.First())
          .ToList()

or in VB.Net:

Dim highways = roads.OfType(Of Highway)().
       GroupBy(Function(road) road.Id).
       Select(Function(x) x.First()).
       ToList()

Answer 3

'Road model

Public Class Road
Public Property Id As Integer
Public Property Name As String
Public Property RoadType As RoadType
End Class 

'Highway model

Public Class Highway
Public Property Id As Integer
Public Property Name As String 
End Class

'RoadType Enum

Public Enum RoadType

Residential
Highway
OffRoad
End Enum

'Highway comparer (used in the distinct clause)

Public Class HigwayNameComparer
Implements IEqualityComparer(Of Highway)
Public Function Equals(ByVal x As Highway, ByVal y As Highway) As Boolean Implements IEqualityComparer(Of Highway).Equals
    Return x.Name = y.Name
End Function

Public Function GetHashCode(ByVal obj As Highway) As Integer Implements IEqualityComparer(Of Highway).GetHashCode
    Return obj.Name.GetHashCode()
End Function
End Class

'Console app

Sub Main()
    Dim roads As New List(Of Road)
    roads.Add(New Road() With {.Id = 1, .Name = "Barclays Road", .RoadType = RoadType.Residential})
    roads.Add(New Road() With {.Id = 2, .Name = "Effie Road", .RoadType = RoadType.Residential})
    roads.Add(New Road() With {.Id = 3, .Name = "Out Road", .RoadType = RoadType.OffRoad})
    roads.Add(New Road() With {.Id = 4, .Name = "M4", .RoadType = RoadType.Highway})
    roads.Add(New Road() With {.Id = 5, .Name = "M4", .RoadType = RoadType.Highway})


    Dim results = (From road In roads Where road.RoadType = RoadType.Highway
            Select New Highway With {.Id = road.Id, .Name = road.Name}).Distinct(New HigwayNameComparer())


    For Each highway As Highway In results
        Console.WriteLine("{0}", highway.Name)
    Next

    Console.ReadLine()


End Sub 

--- Output : M4

Answer 4

First, you could take a look on this blog post: http://blogs.msdn.com/b/csharpfaq/archive/2009/03/25/how-to-use-linq-methods-to-compare-objects-of-custom-types.aspx . This indicates that you need to create EqualityComparer<T> and than simply pass it to Distinct operator.

roads.Where(r => r.RoadType == RoadType.Highway).Distinct(myComparer);

Assuming that myComparer actually implements EqualityComaprer<T> correctly.

Answer 5

You can use a custom equality comparer for Road to use Distinct() :

public class RoadComparer : IEqualityComparer<Road>
{
    public bool Equals(Road x, Road y)
    {
        return x.Id == y.Id;
    }

    public int GetHashCode(Road obj)
    {
        return obj.Id.GetHashCode();
    }
}

var results = myRegion.Roads
                      .OfType<Highway>()
                      .Distinct(new RoadComparer())
                      .OrderBy( road => road.Id)
                      .ToList();