Monday, July 27, 2020

Delphi tips and tricks: Unknown custom attribute

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RTTI attributes are a Delphi language feature that allows you to add metadata information
to types and type members (comparable to C# attributes or Java annotations, to be clear). 
By attributes, you can add metadata information in a declarative way. An attribute is depicted
by square brackets ([ ]) placed above the element it is used for, the attribute can be queried at runtime by using RTTI techniques and you take appropriate actions when you think they are required.

That is just a short introduction to Delphi RTTI Attributes and because it is not the central topic of this post, you can find more information about it here:

As for every Delphi construct you have to import it: put the unit in the uses section to use a specif class, interface, function and so on... This is not really true for Attributes!

Let's take a look at this code, a simple console application:

As you can see the [MyCustomAttribute] it's not declared anywhere ( and trust me it's not declared in the System.SysUtils unit).
So what happens if we try to build the project? Curious?
We are able to build it with success:


This because the compiler produces a warning and not an error around the situation just described (using Delphi 10.4):

[dcc32 Warning] Project1.dpr(15): W1074 Unknown custom attribute

In some cases, you might write an incorrect attribute name, or not import the unit
containing that attribute and this could have terrible consequences at run-time: at first you could think that the RTTI Attribute is not working, the framework is not working and so on... but the real fact is that we didn't include the right unit or wrote an incorrect name and probably we didn't have a look to warnings. 

How can we be sure to have an error instead of a warning?

Up to Delphi 102, if you used a custom attribute that was not known to the compiler (because you typed it incorrectly or a unit was missing in the uses statement), you'd receive the warning:

Warning: W1025 Unsupported language feature: 'custom attribute'

The only way to turn it into an error was to set the Unsupported language feature to error. You had to go to Project | Options | Delphi Compiler | Hint and warnings | Unsupported Language Feature and set the value to error. From now on, every Unsupported language feature generates a compiler error. but that could have included also other unwanted scenarios.

Starting with 10.3, the same code will trigger a new specific warning as we have seen in the messages of the example project above:

Warning: W1074 Unknown custom attribute

Then now it is easier to turn this warning into an error by using the directive:


If you add the above line in the previous project and try to build it again you will receive this error and the build will fail:

This is a really nice tip to know as it can save you a lot of time around troubleshooting, just because you didn't have a look to warnings after you write an incorrect attribute name, or not import the unit
containing that attribute. I saw so many times developers in forums and OS projects complaining about the behavior of a framework/feature when the real "mistake" was in a missed unit or misspelled name.

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