When getting DateTime from Introspection then it will cause cast error.

einhugur

When getting DateTime from Introspection then it will cause weird cast error.

code would be something like:

var dt as DateTime = DateTime(prop.GetValue(instance))

bgrommes

einhugur I got "Cannot assign Dictionary to variable of type Dictionary" here -- wonder if they're related.

got https://forum.objo.dev/d/452-unhelpful-error-messages-involving-dictionary

PaulS

@einhugur
Have you tried

var dt as DateTime = prop.GetValue(instance)

bgrommes

PaulS GetValue() would return an Object which should have to be cast to the desired type. If this is automatic here, inferred from the type of the target, that is arguably great, but an explicit cast should work also. Some of us try to keep these things straight in our own heads without relying on syntax sugar ;-)

The error message suggests that GetValue() IS returning a DateTime which is contrary to the documentation. And even if that's the case one could argue that casting a DateTime to a DateTime should just be no-op, not an error.

Personally IF there is something going on here to "help out" the person writing the code, I'd rather not get that sort of help imposed. Either require me to do a cast or document clear rules about any implicit casting going on, and then it'd be my preference to do my own casting if I so choose.

As you might guess, I don't like code with hidden intent. I like it to be explicit. VB.NET was annoying in this regard sometimes. ISTR it accepting strings to assign to numbers so long as they would survive an implicit Int32.Parse() and I felt that encourages sloppy design / thinking. I assumed that was some sort of sop to VB6 legacy behavior but with Objo we don't really have to perpetuate every questionable decision Objo made in the past; we're not pretending to be code-compatible.

einhugur

PaulS

Doing that will not compile and will just emit the following error:

[Error] Class1.Test Line 23: Cannot assign 'Object' to variable of type 'DateTime'.

PaulS

bgrommes Personally IF there is something going on here to "help out" the person writing the code, I'd rather not get that sort of help imposed.

My excuses for trying to "help". This obviously isn't appreciated. I'll shut my mouth.

bgrommes

PaulS Please don't take it personally -- it wasn't meant that way, and I certainly did not mean to demean your input, which I had zero problem with; in fact, it was absolutely a good suggestion. I was talking (possibly too much) about my personal preferences about what choices the language grammar and runtime impose -- I don't think it should be overly "helpful" if it masks realities that people new to the craft need to understand, at least at some point, and I was just cautioning about creating exceptions or inconsistencies in type conversion. And if it ends up being more "helpful" than I'd prefer, I would still like to be explicit when I think the code is clearer that way.

That is just my input -- which no more or less valid than yours or anyone else's.

As it turns out, based on einhugur's response, I'm not sure you CAN assign to DateTime from Object, even with or without a cast, so it's probably NOT an implicit cast or conversion, but rather just a bug of some sort.

bgrommes

einhugur So just to prove it has nothing to do with introspection:

var test As DateTime = DateTime.Now
var oTest As Object = test
var test2 As DateTime = DateTime.MinValue
test2 = test # works
test2 = DateTime(oTest) # Can't assign DateTime to variable of type DateTime
test2 = oTest # Can't assign Object to variable of type DateTime

Line 6 is fine, I agree that line 5 should not be happening.

Garry

Thanks, this is fixed for the next build: https://feedback.objo.dev/bug/554

The issue was not specific to introspection. PropertyInfo.GetValue() returns Object, and the actual runtime value inside that Object can be a native value type such as DateTime.

This should work:

Var dt As DateTime = DateTime(prop.GetValue(instance))

but the VM’s runtime cast path had drifted from the IsA path. IsA knew that a runtime DateTime value was a DateTime, but Cast did not recognise all of the same VM-native value types. That’s why the error looked paradoxical:

Cannot cast 'DateTime' to 'DateTime'

For the next build, IsA and explicit runtime casts now share the same runtime assignability logic. This fixes boxed DateTime values and also covers the same class of issue for Colour, native heap values, class/interface casts, and supported built-in interfaces.

So this remains a compile-time error by design:

Var dt As DateTime = prop.GetValue(instance)

because that is an implicit Object -> DateTime downcast.

But this now works as intended:

Var dt As DateTime = DateTime(prop.GetValue(instance))