Late bound overload resolution and structures

I was just reviewing some notes I made about updating the language specification and I came across this little gem: What do you think the following code does?

Option Strict Off
Structure MyStruct
Public MyField As Integer

Public Sub Mutate(ByVal x As Byte)
MyField = x
End Sub

Public Sub Mutate(ByVal x As Short)
MyField = x
End Sub
End Structure

Module Module1

Sub Main()
Dim o As Object = 5S
Dim s As MyStruct

s.MyField = 10
s.Mutate(o)
MsgBox(s.MyField)
End Sub

End Module

Yes, that’s right, it prints 10. Why? Well, it has to do with how we handle overload resolution and loose-typing.

When we were originally coming up with the rules for overload resolution, we noticed a particular problem with loosely-typed code (that is, code that doesn’t explicitly state the types of its variables). Because all the variables in a loosely-typed program are typed as Object, there are a lot of places where the regular overload resolution rules fall down. Take the example above: if you follow the normal rules of overload resolution, the call to s.Mutate is ambiguous. That’s because the argument type is Object and the two parameter types are Byte and Short. Object has narrowing conversions to both Byte and Short, so we can’t actually choose between them.

However, we thought, at run-time all values become strongly typed. So even though the variable may be statically typed as Object at compile-time, if we defered the overload resolution to run-time, we could probably resolve the overloading correctly. (I should point out again that this only applies to loosely-typed programs: in a strongly typed program, you’d just insert a cast to resolve the ambiguity.) So we added a special “loosely typed overload resolution rule” to handle this situation: if overload resolution produces an ambiguous result solely because of narrowing conversions from Object, then we defer the resolution until run-time. Or, in other words, we make the call late bound. This isn’t such a bad thing because if you’re doing loosely-typed programming, by definition you’re going to be doing a lot of late binding anyway.

OK, so all is well and good, no? Well, not exactly. You see, the problem in the above situation is that to make the call to Mutate late-bound, we have to call some helper functions at run-time. And those helper functions take the target of call as a value typed as Object (since you can late-bind against any type). Which means that we have to cast the target of the call to Object. Which means that, since MyStruct is a structure, we have to box the value. Which means that the target of the call is no longer the stack location indicated by s, but instead some heap location where we boxed the value to. Which means that Mutuate changes the heap location instead of the stack location, and so the change is lost when the method returns.

This is an unfortunate subtle interaction between two features. The good news is that you really only will get into trouble when mixing strongly-typed with loosely-typed code. If, in the example above, you’d typed s as Object and assigned it a new instance of MyStruct, everything would have worked as expected because the structure would always have been boxed. On the other hand, if you turn Option Strict On, the above example will give you an error on the overloaded call to Mutate and will require you to resolve the ambiguity with a cast. So the chance of anyone running into this is thankfully remote.

Leave a Reply