Delphi - Component Debug

OK .. you have designed a component and it doesn't work.

Now what?

This page contains a few of the more difficult design problems that I have had to work through. I hope it helps.

Constructor Missing "inherited" | Wrong *Package* Name

Constructor Missing "inherited"

If you forget the required inherited in the constructor, then the component will be created and work just fine .. unless .. you place it on the form at design time. In that case, the variable that makes the component available will not be set. Using the following code, I was able to verify that when the program started, an object of type T_xyz was created (verified by setting a breakpoint in the constructor), yet the variable xyz1 was not assigned a value (was equal to nil - verified by setting a breakpoint) when FormCreate was called. Adding inherited to the constructor fixed the problem. I found this by accident .. after single stepping through source code for about 5 hours. I simply noticed that the command was missing.

Wrong *Package* Name

When creating a new group of related components, I don't want to place the new stuff in with items I have already tested and released. Instead, I create a new package and use that to test while developing. When the testing is done, sometimes I merge the components into an existing package and sometimes not.

The basic (and wrong) procedure is

This produces a default package name of

At this point you can select your new components from the tool bar and place them on a form. If you want to change the code,

OK, so now you save the package and give it a proper name. (Wow, are you screwed!!)

That's right, the old package is still there! You can write, compile, change whatever you want, and nothing is going to get fixed.

After 2 days of debugging a (very serious) problem, I decided to try and click Install again. That's when I saw the error

(Boy, was I surprised)

Of course, the solution was simple

There was another clue, (I know, sometimes I'm slow) I had just assumed that "Delphi was mysterious" and ignored this clue. (You have to make some mistakes before the symptoms make sense.)

Correct Procedure

As explained in more detail at Creating a Package, the "more correct" procedure is My problem was that I installed the package before saving (and therefore - naming) it. As a result, the package was originally installed using a default name and all source code changes made after the save were ignored.


By the way, the error I was getting (a Google search found nothing) was caused while testing the proper operation of my component at design time. (It worked perfectly at runtime.) Basically, the new component has a published property that holds a pointer to another component. At design time, the property was set using the Object Inspector .. and everything worked fine. The problem occurred when I deleted the second component. Normally, this causes the property value to be cleared (which verifies that the notify flags are properly set) .. when there is a problem .. Delphi crashes. (This is a standard test that must be tried for every property that can hold a pointer to an object on the form.)

However, since this is a design time failure, there are no useful debug tools (that I know of) that simplify problem identification. As a result, I slugged through the source code trying to see what was wrong. In desperation, I tried to reinstall the package .. and found the problem.

Deleting the old Package

To delete the a package, there are 2 methods. From this point, both methods are the same - simply select the bad package and click Remove.

In this case, I selected

None of the files were deleted, but the package was uninstalled and the icons on the toolbar were removed.

When I installed the new (properly named) package, the icons were restored and the design time error was fixed. In the final analysis,

Author: Robert Clemenzi
URL: http:// / Languages / Delphi / ComponentDebug.html