Sum object properties by its attribute using LINQ from multiple list

Question

Hallo I would like to write a LINQ query to sum object properties based on their attribute .

I have Datagridview like below. I want to loop through Datagridview Category column and write a sum of weights from two bindinglists. 

Public Class Category1    
    
    <Category("WeightVegitables")> _
    Public Property Tomatto() As Integer

    <Category("WeightFruits")> _ 
    Public Property Apple() As Integer

    <Category("WeightFruits")> _ 
    Public Property Banana() As Integer
   
    <Category("WeightMeats")> __
    Public Property Pork() As Integer

    <Category("WeightMeats")> __
    Public Property Fish() As Integer
       
End Class
Public Class Categories1
	Inherits System.ComponentModel.BindingList(Of Category1)
End Class




Public Class Category2    
    
    <Category("WeightVegitables")> _
    Public Property Brinjal() As Integer

    <Category("WeightFruits")> _ 
    Public Property Orange() As Integer
   
    <Category("WeightMeats")> __
    Public Property Chicken() As Integer
       
End Class
Public Class Categories2
	Inherits System.ComponentModel.BindingList(Of Category2)
End Class

Or is there any alternative method to the example but without attributes? 

Can i have more than 1 attribute to a property?

Thanks

Answer 1

Hi Shan1986,

Thank you for posting here.

I make a test based on your description, and here's my custom attribute 'Category'.

<System.AttributeUsage(System.AttributeTargets.Property)>
Public Class Category
    Inherits System.Attribute

    Public weightCategory As String
    Sub New(ByVal category As String)
        weightCategory = category
    End Sub
End Class

The following example calculates the sum of ‘WeightVegitables’, ‘WeightFruits’ and ‘WeightMeats’ from a given ‘Categories1’.

    Private WeightVegitablesCount As Integer
    Private WeightFruitsCount As Integer
    Private WeightMeatsCount As Integer
    Private lst As Categories1 = New Categories1()
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        lst.Add(New Category1() With {
            .Tomatto = 10,
            .Apple = 20,
            .Banana = 30,
            .Fish = 40,
            .Pork = 50
        })
        lst.Add(New Category1() With {
            .Tomatto = 11,
            .Apple = 22,
            .Banana = 33,
            .Fish = 44,
            .Pork = 55
        })
        lst.Add(New Category1() With {
            .Tomatto = 12,
            .Apple = 23,
            .Banana = 32,
            .Fish = 46,
            .Pork = 57
        })
    End Sub
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        For Each prop In GetType(Category1).GetProperties()
            Dim ca = CType(prop.GetCustomAttributes(GetType(Category), False).FirstOrDefault(), Category)
            Dim value As Integer = lst.Sum(Function(x) CInt((x.GetType().GetProperty(prop.Name).GetValue(x))))

            Select Case ca.weightCategory
                Case "WeightVegitables"
                    WeightVegitablesCount += value
                Case "WeightFruits"
                    WeightFruitsCount += value
                Case "WeightMeats"
                    WeightMeatsCount += value
            End Select
        Next
        MessageBox.Show($"WeightVegitables: {WeightVegitablesCount} WeightFruits:{WeightFruitsCount} WeightMeats:{WeightMeatsCount}")
    End Sub

The above code also applies to 'Categories2'.

Hope it could be helpful.

Best Regards,

Xingyu Zhao

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Answer 2

Hallo Xingyu. Thanks for the answer. I get error when i run it one this line ca was nothing and 

 "Select Case ca.weightCategory " 

System.NullReferenceException: 'Object reference not set to an instance of an object.'

Thanks.

Answer 3

Hi,

Did you use the custom 'Category' I provided to make the test?

I can get the correct test result.

Best Regards,

Xingyu Zhao

---

Visual Basic and CLR forum will be migrating to a new home on Microsoft Q&A! (VB.NET and CLR) We invite you to post new questions in the new home on Microsoft Q&A ! For more information, please refer to the sticky post(VB.NET and CLR).

Answer 4

Thanks Xingyu. It worked my mistake.