Category Archives: Uncategorized

Code annotations in AL – Please, don’t!

One of good practices of writing C/AL Code for Microsoft Dynamics NAV since the dawn of civilization was annotating (commenting) code changes. If you are not sure what I mean by that, this is what I am talking about:

While standards varied about > vs +, or < vs -, or what the annotations should include, there was absolute consensus that annotating changes is an absolute must.

And it was a must. It was such an important rule that everyone followed it without questions asked. In my career, I’ve seen one or two situations of somebody changing or deleting a line of code without leaving any comment, and I’ve seen quite a lot of code, believe you me. It was that important.

It was that important in fact that it was one of the first things developers learned when they signed up for the job, and it was one of the rules they all followed from their first day.

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Codeunit Interfaces – this time for real!

There is a particular programming language feature that I’ve always missed in AL (and C/AL for that matter): interfaces. I’ve dreamt about it, then blogged about it, then finally created a feature request in Microsoft’s AL GitHub repository. My GitHub feature request caught quite some traction and became the most upvoted feature request in less than a day.

Now, I don’t know if or to what extent it was my submission that influenced Microsoft, but it doesn’t really matter – what matters is that it’s becoming reality. Last week at NAV TechDays, Microsoft has presented an amazing new feature that will be coming to AL pretty soon: interface object type. And it’s official, or at least as official as Microsoft’s keynote is, and you can check Microsoft’s demo yourself at

The feature is far more comprehensive than anyone could have hoped for, and it can be used for codeunits and enums, and in the future maybe even for a few more things. Who knows, or dares to dream. I dared, and I am pretty happy I did


Now that you are done through this mouthful of the title, you may recognize that it’s the method you invoke when you want to run a control add-in trigger in AL from JavaScript.

There is nothing new about this method itself, it’s just that most people aren’t aware of full power of this method, and they are using this method in a very inefficient way. In this blog, I want to show you what this method can do for you that you may have not been aware of. And I hope I get you to change your habits.

The syntax is this:

It takes at minimum two arguments, and this is what most people invoke it. Obviously, name is the name of the event as declared in the controladdin object that you also must implement as a trigger inside the usercontrol in the page that uses it. Also, args is the array of arguments you want to pass to AL.

Imagine this is how you declare your event:

Then from JavaScript you would invoke it like this:

So far, very simple, obvious, and easy. But here we get to the biggest mistake most people do when invoking the Microsoft.Dynamics.NAV.InvokeExtensibilityMethod method. They invoke it directly. The reason why it’s a mistake is because most often you’ll want to synchronize the invocations between JavaScript and AL as much as you can, and this method – as anything in JavaScript that invokes stuff outside JavaScript – is asynchronous. If you have this:

… you will see the “You see me immediately” message before AL even gets a chance to start executing.

Yes, you can take advantage of more arguments here to make it behave differently. So, let’s take a look at the remaining two arguments.

The skipIfBusy argument tells the control add-in JavaScript runtime to not even invoke your event in AL if the NST session is currently busy doing something else. If you omit it, the skipIfBusy parameter defaults to false so it means your AL event will be raised, and if AL is already busy, it will be raised as soon as AL stops being busy.

The callback argument, though, is where cool stuff happens. This arguments is of function type (you can imply as much from its name), and it is invoked as soon as AL has finished doing whatever you just made it busy with. So, if you want some JavaScript code to happen after the SayHello event completes its work, you can do it like this:

However, that’s not really the most beautiful way of writing JavaScript. That’s how you would write it in late 1990’s, we are now nearly a quarter century ahead. Let’s write some at least tiny little bit less outdated JavaScript, and let’s introduce Promises. Promises are objects which allow you to synchronize asynchronous calls in a syntactically less offensive way than callbacks.

Let’s take a look at why promises are superior to callbacks.

Imagine you want to structure your code nicely, and you don’t want to just call your extensibility methods out of a blue, so you decide to wrap your call into a function, like this:

The syntax of the sayHello invocation is not really that easy to follow. However, we could translate the entire example to promises:

… and suddenly it becomes more readable. (Okay, a part of it being more readable is that I used arrow functions, but that’s because they are both supported at the same language level of JavaScript, and if your browser supports Promises, it will support arrow functions too, and if it doesn’t support Promises, it won’t support arrow functions either).

Apart from this readability benefit, there is another, far bigger benefit of wrapping your Microsoft.Dynamics.NAV.InvokeExtensibilityMethod invocations into promise-returning wrapper functions: it’s the fact that all promises are awaitable in JavaScript.

