Warn about EventSubscriberInstance = Manual on SingleInstance Codeunits

[jwikman]

According to https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/performance/performance-developer#limit-your-event-subscriptions, among others, it is recommended to have event subscriber codeunits in SingleInstance codeunits.
Use single instance codeunits for subscribers, if possible.

In the same paragraph, it also says that Static automatic has a cost over manually binding.

But combining those two in the same codeunit like this could have unexpected behavior:

    EventSubscriberInstance = Manual;
    SingleInstance = true;

The manual binding of a subscriber has the same scope as the variable instance that it is being used in the BindSubscription() call. The binding is active as long as the codeunit instance is active, or until a UnbindSubscription() is called on the instance.

Example 1 - normal codeunit

codeunit 50100 MyManualSubscribers
{
    EventSubscriberInstance = Manual;
    SingleInstance = false;

In below sample, the subscribers in MySubs is inactive after the error is thrown.

    procedure DoSomething()
    var
        MySubs: codeunit MyManualSubscribers;
    begin
        BindSubscription(MySubs);
        SomeCode()
        Error('Ooooops!');
        UnbindSubscription(MySubs);
    end;

Example 2 - SingleInstancecodeunit

codeunit 50100 SingleInstanceSubscribers
{
    EventSubscriberInstance = Manual;
    SingleInstance = true;

In below sample, the event subscribers in SingleInstanceSubscribers are still active after the error has been thrown, leaving them active until end of session (or if some other code is calling UnbindSubscription(MySingleInstanceSubs))

    procedure DoSomething()
    var
        MySingleInstanceSubs: codeunit SingleInstanceSubscribers;
    begin
        BindSubscription(MySingleInstanceSubs);
        SomeCode()
        Error('Ooooops!');
        UnbindSubscription(MySingleInstanceSubs);
    end;

Suggestion for new rule

Please add a new rule that warns if EventSubscriberInstance = Manual; and SingleInstance = true; is set in the same codeunit.

There are some scenarios where above setup is OK, but pragmas should be ok to use in those cases.