Question

I have an array of objects, I find the item by index, assign a value but then looking at the array the item doesn’t show the updated value.

Public Structure Cheque
    Public Id As String
    Public Status As Byte
    Public Amount As String
    Public WarrantNo As String
End Structure

Public Class ChequeCollection

    Private chequeColl() As Cheque 'This is populated ok

Public Sub UpdateChequeAmount(ByVal Id As String, ByVal Amount As String)
        SyncLock lockObject
            Dim idx As Integer = Get_idx(Id) 'Finds it ok
            If idx <> -1 Then
                Dim cheque As Cheque = chequeColl(idx)
                cheque.Amount = Amount 'Updates value ok but if you look in chequeColl the value isn't there
            End If
        End SyncLock
    End Sub

End Class

Answer 1

Because value types are copied everywhere they’re used – you’re updating your copy of the value type that’s in the cheque variable, as opposed to the copy within the array.

You’d need to update the copy in the array:

            Dim cheque As Cheque = chequeColl(idx)
            cheque.Amount = Amount 'Updates value ok but if you look in chequeColl the value isn't there
            chequeColl(idx) = cheque

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And of course, always worth reading “The Truth About Value Types” by Mr. Lippert