In newer versions of JavaScript (EcmaScript 2017 and newer) there is a concept of async functions. Async functions perform some asynchronous work, and you can await on them to make your code look and behave as if it were synchronous.

For example, if you have a function declared as this:

… then you can invoke it like this:

Cool thing about async/await is that it’s nothing more than syntactic sugar for Promises. Every async function implicitly returns a Promise, and you can invoke it either with await syntax, or with .then() syntax. Cosenquently, if a function explicitly returns a Promise, you can await on it as if it were declared as async.

In short, in our earlier example, we could easily do this:

… and it would have exactly the same meaning as the earlier example, except that this time it’s far more readable.

At this stage, our sayHello function is a handy asynchronous wrapper around Microsoft.Dynamics.NAV.InvokeExtensibilityMethod method invocation, but we can do better than that. Instead of having to write wrappers for every single event declared in your controladdin object, you could write something like this:

When you have that, you can use it like this:

Cool, isn’t it? You now not only never have to write that wordy Microsoft.Dynamics.NAV.InvokeExtensibilityMethod ever again (and risk making typos), you also have it fully synchronizable using the await syntax. But we can get even cooler – way cooler – than that. Hold my beer.

You know already that event invocations in AL are void, or that they cannot ever return a value. Your JavaScript cannot invoke AL and have AL return a value to it, that’s just not how AL/JavaScript integration works. At the heart it’s because it’s all asynchronous, but at the end of it, it’s just because Microsoft never cared enough to make it fully synchronized through an abstraction layer that could make it possible. Now that we’ve got it to an awaitable stage, let’s take it to another level by allowing AL to actually return values to your JavaScript wrappers.

Imagine that you declare this event in your controlladdin:

You pass a customer number to it, and you want it to return a JSON object containing your customer record information by its primary key. Ideally, you’ll want to invoke it like this:

Of course, that won’t work, because your GetCustomer trigger in AL – once you implement it in a page – cannot return values. You’d have to have a method declared in your controladdin object and then implement that method in the global scope in JavaScript, where you can pass the result of this operation, something you’d declare like this:

However, implementing it as a separate function in your JavaScript would require some acrobatics to allow you to retain your await getCustomer() syntax. But this is only true if you take the traditional approach of implementing methods as global-scope-level functions in one of your scripts. In JavaScript, you can implement methods on the fly, so let’s do it.

Let’s start with the statement that the GetCustomerResult function should be available in JavaScript only during the invocation of GetResult event in AL, and invoking it outside of such invocation would be a bug, and should not be allowed. When you do it like this, then you can write your code in such a way that you create this function in JavaScript just before you invoke the AL event, and you delete this function immediately when AL returns the result, something like this:

You can then do something like this:

How cool is this?

There is an even further level of awesomeness you can add to your event invocations, and it has to do with the skipIfBusy argument, but that’s a topic for a future blog post, I think you have enough to chew on for now. And I know that at this stage, invoking Microsoft.Dynamics.NAV.InvokeExtensibilityMethod directly, instead of through a pattern such as this, seems very stone age.

Tips & Tricks: Going full screen with control add-ins in BC150

Long time no see here, for I don’t know which time. That’s how it is these days, life kicks in, work kicks in, stuff kicks in, and then time flies and months go between posts here.

I am currently delivering my “Developing Control Add-ins using AL language” workshop at the first pre-conf day of NAV TechDays 2019, and while my group is busy developing their control add-in, I decided to answer the question I got five minutes ago: “can we please have this piece of code”?

So, here it goes. This little trick will allow you to go full screen with your control add-in in BC150. Having a full-screen control add-in is not something that you can do with just setting properties on your controladdin object, but it’s absolutely possible. Also, this is not something you can only do with BC150 – as a matter of fact you can do it in every single version of the Business Central or Dynamics NAV web client (or tablet, or phone client for that matter). However, every single client and every single version has a different overall HTML structure, so this particular trick applies to BC150. It may work on 140, it may work on 160 in the future, but it also may not.

The principle is the same, though: you hide unnecessary screen elements, and you make your control add-in iframe element full width and full height, and that’s it.

This is what your controladdin object must declare:

And then, this is what your control add-in initialization JavaScript code should do (typically you’d put this into your startup script):

And that’s it.

Good luck full-screening!

Codeunit interfaces in AL

Nearly two and a half years ago I had a dream. It was about codeunit references and codeunit interfaces. And today, nearly two and a half years later I am still here, still having the same dream. We still cannot do codeunit interfaces in AL. But it doesn’t mean we don’t need them. We do. We badly do.

Two things triggered me to write this post about how badly we need codeunit interfaces, or any kind of polymorphism: a NAV TechDays talk, and a github project I saw.

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