VB typeof operator with generics - vb.net

Question

When comparing types in VB the following works as expected and enables the current instance to be compared against a specific inherited class, in this case returning False (snippet from LINQPad)
Sub Main
Dim a As New MyOtherChildClass
a.IsType().Dump()
End Sub
' Define other methods and classes here
MustInherit class MyBaseClass
Public Function IsType() As Boolean
Return TypeOf Me Is MyChildClass
End Function
End Class
Class MyChildClass
Inherits MyBaseClass
End Class
Class MyOtherChildClass
Inherits MyBaseClass
End Class
However when generics are introduced the VB compiler fails with the error Expression of type 'UserQuery.MyBaseClass(Of T)' can never be of type 'UserQuery.MyChildClass'.
' Define other methods and classes here
MustInherit class MyBaseClass(Of T)
Public Function IsType() As Boolean
Return TypeOf Me Is MyChildClass
End Function
End Class
Class MyChildClass
Inherits MyBaseClass(Of String)
End Class
Class MyOtherChildClass
Inherits MyBaseClass(Of String)
End Class
The equivalent code in C# compiles and allows the comparison, returning the correct result
void Main()
{
var a = new MyOtherChildClass();
a.IsType().Dump();
}
// Define other methods and classes here
abstract class MyBaseClass<T>
{
public bool IsType()
{
return this is MyChildClass;
}
}
class MyChildClass : MyBaseClass<string>
{
}
class MyOtherChildClass : MyBaseClass<string>
{
}
Why does the VB compiler not allow this comparison?

Answer 1

You raise an interesting point about VB/C# compilation that I can't really speak to. If you're looking for a solution, here's a way to do it from the question How can I recognize a generic class?
Define these functions:
Public Function IsSubclassOf(ByVal childType As Type, ByVal parentType As Type) As Boolean
Dim isParentGeneric As Boolean = parentType.IsGenericType
Return IsSubclassOf(childType, parentType, isParentGeneric)
End Function
Private Function IsSubclassOf(ByVal childType As Type, ByVal parentType As Type, ByVal isParentGeneric As Boolean) As Boolean
If childType Is Nothing Then
Return False
End If
If isParentGeneric AndAlso childType.IsGenericType Then
childType = childType.GetGenericTypeDefinition()
End If
If childType Is parentType Then
Return True
End If
Return IsSubclassOf(childType.BaseType, parentType, isParentGeneric)
End Function
Call like this:
Dim baseType As Type = GetType(MyBaseClass(Of ))
Dim childType As Type = GetType(MyOtherChildClass)
Console.WriteLine(IsSubclassOf(childType, baseType))
'Writes: True
Here's a Microsoft Connect Ticket that might deal with this issue and give some explanation as to whether this was a feature or a bug of generic typing.
Although this case doesn't seem supported by the Type Of documentation which states that for classes, typeof will return true if:
objectexpression is of type typename or inherits from typename

Answer 2

I'm familiar with C# but less so with VB. However, the example VB code and example C# code appear to be different. In the VB example you use Return TypeOf Me Is MyChildClass, which in C# would be return typeof(this) is MyChildClass;. But the (supposedly working) C# example just has return this is MyChildClass;.
I would expect that TypeOf Me Is MyChildClass is asking whether the instance expression on the left (which is a Type) can be assigned to a variable declared as the type on the right (MyChildClass). Since the framework class Type has no connection to your MyChildClass this is impossible and thus a likely mistake which the compiler can catch with a warning or error--possibly the one you're getting.
Instead, I would think that the VB code should be Return Me Is MyChildClass to match the C# example, which should correctly ask if the instance Me can be assigned to a variable declared as MyChildClass. Does VB still object if this syntax is used, or does that fix the error and get the correct behavior?