FormObservableLink works in data source extensions

FormObservableLink class is useful for refreshing display and edit methods in D365FO. You create an instance variable of FormObservableLink type in a form, initialize it and call its observe() method in display/edit methods that you need to refresh on demand. When the refresh is required, you call markChanged() method of FormObservableLink and the system will rerun the methods and display new values. You can find more details in AX 7. Display methods and Form Observability, for instance.

I wondered if I can do the same in a data source extension and I wasn’t really sure that it would work. But it does – and the implementation is virtually the same as when building a whole form.

Here is a simple example:

[ExtensionOf(formDataSourceStr(smmContactPerson, ContactPerson))]
final class MySmmContactPerson_ContactPersonDs_Extension
    // Declare and initalize FormObservableLink
    private FormObservableLink observableLink = new FormObservableLink();
    edit NoYes myMethod(boolean _set, ContactPerson _contactPerson, NoYes _newValue)
        // This says that observableLink will be able to refresh myMethod()
    void refreshMethodsWhenSomethingHappened()
        // Here we trigger a refresh

‘is’ and ‘as’ operators with .NET types

I recently ran into an unfortunate limitation of .NET Interop from X++ (in D365FO).
I wanted to check if an X++ object is of a given type, nevertheless the type used for the variable declaration was a .NET interface. Here is an example:

using Microsoft.Dynamics.ApplicationSuite.FinancialManagement.Currency.Framework;
void demo(IExchangeRateProvider _provider)
    if (_provider is ExchangeRateProviderCBOE) {}

Exchange rate providers are X++ classing implementing IExchangeRateProvider interface and I wanted to check if the object I received was a particular provider (namely ExchangeRateProviderCBOE class or its child). Unfortunately this ended up with a compilation error:

The operand on the left side of the ‘is’ or ‘as’ operator must be a table, class, or form.

As I tested, neither managed interfaces nor managed classes (such as System.Object) can be used on the left side of ‘is’ and ‘as’ operators.

I solved the problem by using Type.IsAssignableFrom() method. Simply using the ‘is’ operator would be nicer, but this does the job too.

void demo(IExchangeRateProvider _provider)
    System.Object providerObj = _provider;
    if (providerObj.GetType().IsAssignableFrom(new ExchangeRateProviderCBOE().GetType())) {}

Note that the problem is only with the left side of the operators, e.g. when you want to check whether a managed type is this or that. Using the ‘is’ operator to check if an instance of an X++ class implements a managed interface works without problems.

Object obj = new ExchangeRateProviderCBOE();
if (obj is IExchangeRateProvider) {}

Exception handling in PU31

In 2018, I wrote the blog post Throwing managed exceptions from X++ in D365FO, where I pondered upon how throwing proper exceptions objects in X++ would be beneficial. This is still true. I also showed a proof of concept how it can be done despite the fact that X++ doesn’t directly support it. But this has changed! Platform Update 31 has introduced the possibility to throw managed (CLR) exceptions directly with the throw statement.

Therefore you can now do things like this right from X++:

throw new System.ArgumentException('A value must be set', 'FromDate');

Then you can react to this particular type of exception and you’ll also get a lot of context, such as which argument is wrong. For example:

System.ArgumentException argEx;
catch (argEx)
    warning(strFmt("Please provide a valid value for the parameter '%1'.", argEx.ParamName));

If you want to define your own exception class (e.g. FieldEmptyException from my previous blog post), you still can’t do it in X++ – you need a C# project or something. And it’s probably not going to change. But it shouldn’t be a big problem, because working with C# projects in D365FO is very easy.

PU31 added one more ability of throw. Imagine that you want to log an exception, but you don’t want to handle it. You catch it, log it and then you can throw another exception. But this new exception won’t have the same properties as the original one, e.g. its stack trace will show that it was thrown from your catch clause. To rethrow the same exception, use throw without any argument. Like this:

catch (ex)

Compare records in code

When saving a record, I had to check which fields had changed and react in a special way if only certain fields (and not any other) changed their value.

I could iterate all fields and compare their values in two table buffers (the original and the updated one), but I thought that D365FO might already have such logic. And indeed it has: in VersioningCompareRecordVersions class. (It exists in AX 2012 too and maybe even in older versions.)

The following example prepares two slightly different records, compares them and shows which fields differ:

// Prepare records to compare
PurchLine origLine;
PurchLine modifiedLine;
select firstonly origLine;;
modifiedLine.PurchQty += 1;
modifiedLine.Name += " updated";
// Compare records
var comparer = VersioningCompareRecordVersions::newTableId(tableNum(PurchLine));
container changes = comparer.packChangedFields(VersioningChangeType::Updated, origLine, modifiedLine);
// Show modified fields
for (int i = 1; i <= conLen(changes); i++)
    container changedField = conpeek(changes, i);
    FieldId fieldId = conpeek(changedField, VersioningCompareRecordVersions::posRelatedFieldId());
    info(fieldId2name(tableNum(PurchLine), fieldId));

There is more what you can do with VersioningCompareRecordVersions, and there are other related classes as well (such as VersioningComparePurchLine), but this is a good starting point.

Thoughts on element prefix vs. suffix

When I read Evaldas Landauskas’s blog post Development guidelines: Prefix Vs. Suffix, I thought I would write a comment below the post and share a few ideas, but then I decided that it’d be better to write my own blog post. Here I have a much better control over formatting and the content will be more visible than if it’s hidden in a mere comment.

I’m not going to dispute the argument that prefixes are better; I’ve seen teams using various approaches and I’m fine with most of them. On my current projects, we also use prefixes for most things. But I want to add a few more things to consider.

The statement that you can’t search objects by suffix because you can’t handle those ending with _Extension is underestimation of regular expressions. For example, if I want to find elements ending with either Xyz or Xyz_Extension, I can use Xyz(_Extension)?$. Is it complicated? Yes, a bit. Is it impossible? Definitely not.

Nevertheless are we doing the right thing in the first place? Isn’t our goal to find elements in our model? If so, searching by a part of name doesn’t really match our intention. We should model:”XYZ project” instead.

Finding all elements starting with a company prefix is nice, but it’s not what people usually need. A more common task is finding extensions of an existing element, such as a table. I think this should be really easy, because it’s both common and important, but we lack really good tools for that. As a workaround, some people choose starting names of extension classes with the name of the original object, which allows them to see all extensions together. We can use regular expressions again to deal with prefixes as well, such as searching for something like type:class SalesTable.*_Extension$, but any solution based on name simply isn’t good enough, in my opinion. And references aren’t great for this purpose either. But we have to live with what we have, or to build better tools.

Changing the topic, I’m also not sure where this statement came from: “We should follow Microsoft’s pattern and suffix it as well.”. As far as I know, Microsoft doesn’t address vendor prefixes/suffixes at all and general naming conventions say that “A subject area specific application object is prefixed with the name of the subject area the object belongs to, for example Cust*, Invent*, Ledger*, Proj*, Vend*”. Therefore names like RevRecAmountPercentList look correct to me.

We surely can find many places where Microsoft isn’t following best practices, just note that SalesFormLetter extending FormLetterServiceController is a special case. It used to extend FormLetter class, but it has changed when SysOperation framework was introduced. One could argue that the name should have been FormLetter_Sales instead of SalesFormLetter, but naming conventions say that it can be the case and not that it must.

I would like to thank Evaldas for giving me something to think and write about. 🙂