Windows.Services.Store WPF fails at subscriptionStoreProduct.RequestPurchaseAsync

Target is .NET 7.0. Target OS: 10.0.22621.0 Min Supported OS: 10.0.17763

App was certified in the windows store before adding the subscription code. On Save, it checks subscription status.

We followed https://learn.microsoft.com/en-us/windows/uwp/monetize/enable-subscription-add-ons-for-your-app converted to VB.

MS Store reports that the code gets available subscription and works until subscriptionStoreProduct.RequestPurchaseAsync. Here it failed saying "Invalid window handle. (0x80070578) Consider WIndowNative, InitializeWithWindow". We added code for InitialzeWithWindow and now it fails saying "Specified cast is not valid". Extra code is enclosed by ''''' comments

There doesn't appear to be a sandbox for troubleshooting this so we have to keep resubmitting the app for certification and waiting for it to fail.

How do we fix this? Code posted below:

Private context As StoreContext
Private subscriptionStoreId As String = "************"
Private subscriptionStoreProduct As StoreProduct

Public Async Function SetupSubscriptionInfoAsync() As Task
    Try
        context = StoreContext.GetDefault()
        Dim userOwnsSubscription As Boolean = Await CheckIfUserHasSubscriptionAsync()
        If userOwnsSubscription Then
            AppGood = True
            Exit Function
        End If
        subscriptionStoreProduct = Await GetSubscriptionProductAsync()
        Dim sku As StoreSku = subscriptionStoreProduct.Skus(0)
        If sku.SubscriptionInfo.HasTrialPeriod Then
            MessageBox.Show("7 day trial available", "Trial")
        Else
            MessageBox.Show(sku.SubscriptionInfo.BillingPeriod & " (Subscription Period)" & vbCrLf & sku.SubscriptionInfo.BillingPeriodUnit & " (Subscription Period Unit)", "Subscription")
        End If
        Await PromptUserToPurchaseAsync()
    Catch ex As Exception
        MessageBox.Show("Error accessing Windows Store for subscription status.", "SetupSubscriptionInfoAsync Error")
    End Try
End Function

Private Async Function CheckIfUserHasSubscriptionAsync() As Task(Of Boolean)
    Try
        Dim appLicense As StoreAppLicense = Await context.GetAppLicenseAsync()
        For Each addOnLicense In appLicense.AddOnLicenses
            Dim license As StoreLicense = addOnLicense.Value
            If license.SkuStoreId.StartsWith(subscriptionStoreId) Then
                If license.IsActive Then
                    Return True
                    Exit Function
                End If
            End If
        Next
    Catch ex As Exception
        MessageBox.Show(ex.Message, "CheckIfUserHasSubscriptionAsync Error")
    End Try
    Return False
End Function

Private Async Function GetSubscriptionProductAsync() As Task(Of StoreProduct)
    Try
        Dim result As StoreProductQueryResult = Await context.GetAssociatedStoreProductsAsync(New String() {"Durable"})
        For Each item In result.Products
            Dim product As StoreProduct = item.Value
            If product.StoreId = subscriptionStoreId Then
                Return product
                Exit Function
            End If
        Next
    Catch ex As Exception
        MessageBox.Show(ex.Message, "GetSubscriptionProductAsync Error")
        Return Nothing
    End Try
    Return Nothing
End Function

Private Async Function PromptUserToPurchaseAsync() As Task
    Try
'''''these 3 lines were added after inital error
        Dim mywin = New Window
        Dim hwnd = New WindowInteropHelper(mywin).Handle
        InitializeWithWindow.Initialize(subscriptionStoreProduct, hwnd)
'''''

        Dim result As StorePurchaseResult

        result = Await subscriptionStoreProduct.RequestPurchaseAsync()

        Select Case result.Status
            Case StorePurchaseStatus.Succeeded
                MessageBox.Show("Subscription purchase successful.", "Success")
                AppGood = True
                Exit Function
            Case StorePurchaseStatus.NotPurchased
                MessageBox.Show("Subscription purchase did not complete or was cancelled.", "Fail")
                Exit Function
            Case StorePurchaseStatus.ServerError, StorePurchaseStatus.NetworkError
                MessageBox.Show("Network or server error, please try again later.", "Fail")
                Exit Function
            Case StorePurchaseStatus.AlreadyPurchased
                MessageBox.Show("Subscription already active, no action necessary.", "Success")
                AppGood = True
                Exit Function
        End Select
    Catch ex As Exception
        MessageBox.Show(ex.Message, "RequestPurchaseAsync Error")
    End Try
End Function

Private Async Sub btnSave_Click(sender As Object, e As RoutedEventArgs) Handles btnSave.Click
    Await SetupSubscriptionInfoAsync()
    If AppGood Then
        Dim MainWin As New MainWindow
        MainWin.Show()
        Me.Close()
    Else
        MessageBox.Show("No valid license.", "No license")
    End If
End Sub

You cannot get a valid window handle from WindowInteropHelper.Handle until the Window is initialized. Use existing opened Window, if any, to get a window handle for IInitializeWithWindow.Initialize method.

Or you can use WindowInteropHelper.EnsureHandle method to make the Window to create its window handle and retrieve it.

Dim mywin = New Window
Dim hwnd = New WindowInteropHelper(mywin).EnsureHandle