Nothing is private in the debugger (part 2)

In a previous post I discussed how accessibility is ignored when evaluating expressions in the debugger and the unexpected scenarios that it creates.  One case I neglected to mention in that article is how this behavior works with the VB late binding engine. 

The expression evaluator only relaxes accessibility rules when binding an expression.  This is possible because the expression evaluator effectively hosts the compiler and can override items like accessibility checks.

In the case of late binding the compiler only participates in building an expression that will call into the VB runtime late binding engine.  The accessibility determination for the target of late bound call occurs in the VB runtime and is not (currently) overidable by the expression evaluator. 

Late bound access combined with static access can lead to additional confusing behavior.  For example.

 Class C1
    Private Field1 As Integer
    Public Property Property1 As Integer
End Class

Module Module1

    Sub Main()
        Dim v1 As New C1
        Dim v2 As Object = v1
        Stop
    End Sub

End Module

When the following code is run, all members are accessible from the v1 local.  It is statically typed to C1 and hence the expression evaluator can ignore accessibility and access the values.

The local v2 references the same object instance so it’s reasonable to assume it can access the same values.  However because it’s statically typed as object, calls like v1.Field1 actually turn into late bound calls and hence are subject to the rules of the late binder.

Evaluation of Field1 fails here because the late binder does not allow access to private fields.  Property1 evaluates just fine though because it’s public.

We are considering changing this behavior in a future release.  As usual no promises.