Question

I have a class which I have marked as MustInherit (called BasePage), with a generic method that is marked as MustOverride:

Protected MustOverride Function SaveData(Of T As {BaseClass})(ByVal item As T) As T

What I want to do is force the users of this method to only supply a type of BaseClass, or anything derived from it. Also, when a class derives from BasePage, it should work on only one derived class from BaseClass:

Protected Overrides Function SaveData(Of T As BaseClass)(ByVal item As T) As T
  Dim grad As DerivedClass = CType(item, DerivedClass)

  Return grad
End Function

However, when I try to do the cast, it flags up the following error:

Value of type 'T' cannot be converted to 'DerivedClass'.

All the documentation I have read suggests that this should work. However, it’s not a big problem if it doesn’t work, as I can work around by making a non-generic method that only accepts BaseClass.

Any ideas?

Answer 1

All the documentation I have read suggests that this should work.

On the contrary: it can’t work. The type T derives from BaseClass – but nothing in your code tells the compiler that it is convertible to DerivedClass. For example, it could be of type IndependentlyDerivedClass which is a sibling of DerivedClass.

However, the following cast works:

Dim grad As DerivedClass = DirectCast(DirectCast(item, BaseClass), DerivedClass))

Notice that I’m using DirectCast in place of CType. This is a best-practice when casting in class hierarchies since DirectCast only allows such casts (these, and boxing/unboxing conversions) so you minimize the risk of accidentally calling a conversion operator (which can happen when you’re using CType on non-related types).