InteractiveChange and ProgrammaticChange Oddness

Hi folks,

There are actually two odd issues here, but the second one, for which this subject is named, seems even stranger than the first:

First Issue

I have a textbox, in whose Init() I have this:

THIS.AddProperty("OldValue",{})

THIS.Value = {}

(Let's leave aside, for now, that I could set the default value in the property sheet instead of doing this, and I could use the Format property to enforce good Date types. I don't do that, and haven't in nearly 10 years of use.)

There are some programmatic methods that can set this textbox's value to a Date coming from a Date field in one specific table without any "translation", or to an empty date, as I do above. The table does not accept NULLs.

The InteractiveChange() method of this control (fires when the user is typing in the control) will compare THIS.Value to THIS.OldValue and if they are not the same, THIS.Value will be copied to THIS.OldValue, like so:

IF THIS.Value <> THIS.OldValue THIS.OldValue = THIS.Value ENDIF

There is identical code in the control's ProgrammaticChange() method, so that the two properties are kept in synch when some other piece of code changes the control's Value.

I have verified that no code ever can change the control's Value or OldValue properties to anything that is not a Date. I have also confirmed that there is no somehow corrupted value in the relevant table's date column that is not a valid Date data type. Further, if the user tries to type anything other than a digit or a slash into the control, nothing happens, including that the InteractiveChange() code does not run. The cursor just sits there, waiting for the user to type a valid character. (Not directly relevant but just in case you ask, I also can confirm that the standard un-trappable "Bad date" error will occur if the user's entry consists only of dights and slashes that do not correspond to a valid date.)

This is ancient code and it has not been changed in many, many years.

But today, for the first time (at least since my program started emailing me when it crashes about a year ago) the line:

IF THIS.Value <> THIS.OldValue

triggered Error 107 ("Operator/operand type mismatch") in the InteractiveChange() method.

That is the first odd issue. I cannot figure out for the life of me how that could happen.

Second Issue

The code in the InteractiveChange() and ProgrammaticChange() methods of this textbox was originally identical.

In order to add an error handler that displays a message, resets the values to empty dates, and does not crash the program, I added:

IF TYPE("THIS.Value") <> "D" OR TYPE("THIS.OldValue") <> "D" *Issue some message THIS.Value = {} THIS.OldValue = {} ENDIF

to the InteractiveChange() method before it gets to the property value-comparison line.

Then I inserted

THIS.Value = "Foobar"

Above that block to test my change and ran the program.

I began typing into the textbox. I immediately got Error 107 from the **ProgrammaticChange()** method.

Yes, I had only made the change in the InteractiveChange() method. But I expected that method to generate the error message. Instead, somehow, when I started typing, InteractiveChange() ran to the point where I set the control's Value to a string, executed that line, and then, instead of generating the error message, execution somehow jumped to the ProgrammaticChange() method, where Error 107 occurred on:

IF THIS.Value <> THIS.OldValue

????

I don't believe that InteractiveChange() is supposed to call ProgrammaticChange() or vice-versa. That's why there are two separate methods.

I have often said that the VFP order of events does not always proceed as advertised, and I guess here is proof.

Any thoughts?

Thanks and I hope you are amused.

Ken Dibble www.stic-cil.org