Implement IComparer(Of DateTime) on list(of X) property

Question

I want to sort all items in the list of the property reviews of the class CompanyTotalReview by the property reviewdatetime in descending order.

Here's what I have:

Dim allReviews As RestService.CompanyTotalReview = rst.getReviews(type, objectId, _lang)

Dim dc As New ReviewComparer
allReviews.reviews.Sort(dc)



Public Class CompanyTotalReview
    Public Property totalreviews() As Integer
        Get
            Return _totalreviews
        End Get
        Set(value As Integer)
            _totalreviews = value
        End Set
    End Property
    Private _totalreviews As Integer

    Public Property averagereview() As String
        Get
            Return _averagereview
        End Get
        Set(value As String)
            _averagereview = value
        End Set
    End Property
    Private _averagereview As String


    Public Property reviews() As List(Of Review)
        Get
            Return m_reviews
        End Get
        Set(value As List(Of Review))
            m_reviews = value
        End Set
    End Property
    Private m_reviews As List(Of Review)

End Class


Public Class Review
    Public Property overallscore() As Integer
        Get
            Return _overallscore
        End Get
        Set(value As Integer)
            _overallscore = value
        End Set
    End Property
    Private _overallscore As Integer

    Public Property reviewtext() As String
        Get
            Return _reviewtext
        End Get
        Set(value As String)
            _reviewtext = value
        End Set
    End Property
    Private _reviewtext As String

    Public Property reviewdatetime() As DateTime
        Get
            Return _reviewdatetime
        End Get
        Set(value As DateTime)
            _reviewdatetime = value
        End Set
    End Property
    Private _reviewdatetime As DateTime


End Class

updated class based on comments

Public Class ReviewComparer
    Implements IComparer(Of RestService.Review)

    Public Function Compare(xr As RestService.Review, yr As RestService.Review) As Integer Implements System.Collections.Generic.IComparer(Of RestService.Review).Compare

        Dim x As DateTime = xr.reviewdatetime
        Dim y As DateTime = yr.reviewdatetime

        If x = Nothing Then
            If y = Nothing Then
                Return 0
            Else
                Return -1
            End If
        Else
            If y = Nothing Then
                Return 1
            Else
                Dim retval As Integer = x.CompareTo(y)

                If retval <> 0 Then
                    Return retval
                Else
                    Return x.CompareTo(y)
                End If
            End If
        End If

    End Function
End Class

your comparer needs to take the type you are comparing, not the type of the property you want to compare. E.g., `Of RestService.Review`, then in your compare method you can compare the x and y reviews' date properties with each other. — ps2goat, Nov 26 '13 at 21:00

score 1 · Accepted Answer · answered Nov 26 '13 at 21:07

You are sorting Review objects, not datetime objects. Use a review comparer like this.

Public Class ReviewComparer
    Implements IComparer(Of Review)

    Public Function Compare(xr As Review, yr As Review) As Integer Implements System.Collections.Generic.IComparer(Of Review).Compare

        Dim x As DateTime = xr.reviewdatetime
        Dim y As DateTime = yr.reviewdatetime

        If x = Nothing Then
            If y = Nothing Then
                Return 0
            Else
                Return -1
            End If
        Else
            If y = Nothing Then
                Return 1
            Else
                Dim retval As Integer = x.CompareTo(y)

                If retval <> 0 Then
                    Return retval
                Else
                    Return x.CompareTo(y)
                End If
            End If
        End If

    End Function
End Class

I updated my question with your code. However, I would expect that after this line: `allReviews.reviews.Sort(dc)`, all reviews would be sorted so that when I access this review `allReviews.reviews(0).reviewtext` I would get the latest review first. Or am I sorting ascending right now? If so, how do I sort descending? Thanks! — Adam, Nov 26 '13 at 21:15
I didn't check your logic but maybe try just a simple Return x.CompareTo(y) in your function. — Ceres, Nov 26 '13 at 21:30

Implement IComparer(Of DateTime) on list(of X) property

1 Answers1