Question

I’m sure there is an answer to this somewhere but I’m clearly using the wrong terminology in my searches, so I apologise in advance for this inevitably being a duplicate.

Take the function CType. Clearly I can cast (or at least try) a given object to a given reference type. The function will not work if trying to cast to a structure, i.e.

CType(myObject, Integer)

…will generate a compiler error. This I’m sure most often crops up when working with generics:

Public Function GetResults(Of T)() As T
    Dim instance As T
    Return CType(GetData(instance), T)
End Function

Public Function GetData(ByVal param As myClass) As myClass
    'do stuff
    Return param
End Function

Public Function GetData(byval param As Integer) As Integer
    'do stuff
   Return param
End Function

Public Function GetResults(ByVal param As Object) As Object
    Throw New NotImplementedException
End Function

Probably not the best of examples but hopefully shows what I mean. The following will work:

Dim result = GetResult(Of myClass)

The following will fail on the CType

Dim result = GetResult(Of Integer)

I’m not a big fan of using exceptions/Try-Catch if logic can be applied, so although I could Try-Catch the CType, I’d rather find a way to route it through different code. I know with Generics I can use

Public Function GetResults(Of T As Structure) As T

What is the opposite? I want only reference types so that CType doesn’t fail… I can’t overload the T As Structure with a plain T because it considers them identical signatures, so surely there’s a keyword I’m missing somewhere? ^^

Long question, probably only requries a one word answer (sorry)… Thanks in advance!

Answer 1

Public Function GetResults(Of T As Class) As T