Calculate SUM of cost from object instances VB.Net

Question

Hallo i have a BindingList of Work and i want to make one of the object instance is a total of all other object instances.

for example i have property "ID" , ID from 1 - 99 are reserved for calculating cost and instance with ID 100 is a sum of all. I would like to know how it is normally done or is there any alternative?

Public Class Work

    Public Property ID As Integer
    Public Property Description As String
    Public Property Quantity As Integer
    Public Property UnitCost As Integer
    Public ReadOnly Property Cost As Integer
        Get
            Return GetCost()
        End Get
    End Property
    Public Function GetCost() As Integer

        Dim CalCost As Integer

        Select Case ID

            Case 1 To 99
                CalCost = UnitCost * Quantity

            Case 100 ''100 is the ID for Total
                CalCost = Sum() Of Cost  "totalcost"???  SUM of all Object In this collection 

        End Select

        Return CalCost

    End Function

End Class

Thanks

Answer 1

Hello,

It's best to keep the total logic outside of the Work class e.g.

• PopulateButton loads mocked data
• TotalButton gets totals from 1-99 identifiers
• ChangeButton to change quantity property of the current item
• Changes in the DataGridView are reflected via INotifyPropertyChanged

  Revised Work class

  Public Class Work
  Implements INotifyPropertyChanged

  Private _id As Integer  
  Private _description As String  
  Private _quantity As Integer  
  Private _unitCost As Integer  
  
  Public Property Id As Integer  
      Get  
          Return _id  
      End Get  
      Set  
          _id = Value  
      End Set  
  End Property  
  
  Public Property Description As String  
      Get  
          Return _description  
      End Get  
      Set  
          _description = Value  
      End Set  
  End Property  
  
  Public Property Quantity As Integer  
      Get  
          Return _quantity  
      End Get  
      Set  
          _quantity = Value  
      End Set  
  End Property  
  
  Public Property UnitCost As Integer  
      Get  
          Return _unitCost  
      End Get  
      Set  
          _unitCost = Value  
      End Set  
  End Property  
  
  Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged  
  Protected Overridable Sub OnPropertyChanged(<CallerMemberName> Optional memberName As String = Nothing)  
      RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(memberName))  
  End Sub  
  
  Public Overrides Function ToString() As String  
      Return $"Id: {Id} Description: {Description}"  
  End Function  
  

  End Class

Extensions

For obtaining totals

Public Module Extensions  
    <Extension>  
    Public Function ToList(Of T)(collection As IEnumerable(Of T)) As List(Of T)  
        Return New List(Of T)(collection)  
    End Function  
    <Extension>  
    Public Function Total(sender As List(Of Work)) As Integer  
        Return sender.Where(Function(work) work.Id <= 99).Sum(Function(work) work.Quantity * work.UnitCost)  
    End Function  
End Module  

Form code

Public Class Form1  

    WithEvents mBindingSource As New BindingSource  
    Private mBindingList As New BindingList(Of Work)  


    Private Sub PopulateButton_Click(sender As Object, e As EventArgs) Handles PopulateButton.Click  

        Dim list As New List(Of Work) From {  
            New Work With {.Id = 1, .Description = "First", .UnitCost = 100, .Quantity = 2},  
            New Work With {.Id = 2, .Description = "Second", .UnitCost = 10, .Quantity = 1},  
            New Work With {.Id = 99, .Description = "Ninety nine", .UnitCost = 200, .Quantity = 1},  
            New Work With {.Id = 100, .Description = "One hundred", .UnitCost = 100, .Quantity = 5}  
        }  

        mBindingList = New BindingList(Of Work)(list)  
        mBindingSource.DataSource = mBindingList  
        DataGridView1.DataSource = mBindingSource  

    End Sub  

    Private Sub TotalButton_Click(sender As Object, e As EventArgs) Handles TotalButton.Click  
        If mBindingSource.DataSource IsNot Nothing Then  
            Dim total = mBindingList.ToList().Total()  
            TotalLabel.Text = $"Total: {total}"  
        Else  
            MessageBox.Show("No work items")  
        End If  
    End Sub  

    Private Sub ChangeButton_Click(sender As Object, e As EventArgs) Handles ChangeButton.Click  

        If mBindingSource.DataSource IsNot Nothing Then  

            Dim quantity As Integer  

            If Integer.TryParse(QuantityTextBox.Text, quantity) Then  
                mBindingList(mBindingSource.Position).Quantity = quantity  
                mBindingSource.ResetCurrentItem()  
            End If  

        Else  
            MessageBox.Show("No work items")  
        End If  

    End Sub  
End Class  

Screen shot