2010-02-24-Vb20xx-reflapi-Karl-Michael Beck To LW

Keywords

• Type safe reflection api.
• Inheriting from Generic-type-arguments

version: 2010-02-24T23:15

Hi L,
I would suggest to implement an extended type system for compiletime checked type safety of code generators and other code using reflection.

1. Problem: Reflection APi not type safe.

A language integrated code generator and code based on reflection may want to use a reflection-api that supports compile-time checks. Using reflection is not type safe. If you accept an type object there are very often constraints to type objects valid for specific use cases. These constraints can be checked at runtime but there is no compiletime check for it.

2. Problem: Inherit from generic-template-arguments

As in c++ a class can be inherited from template arguments users may want to inherit from generic type parameters in vb. As vb supports type constraints for generic type arguments working with base type members inherited from generic type arguments can be compile-time checked.

2. Solution: Provide a type safe reflection api.

' Begin Vb20xx
Class B
End Class
Class C
  Inherits B
End Class
Class Demo
  Sub S
   Dim ob As B = New C ' valid cast: C is kind of B
   Dim oc As C = New B ' compiler error: B is not kind of C
   Dim tc As Type = GetType(B)
   Dim tb As Type(Of B) = GetType(C) ' Valid cast: C is kind of B
   Dim tc As Type(Of C) = GetType(B) ' Compiler error: B is not kind of C
  End Sub
End Class
' End Vb20xx

2. Solution: Inherit from generic-type-arguments.

The mechanism needed to implement solution 1 has something todo with the ability to inherit from generic-type-arguments. This feature may also be public for the users. However, there must be some new layers for type-system implemented for the final type system requires to use this service.

' Begin Vb20xx
Class C
End Class
Class G(Of T As C)
  Inherits T ' inherits any member in C and any member in T.
  Sub New()
   Dim c As C = Me ' Valid cast: T is constrained to C.
  End Sub
End Class
' End Vb20xx

Note: A Problem arising here is, that each member in the generic type must be implicitely declared as shadows. But i guess, together with a homogenous naming system this case is quite strange.