How to do it...

1. Open your AL project folder in Visual Studio Code.
2. First, we need to create the new logic that we will implement.

In Visual Studio Code's Explorer pane, right-click and create a new file named Check Customer Television Shows.al. In the Editor tab, add the following code, which checks to make sure that there is at least one television show defined as the favorite for the given customer:

codeunit 50100 "Check Cust. Television Shows"
{
    procedure CheckCustomerTelevisionShows(CustomerNo: 
    Code[20])
    begin
        ValidateFavoriteShowExists(CustomerNo);
    end;

    local procedure ValidateFavoriteShowExists(CustomerNo: 
    Code[20])
    var
        CustomerTelevisionShow: Record "Customer Television 
        Show";
        NoFavoriteShowErr: Label 'You need to define a favorite 
        television show for Customer %1.';
    begin
        CustomerTelevisionShow.SetRange("Customer No.", 
        CustomerNo);
        CustomerTelevisionShow.SetRange(Favorite, true);
        if CustomerTelevisionShow.IsEmpty() then
            Error(NoFavoriteShowErr, CustomerNo);
    end;
}

1. Now that we have our new logic, let's hook it into the sales posting routine. In Visual Studio Code's Explorer pane, right-click and create a new file named Sales Post Subscribers.al. In the Editor tab, add the following code:

codeunit 50101 SalesPostSubscribers
{
    [EventSubscriber(ObjectType::Codeunit, Codeunit::"Sales-Post 
    (Yes/No)", 'OnAfterConfirmPost', '', false, false)]
    local procedure OnAfterConfirmPost(SalesHeader: Record "Sales 
    Header")
    var
        CheckCustomerTelevisionShows: Codeunit "Check Cust. 
        Television Shows";
    begin
        CheckCustomerTelevisionShows.CheckCustomerTelevisionShows
        (SalesHeader."Sell-to Customer No.");
    end;
}

1. Time to test! Press F5 to build and publish your application. When your browser opens and you log in to your sandbox, perform the following steps:
   1. Use the  icon and search for sales invoices to go to the Sales Invoices page.
   2. Select an existing sales invoice from the list or create a new one:
      • Make sure you use an invoice for a customer that has no television shows defined!
   3. Post the sales invoice.

After confirming that you want to post the document, you should see an error similar to the following one:

If you want to test further, repeat the preceding process but before you post the invoice, add at least one television show to the customer and mark it as a favorite. Your sales invoice should then post without any error.